Anthozoan Chemical Defenses: Integrating Compounds, Enzymatic Activities, and Omics-Based Discoveries

TL;DR

This paper reviews chemical defenses in anthozoans, highlighting compounds and enzymes that may have biotechnological and pharmaceutical applications.

Contribution

The paper integrates chemical, enzymatic, and omics-based discoveries to explore anthozoan chemical defenses for bioprospection.

Findings

Anthozoans produce diverse compounds with potential antiviral, antibacterial, and anticancer properties.

Genomic and transcriptomic studies reveal genetic elements aiding anthozoans in surviving marine stressors.

Defensive enzymes in anthozoans and their symbionts contribute to survival in hostile environments.

Abstract

Anthozoa is a species-rich class with an innate immune system that acts as a defensive tool and shares many of its cellular pathways with mammalian immune responses. In addition to immune-related strategies (e.g., allorecognition and xenorecognition), anthozoans have evolved to use compounds or toxins for chemical communication, defense, or predation, which may exhibit biological activities useful for human health, mainly antiviral, antibacterial, anti-inflammatory, anticancer, and antitumor properties of pharmaceutical interest. These compounds/toxins can be alkaloids, amino acids, proteins, ceramides, diterpenes, and sesquiterpenes and are mainly distributed into Hexacorallia and Octocorallia. Anthozoans are enriched in defensive enzymes, which can either be found in anthozoan species or their symbionts and help them survive in hostile conditions. Studies related to genomics and transcriptomics using advanced sequencing efforts revealed the presence of genetic elements in anthozoans that help them survive against abiotic and biotic stressors in the marine environment. This review presents developments and highlights the current state of knowledge about anthozoans’ chemical weaponry that can drive further bioprospection of anthozoan species producing compounds and toxins which may be useful in biotechnological applications. Omics research in Anthozoa is still nascent, and more efforts are required to fully understand the chemical ecology, diversity, and possible biotechnological applications of cnidarian genes and their products.

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1. Introduction

Cnidarians constitute one of the primitive diverging phyla in the animal kingdom, and knowing about their origin and diversification is crucial to understanding metazoan evolution. Phylum Cnidaria demonstrated divergence between two large clades, namely Anthozoa (corals and sea anemones) and Medusozoa, containing scyphozoans (true jellyfish), cubozoans (box jellies), hydrozoans (hydroids, hydra, and hydromedusae), and staurozoans (stalked medusae) [1,2]. These soft-bodied, mostly sessile organisms have witnessed the emergence (and disappearance) of a large number of life forms since their origin, at least 700 Ma during the Precambrian era [2,3]. Cnidarians have emerged as successful in thriving in all aquatic environments, protecting themselves from predators, remaining efficient in catching prey, and performing other biological roles necessary for survival. These coelenterates (cnidarians) have developed the ability to produce numerous toxins and bioactive molecules as defensive tools and for prey capture [4]. The members of phylum Cnidaria, including jellyfish, sea anemones, and hydrozoans, are all equipped with cnidocytes (nematocytes), which are stinging cells used as chemical arsenals for avoiding predators and capturing their prey [5]. These cells contain an organelle called cnida or cnidocyst, a product of extensive Golgi secretions, which is possibly the most sophisticated organelle in nature, and their explosive secretion is one of the fastest biomechanical reactions documented in the animal kingdom [6,7]. Cnidarians (e.g., hydra and sea anemones) are mainly dependent on allomones which are distributed through the whole organism, both in body tissues and in specialized stinging cells (nematocytes) [8]. Thus, the evolutionary success of cnidarians can be associated with their well-developed defensive and predatory behaviors, particularly their specialized cells and toxins, which have helped them survive and diversify across multiple aquatic environments for millions of years.

Phylogenetic analysis shows that Anthozoa have appeared earlier in evolutionary history than other classes of Metazoa as they possess circular DNA compared to Cubozoa, Scyphozoa, and Hydrozoa, which carry linear DNA [9], a key position in evolution, which highlights their significant ecological role in marine ecosystems and in the food web [10]. Key evolutionary events that have led to the ecological success of anthozoans across the Phanerozoic include modular and colonial form, the ability to deposit a skeleton of crystalline aragonite or calcite, and the well-developed symbiosis with photosynthetic dinoflagellates. These traits support anthozoans in generating biogenic structures, which maintain the whole reef ecosystems in both shallow and deep waters [11].

Anthozoa is considered the most species-rich class with about 12,505 species in the phylum Cnidaria, which are mainly contained in two subclasses: Hexacorallia (8692 species) and Octocorallia (3671 species), as reported by World Register of Marine Species (WoRMS) (www.marinespecies.org, Accessed 8 April 2025). Hexacorallia (also known as Zoantharia) is further divided into Scleractinia (hard corals, stony corals, true corals, etc.), Zoanthidea (Zoanthids and gold coral), Antipatharia (black corals, whip corals, wire corals, and thorny corals), and Actinaria (sea anemones) orders, while Octocorallia is composed of members of soft corals, gorgonians, sea fans, sea whips, sea feathers, precious corals, pink coral, red coral, golden corals, bamboo corals, leather corals, and horny corals [12,13,14]. Anthozoans are immunologically sophisticated organisms with larger genomes and gene families showing resemblance to those of Bilateria. It is difficult to explain that cnidarians have survived so efficiently with only an innate immune system that acts as a defensive tool against infectious agents. However, these organisms are of great interest because many of their cellular pathways in innate immunity are similar to mammalian immune responses which are absent in other basal invertebrates [15].

Previously, venom was thought to be used mainly for predation in Cnidaria [16,17]. However, cnidarians are now considered one of the two phyla using venom for three prominent ecological functions, which are predation, defense, and intraspecific competition [18]. The extent of the use of venom as a defense tool is highly variable even within closely related groups [19,20]. For instance, in Hexacorallia, nematocyst discharge, when exposed to mechanical and chemical stimuli, was observed in all actiniarian species, with nematocyst secretions in only 40% of zoanthid species and no discharge in the corallimorphians that were tested. These results were reinforced by on-field observations where reef fish showed consistent refusal to consume cnidarians with defensive nematocysts but not to defenseless cnidarians [19]. Another alternative chemical defense strategy in Anthozoa is mediated via poisonous secondary metabolites, particularly in species that dwell in coral reefs [19,20]. In addition, toxin peptides and cells producing nematocysts are distributed across the entire organism, but nematocyte and venom profiles have been seen to vary across morphological structures in species of Actinaria.

A study by Ashwood et al. [21] attempted to understand the relationship between the patterns of toxin expression and the ecological roles of anatomical structures of the sea anemone Telmatactis stephensoni. The results revealed that the regionalization of toxin production aligns with partitioning ecological venom functions across envenomating structures, revealing three major functional regions: tentacles, epidermis, and gastrodermis. Structures serving similar functions exhibited comparable putative toxin profiles and nematocyst types. While no overlap existed between toxins identified via proteomics versus transcriptomics, expression patterns of specific milked venom peptides remained conserved across RNA-sequencing and mass spectrometry imaging datasets. Our data suggest that T. stephensoni may be transcriptionally inactive, containing only mature nematocysts in distal thread portions. These findings indicate that venom profiles of different anatomical regions in sea anemones vary according to their respective ecological functions [21].

Marine sea anemones exhibit diverse ecological strategies for interspecific interactions and food acquisition, shaped by co-evolutionary processes that encompass mutualistic relationships with clownfish and crustaceans, symbiotic associations with zooxanthellae or zoochlorellae, and predator–prey dynamics involving sea slugs. A study by Durán-Fuentes et al. [22] documented feeding behavior and interspecific interactions of Actinostella flosculifera, while characterizing the predatory strategy of the sea slug Spurilla braziliana and the corresponding escape mechanism of A. flosculifera. The study further revealed that shallow tidal pool habitats (~10 cm depth), occupied by A. flosculifera, function as ecological traps, facilitating the prey capture of marine organisms and some biowaste that become stranded during low tide conditions. This is the first documented case of S. braziliana predation on A. flosculifera and describes interspecific associations between A. flosculifera and four crustacean species [22]. These findings collectively demonstrate that sea anemones function as key ecological mediators in marine environments, utilizing specialized venom systems that are anatomically regionalized according to their specific ecological roles—from prey capture and defense to facilitating complex interspecific relationships—and highlight their critical importance as both predators and partners in maintaining the structural and functional integrity of marine community networks.

Calcium carbonate (CaCO_3_) biomineralization represents a 541-million-year evolutionary innovation that has fundamentally shaped species development and global carbon cycling [23]. Within the class Anthozoa, two distinct clades have independently evolved calcification capabilities: Scleractinia (stony corals/Hexacorallia) and Octocorallia (octocorals). Scleractinian corals function as primary reef builders, producing homogeneous aragonite skeletons through well-characterized calcification processes elucidated via skeletal proteomics and immunohistochemistry. Conversely, octocorals exhibit diverse skeletal structures including variable CaCO_3_ polymorphs (aragonite and calcite), organic components such as gorgonin, and distinct sclerite morphologies [24]. This structural diversity in octocorals provides a comparative framework for understanding alternative calcification strategies relative to the established scleractinian model, offering insights into the evolutionary trajectories of biomineralization within cnidarian reef ecosystems. Comparative skeletal proteome analyses revealed that corals with distinguished CaCO_3_ polymorphs utilize a common molecular toolkit that contains cadherin, von Willebrand factor type A, and carbonic anhydrase domains for calcified skeleton deposition. On the other hand, significant divergence exists in collagen distribution, with calcite-forming octocorals exhibiting abundant collagen expression, while aragonitic stony corals show minimal collagen presence. In addition, octocoral collagens have evolved specialized domains associated with matrix adhesion and immune function, potentially representing novel genetic adaptations specific to octocoral calcification mechanisms. These comparative findings illustrate the molecular diversity underlying coral biomineralization strategies and provide foundational insights into octocoral skeletal evolution and formation processes [24]. Venom composition differs significantly across cnidarian classes, where only six proteins out of soluble nematocyst proteins are shared among Scyphozoa, Hydrozoa, and Anthozoa, which mainly have housekeeping functions. The proportion of shared protein content is substantially lower for nematocyst proteins (2%) in comparison to the total proteome (15%) [3]. Venoms of scyphozoans and hydrozoans produce similar biochemical effects; however, sea anemone’s venom is unique as it is dominated by peptide neurotoxins [3,25]. It is hard to determine whether the abundance of neurotoxins is characteristic of anthozoans’ venom unless the taxonomic bias in available data is resolved and knowledge of coral venoms is further elucidated. A comparative analysis of soluble nematocyst proteomes across eight cnidarian species revealed that roughly one-third of identified toxin protein families are shared between Anthozoa and Medusozoa, though Staurozoa was not represented in the study. Among the remaining toxin families, four were restricted to a single taxonomic class, while fifteen were absent from at least one class, with no observable correlation between toxin family distribution patterns and phylogenetic relationships. The apparent loss of multiple toxin families in Cubozoa was linked to a flawed phylogenetic reconstruction that incorrectly positioned Cubozoa as external to both Anthozoa and Medusozoa. This misplacement of Cubozoa likely resulted from the phylogenetic analysis relying solely on the presence/absence of data of known toxins from the ToxProt database rather than more comprehensive molecular data [26].

Actinarians in the subclass Hexacorallia show the highest biological and anatomical diversity [13]. Sea anemones inhabit virtually all marine environments, ranging from deep ocean depths to intertidal coastal areas and from tropical regions to Antarctic waters [13,27], and their widespread distribution lies partly in their capacity to adapt to diverse environmental pressures [28]. In actiniarians, ecological interactions and environmental conditions primarily drive toxin gene expression rather than influence the retention and expansion of toxin gene families themselves [29,30,31]. Comparative studies demonstrate that environmental factors have minimal effects on toxin gene repertoires, showing that phylogenetically related cnidarian species possess more similar toxin gene complements than species sharing the same ecological niche [31]. Additionally, phylogenetic analyses of cnidarian toxin gene sequence variation consistently demonstrate that toxin gene distribution patterns correlate with species evolutionary relationships [32,33,34,35]. These findings indicate that speciation serves as a major driving force in shaping both toxin gene complement and sequence diversity. Nevertheless, ecological factors influencing toxin expression create dynamic spatial and temporal patterns in venom composition [29,30,31,36].

Anthozoans are able to eliminate dangerous microorganisms and also take advantage of associated microbial communities for metabolism, immune defense, development, and behavior [15]. Parisi et al. [15] reviewed knowledge on anthozoan immunity, reporting that they have mechanisms of self-/non-self-recognition, missing adaptive immunity, and discussed signaling pathways and gene transcription activation for defense against pathogens and maintaining homeostasis.

Previous available reviews focused on the phylogenetic relationship of sea anemone toxins, their genes, 3D structures of toxins [25], toxins in corals [37] or cnidarians in general [32,38], cnidarian immunity and defense mechanisms [15], bioactive compounds in zoanthids (sessile colonial anthozoans) with emphasis on alkaloids [39], bioactive metabolites from sea anemones [40], structural overview of sea anemone toxins [41], cytolytic toxins from sea anemones [42], and genomics and transcriptomics of cnidarians, primarily targeting developmental regulatory genes including key bilaterian traits such as mesoderm, nervous system, and bilaterality [43].

In this review, we examined research articles, review articles, and online databases (NCBI) about anthozoan toxins and compounds, chemical defenses, defensive enzymes, and progress in omics for the bioprospection of genomes and transcriptomes in Anthozoa. Our search was performed on Google Scholar, PubMed, and Web of Science, and search parameters were extended to the ‘related articles’ functions.

The aim of the current review is to summarize the compounds and toxins identified in different groups of anthozoans and explore their possible pharmaceutical applications (Figure 1). The review further discusses defense strategies in anthozoans with insights into antioxidant enzymes, defensive enzymes in symbionts, and species-specific enzymatic responses. This work also highlights the current state of the art in genomics and transcriptomics of anthozoans, targeting genetic sequences that may be associated with anthozoans’ responses to various stressors, including predation, bleaching events, and climate change.

The efforts to develop a market impact of biomolecules from anthozoans are still in the nascent stages and will need more research and time. However, some anthozoans, for instance, sea anemones, are making progress in the experimental phases and even in clinical trials. A successful example in clinical trials is the peptide ShK, which was first isolated from the sea anemone Stichodactyla helianthus. An analog of this peptide, ShK-186 [44], known as Dalazatide (Figure 2), is currently in Phase 1b-2a clinical trials for treating autoimmune diseases, including multiple sclerosis and rheumatoid arthritis. The selectivity of ShK-186 for voltage-gated potassium channel Kv1.3 over Kv1.1 is reported to be >100-fold greater than ShK [45,46]. ShK-186 has also shown promising results in vitro by modulating CD4^+^T_EM_ cell activity via Kv1.3 blockade and may offer a possible treatment strategy for patients with granulomatosis with polyangiitis (GPA) with high specificity and fewer side effects; Dalazatide is also reported in a Phase 1b trial for the treatment of plaque psoriasis [47,48].

Defense System in Anthozoans

Cnidarians have no specialized immune cells in their arsenal, yet some cnidarians exhibit specific allorecognition features, for instance, the immunocompetence in colonial hydrozoans and anthozoans characterized by specific reactivity to non-self and succeeding cytotoxic behavior at the colony level, and the presence of a specific memory component in an anthozoan coral (Montipora) [49,50]. Allorecognition is believed to provide protection to colonial cnidarians from mixing with genetically dissimilar individuals and to counteract germline parasitism. These phenomena can be performed with a range of effector mechanisms, including contact avoidance through chemical sensing, use of nematocysts, and barrier formation. For instance, the sea anemone Anthopleura xanthogrammica shows tolerance to adjacent coral individuals but also shows aggressive behavior to heterogenic clones with which it may come in contact [51]. Xenorecognition in coral reefs is expressed in the form of distinctive morphological and cytological responses, and colonial reef corals elicit a repertoire of effector mechanisms as a response to xenogeneic contacts [52]. Studies have revealed that single colonies may use simultaneously or separately different effector mechanisms, revealing the capability for the ‘non-self-recognition’ pattern over ‘self-recognition’ [53]. The effector mechanisms in anthozoans during allogeneic or xenogeneic interactions display enormous complexity. The catalog includes chemical sensing to avoid contact, allelopathy, tissue and skeletal outgrowths, barrier formation, developing sweeper tentacles, recruiting mesenterial filaments, forming pseudofusions, retarded growth rates, bleaching, nematocyst firing, onset of delayed responses, necrosis, tissue growth with no calcification, attracting motile phagocytes, and many more (details in Rinkevich [54]. Despite the absence of specialized immune networks, cnidarians, including anthozoans, have evolved well-developed allorecognition and xenorecognition strategies to help them discriminate between the self and non-self, safeguarding colonial integrity via behavioral and chemical adaptations.

In addition to immunity-related responses, anthozoans have evolved to use compounds or toxins as their chemical communication system, defense, or predation tools to ensure their survival in extreme hostile environments. These cnidarians, ranging from pelagic to benthic species, have been shown to be able to produce a repertoire of toxic compounds that may also have antiviral, antibacterial, and anticancer activities [55]. Despite the lack of information about the immune defense system in cnidarians, the tissues and mucus produced by them are involved in defense mechanisms containing a diverse array of peptides, including neurotoxins of sodium and potassium channels, cytolysins, phospholipase A2 (PLA2), and acid-sensing ion channel (ASIC) peptide toxins, among others. These organisms can also benefit from the versatile aspects of some of their toxins; for instance, some bioactive molecules can offer toxicity associated with antimicrobial activity [55]. The interest in investigating anthozoans is not merely to study toxins and venom, but these animals can also be the source of new molecules of considerable interest for biotechnological and pharmaceutical applications.

2. Compounds and Toxins

2.1. Hexacorallia

Hexacorallia represent a diverse class of anthozoans, and their species diversity is also translated into chemical diversity. This anthozoan sub-class contains all black corals, scleractinians, sea anemones, and tube anemones grouped into several orders (e.g., Actiniaria, Antipatharia, Ceriantharia, Corallimorpharia, Scleractinia, and Zoanthidea). Most Hexacorallians have hexamerous symmetry (as the name suggests), although eight- or ten-part symmetry can also be seen. All species have spriocysts, a type of cnidia with a single-walled capsule and a tubule composed of tiny entangling sub-treads [56].

2.1.1. Zoanthids/Zoantharian

Zoanthids are colonial sea anemones that possess one of the deadliest toxins ever documented, known as palytoxin. It is believed that highly toxic species are not sold commercially for home aquaria; however, the species Palythoa/Protopalythoa spp. (Zoanthus spp.) were unintentionally introduced into a home aquarium where high concentrations of palytoxin were found, which induced severe respiratory reactions in an individual attempting to remove the contaminated colonies using boiling water [57]. Using genetic analysis of 16S and cytochrome c oxidase (COI), Deeds et al. [58] reported the four zoanthid specimens in three aquarium stores in the Washington D.C. area (Palythoa heliodiscus) that were previously responsible for severe respiratory reactions in home aquariums. It was also tested in mice with a lethal dose (LD) of 300 ng/kg [59]; 2 mg of crude toxin from the combined samples can kill 3000,000 mice (standard mouse size of 20 g). Sawelew et al. [60] characterized palytoxin from an undescribed species of Palythoa (sister species to Palythoa aff. clavata) and found in vitro cytotoxicity (some at picomolar doses) against human cancer cells, including cells from lung carcinoma, glioma, gliosarcoma, and melanoma, making palytoxin among potent anticancer candidates. Palytoxin is also a skin tumor promoter, and apprehending the underlying mechanisms of tumor promotion is mandatory to develop preventive and therapeutic strategies [61]. The ecological function of palytoxin is still controversial, and several hypotheses have been established and debated. The real producer of this toxin appears to be a dinoflagellate; most ecotoxicological studies associated with this compound are focused on the metabolites produced by the genus Ostreopsis and not by Palythoa [62].

In a study by Chen et al. [63], a novel neuropeptide Y-like polypeptide, ZoaNPY from Zoanthus sociatus, was explored for its binding with NPY Y2 receptor (mediating NPY-induced angiogenic response) and proangiogenic activity using an in vitro HUVEC model and an in vivo zebrafish model. Their results revealed that ZoaNPY was able to enhance cell survival, migration, and tube formation in endothelial cells at 1–100 pmol. In addition, ZoaNPY could restore chemically induced vascular insufficiency in zebrafish embryos. The neuropeptide could also enhance the phosphorylation of proteins related to angiogenesis signaling in Western blots. Furthermore, ZoaNPY was able to directly and physically interact with NPY Y2 receptor, showing the pro-angiogenic effects of ZoaNPY involved in activating NPY Y2 receptor, which further activates Akt/mTOR, PLC/PKC, ERK/MEK, and Src-FAK-dependent signaling pathways. These results give possible directions for developing novel pro-angiogenic drugs derived from NPY-like polypeptides in pharmaceuticals [63].

Venoms from marine species have been of interest for mining emerging sources of peptide-based therapeutics, and several peptide toxins from sea anemones have been studied for their pharmacological benefits. Venom complexity can be unlocked with combined approaches of large-scale sequencing and data analysis via integrated transcriptomics and proteomics to annotate new proteins or peptides. Transcriptomic and proteomic analysis of Zoanthus natalensis identified six groups of expressed peptide toxins, including neurotoxin, hemostatic and hemorrhagic toxin, protease inhibitors, mixed function enzymes, auxiliary proteins, allergen peptides, and innate immunity-associated peptides. Molecular phylogenetic analysis confirmed the presence and expression of Kunitz-like peptides (similar to Kunitz peptides from snake and spider) in Z. natalensis proteome and transcriptome. In vitro bioassays of this peptide, named ZoaKuz1, revealed an intrinsic neuroprotective activity in the zebrafish model of Parkinson’s disease by serving as a voltage-gated potassium (Kv) channel blocker, suggesting a therapeutic role to control neural dysfunction via the inhibition of neurodegeneration triggered by ion-channel hyperactivity [64]. Likewise, three Kunitz-like peptides (PcKuz) were identified in P. caribaeorum transcriptome, and in vivo toxicity tests in zebrafish larvae were performed to assess neuroprotective effects. The PcKuz3 isotoxin appeared to be the most neuroactive PcKuz peptide which inhibits 6-hydroxydopamine (6-OHDA) induced-neurotoxicity on locomotive behavior in the zebrafish model and indicates neuroprotective effects of PcKuz3 [65] which may serve as an insightful candidate for treating neurodegenerative diseases.

Marine invertebrates are factories for synthesizing compounds, which have numerous health-improving benefits for humans, and zoanthids are rich in antimicrobial compounds to mitigate some of the problems associated with human pathogens resistant to conventional antibiotics. The antimicrobial potential of the cnidocyst extract from Mediterranean zoanthid coral Parazoanthus axinellae, commonly known as yellow cluster anemone, was explored by Stabili et al. [66]. The cnidocyst extract produced remarkable antibacterial activity against human pathogens, such as Streptococcus agalactiae (GBS) and Coccus sp., against several Vibrio species, including microbial agents for humans and aquaculture, mainly, V. alginolyticus, V. anguillarum, V. fischeri, V. harveyi, and Vibrio vulnificus [66]. The antibacterial potential of the P. axinellae cnidocyst extract against vibrios, chiefly V. alginolyticus, is extremely important for biotechnological applications because the extract might be exploited to combat vibriosis, a significant challenge in aquaculture with heavy economic loss [67,68,69].

In addition, other results have also reported the high antibacterial activity of cnidocyst extract against the bacterial strain Streptococcus agalactiae (GBS), a relatively frequent bacterial strain found in the gastrointestinal and genitourinary tract of females. The vertical transmission of the bacteria from mothers to infants at the time of birth is a critical challenge leading to septicemia, meningitis, sepsis, and neonatal pneumonia [66,70,71]. Among newborns, there is a strong relationship between GBS infection and the risk of intrauterine fetal birth [72]. Therefore, finding antibacterial agents capable of fighting GBS is a difficult task due to its high incidence among pregnant women and their neonates, and established antibiotic resistance [73]. The antimicrobial weaponry of P. axinellae cnidocyst extract can give new possibilities of fighting infectious agents including GBS bacteria. The P. axinellae extract incorporated into nanostructures has demonstrated antimicrobial activity against the Gram-positive strain Staphylococcus aureus and Gram-negative strains Aeromonas hydrophila, Aeromonas sobria, Escherichia coli, and Salmonella enterica [74]. Different extracts of marine zoanthid Palythoa caribaeorum demonstrated antioxidant potential (DPPH radical scavenging assay, ferric-ion chelating assay, and ferric-reducing power), cytotoxicity against Artemia nauplii (brine shrimp), and hemolytic activity (to establish whether cytotoxicity is related to damage to the cell membrane or not) against human erythrocytes at 50 µg/mL [75]. These studies suggest that the extracts could have compounds/antioxidants with potential application in pharmaceuticals.

2.1.2. Scleractinia/Stony Corals

Scleractinian corals (stony corals) are the most abundant reef-forming cnidarians found in coral reefs around the world. Most toxicological studies have been performed on Anthozoa; however, the order Scleractinia is poorly explored [76]. Due to their abundance and ecological significance, the knowledge about the diversity of their toxins (bioactive compounds) and their biological functions is important for marine research. Chemical compositions and biological activities of the aqueous extracts of three scleractinian corals collected in the Mexican Caribbean, namely, Pseudodiploria strigosa, Porites astreoides, and Siderastrea siderea, were evaluated for toxicity to crickets (Acheta domestica), hemolysis, vasoconstriction, and nociceptive activity. It was reported that extracts were lethal to crickets and induced concentration-dependent hemolytic activity in rat and human erythrocytes due to the presence of cytolysins. Furthermore, the extracts also exerted vasoconstrictor effects on the vascular tone of isolated rat aortic rings and produced significant nociceptive behavior. The presence of phospholipases A2 (PLA2) and serine protease activities was also reported which is responsible for toxicity in scleractinian corals [77]. The isolated aortic rate assay was used to determine whether extracts contained components that induced effects on the cardiovascular system or not [78]. It was further revealed that scleractinian corals can produce low-molecular-weight peptides that can induce toxicity and vasoconstriction [77]. It is noteworthy that cnidarian species can inflict moderate to extreme pain when contacted by humans, and the degree of envenomation depends on the composition of venom and its entry pathway to human skin [79]. The study reported that all extracts produced significant nociceptive behavior during the neurogenic phase. The attempt to obtain extracts of nematocysts from these corals is challenging and justifies the lack of research on the toxicity of these organisms [77].

Associated Microbiota

Marine organisms and their associated microbiota are reservoirs of varied chemical compounds of interest in marine biotechnology and have led to a substantial number of research initiatives focused on marine bacteria- and fungi-derived compounds. In this regard, Scleractinia and their associated marine bacteria and fungi are considered at the top of the hierarchy, producing secondary metabolites with promising pharmaceutical applications [80]. About twenty-nine different compounds (eleven xanthones, five sesquiterpenes, four phenyl ethers, five alkaloids, and four other compounds) were detected in the methanol extract of the solid rice culture of fungus Scopulariopsis sp., isolated from the inner tissue of coral Stylophora collected from the Red Sea in Egypt. The ethyl acetate extract of this fungus on the solid rice medium showed cytotoxicity against the mouse lymphoma cell line L5178Y. Further cytotoxicity investigations disclosed isolated compounds against mouse lymphoma cell line L5178Y which were antibiotic AGI-B4, violaceol I, violaceol II, and scopularide A with IC_50_ (half-maximal inhibitory concentration) values of 1.5, 9.5, 9.2, and 1.2 µM, respectively, compared to the reference drug kahalalide F with an IC_50_ value of 4.3 µM [81]. In another study by Bara et al. [82], the same fungus isolated from the same hard coral near the Egyptian coastline in the Red Sea was studied to explore the metabolic potential when grown on white beans instead of rice media to assess changes in fungal metabolites [82]. As a result of this approach, metabolites were isolated only in this condition, strongly supporting the one strain, many compounds (OSMAC) approach [83]. Two new terpenoids, 3β,7β,15α,24-tetrahydroxyolean-12-ene-11,22-dione and 15α,22β,24-trihydroxyolean-11,13-diene-3-one, along with fourteen known compounds, were reported, including triterpenoids, coumarins, sesquiterpenoids, and polyketides. All the studied compounds were examined for cytotoxic activities against the mouse lymphoma cell line L5178Y, and for antibacterial and antitubercular activities; however, none of them exhibited significant activity, even when the dose was raised to 10 µg/mL [84].

The crude extract of the solid rice culture of marine-derived fungus Gliomastix sp., isolated from Stylophora sp., contained eight new hydroquinone derivatives, gliomastins A–D, (Gliomastin A), 9-O-methylgliomastin C, acremonin A 1-O-β-D-glucopyranoside, gliomastin E 1-O-β-D-glucopyranoside, and 6′-O-acetyl-isohomoarbutin, together with seven identified analogs. The extract was able to induce cytotoxicity against the mouse lymphoma cell line L5178Y, with the inhibition of 69.1% at a dose of 10 µg/mL [85].

In the case of scleractinian-associated bacteria, a yellowish aerobic marine bacterium, Erythrobacter flavus strain KJ5 (formerly called Erythrobacter sp. strain KJ5), was found in hard coral Acropora nasuta (family: Acroporidae) in Karimunjawa Islands, Indonesia [86], and carotenoids and non-sulphated carotenoids were isolated from this strain. By using an enzymatic assay to investigate the presence of other compounds, the authors showed the discovery of sulfotransferases that catalyze the conversion of carotenoids into carotenoid sulfates, which may be responsible for antithrombotic, antifouling, antiviral, and anti-inflammatory activities [87,88]. A study by Carlson et al. [89] involved a bioassay-guided fractionation to investigate the culture extracts of Streptomyces sp. SCSIO 41399, isolated from the scleractinian coral Porites sp. (collected from Wenchang, China). This investigation led to the isolation and subsequent identification of eight compounds, which included a novel anthracycline, aranciamycin K, one new tirandamycin analog, isotirandamycin B, and four other anthracycline derivatives. Given the necessity of finding new antibiotics and the preventive role of tirandamycin against vancomycin-resistant Enterococcus faecalis, the compounds were tested against Streptococcus agalactiae. Compounds such as isotirandamycin B, tirandamycin A, and tirandamycin B were underscored as potent bacteriostatic agents, with minimum inhibitory concentration (MIC) values of 5, 2.5, and 2.5 µg/mL, respectively, compared to erythromycin as a reference drug displaying IC_50_ values of 5 µg/mL [89]. In addition, Scleractinia-associated zooxanthellae have been reported to contain several compounds, for instance, marine sterols isolated from cultured zooxanthellae from coral Oculina diffusa [90]. Furla et al. [91] focused their research on the symbiosis between Anthozoa and dinoflagellates, in particular, the dinoflagellate Symbiodinium spp., which is also known as zooxanthellae. This symbiosis has influenced species life in different ways: the host has adopted behaviors to optimize the photosynthesis of the dinoflagellates, evolved the ability to absorb and concentrate dissolved inorganic carbon from seawater to supply the algal photosynthesis, developed systems to absorb inorganic nitrogen (which is generally unusual for a metazoan), and has adopted an antioxidant strategy to protect against the oxygen radicals produced during algal photosynthesis. On the contrary, the symbiont produced and transferred to the host sunlight protective molecules, e.g., mycosporine-like amino acids. These findings highlight an example of animal–plant co-evolution.

2.1.3. Sea Anemones/Actinarians

Sea anemones (order Actinaria), also metaphorically known as the flowers of the sea, comprise another important group of Anthozoa, which distinguishes itself from all other cnidarians due to the lack of free-swimming and contains solitary, sessile, and benthic polyps [41,92]. Sea anemone Actinia equina is a benthic cnidarian commonly found on the Portuguese rocky shores, formed by a smooth column, usually red, green, or brown, with a blue line on the edge of the base [25]. The components identified are chiefly proteinaceous and are classified as neurotoxins, cytolysins, and Kunitz-type peptides, mediating the process of paralysis, immobilization, and death of the prey [93]. To study the venom system of A. equina as a potential source of bioactive compounds with biotechnological opportunities for drug discovery, Alcaide et al. [94] characterized the morpho-anatomy of the venom-shooting apparatus and identified proteinaceous toxins and related bioactive compounds in venom, evaluated toxicity, and compared the venom system between two common morphotypes of A. equina (red and green). The toxicity assays revealed that A. equina is able to secrete toxins that produce adverse effects on the prey with varied efficiency for each morphotype. Venom extracts of Actinia exerted toxicity in zebrafish embryos; green specimen extracts produced a faster toxic effect with lower EC_50_, whereas red specimen extracts caused severe malformations to surviving embryos [95]. The venom’s proteome revealed the presence of proteins of biotechnological interest, toxins being the leading members. A neurotoxin known as Delta-actitoxin-Aeq2a was found in both green- and red-specimen extracts [95], and this protein, also called Ae I, is a type-I sodium-channel inhibitory toxin which, upon binding to voltage-gated sodium channels, delays their inactivation during signal transduction and has exhibited toxicity for crabs and mice [96], suggesting the possible use of toxins (proteins) in biotechnology and biomedicine.

Deep-sea anemones are a rich source of bioactive compounds, and they survive extreme conditions of no light, low oxygen, and high pressure through different biochemical and physiological adaptations that may modify their gene regulation, primary metabolism, and most importantly, the production of secondary metabolites [97,98]. Sea anemones of orders Actiniaria and Corallimorpharia (Coral-like anemones) are prevalent in Oceans, particularly in the Pacific Ocean, and they inhabit the intertidal zones to a depth of over 10 km [99]. A study by Kvetkina et al. [100] identified five species of sea anemones which were collected in the Bering Sea and the Sea of Okhotsk in Russia, and species included Actinostola callosa, Actinostola faeculenta, Stomphia coccinea (family Actinostolidae), Liponema breviocorne (family Liponematidae, order Actiniaria), and Corallimorphus cf. pilatus (order corallimorpharia). The extracts of Liponema brevicorne and Actinostola callosa displayed the highest hemolytic activity, whereas the extract of Actinostola faeculenta showed high cytotoxic activity against murine splenocytes and Ehrlich carcinoma cells. The hemolytic potential of aqueous extracts may be associated with the presence of pore-forming toxins known as actinoporins which may be recruited for health-improving applications in humans.

The extracts of Corallimorphus cf. pilatus were not toxic to mouse spleen cells; however, the extracts produced the greatest cytotoxic effects against Ehrlich carcinoma cells, most probably due to the presence of neurotoxins or pore-forming toxins. Furthermore, sea anemones C. cf. pilatus and Stomphia coccinea represented promising sources of antimicrobial compounds (antimicrobial peptides) against Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus) and antifungal compounds against Candida albicans. The aqueous extracts of all sea anemones studied demonstrated α-galactosidase-activating activity which gives an indication of the presence of effectors of this enzyme in the sea anemones [100]. The inhibition of biochemical pathways involving glycosidases via powerful selective inhibitors underscores the treatment of several infectious diseases, malignant neoplasms, and genetic disorders [101], and such an approach can be the cornerstone for searching for novel natural effectors or inhibitors of these enzymes. Deep-sea organisms can, therefore, be treasured for the bioprospection of compounds that might have hemolytic, cytotoxic, antimicrobial, and enzyme-blocking potentials, including anthozoans.

2.2. Octocorallia

Octocorals (Cnidaria, Anothzoa, and Octocorallia) are magnificent biorepositories of natural compounds with unique chemical structures and bioactivities which are of the utmost significance to medicine and biotechnology [102]. This subclass contains a wealth of novel, unusual terpenoids. For instance, the diterpenoids caribenol A and B and elisapterosin B isolated from Pseudopterogorgia elisabethae (now known as Antillogorgia elisabethae), and bipinnapterolide B isolated from Pseudopterogorgia bipinnata (also known as Colombian gorgonian Octocoral), are promising compounds possessing antituberculosis potential, inhibiting the growth of Mycobacterium tuberculosis in vitro [103,104]. Within Anthozoa, the orders Alcyonacea (soft corals) and Gorgonacea (sea fans) are the ones with the highest number of propitious marine bioactive compounds. The order Alcyonacea contains species that are a potential source of producing secondary metabolites which include diterpenes, sesquiterpenes, furanoditerpenes, terpenoids, capnellenes, and steroids with a diverse range of biological activities (Table 1) that can have a great potential in developing new pharmaceuticals [103].

2.2.1. Soft Corals

Members of the genus Xenia (family xeniidae) are rich in diterpenoids; for instance, Xeniolides I, which was isolated from Xenia novaebrittanniae (Kenyan soft coral), exhibited antibacterial activity against Escherichia coli and Bacillus subtills at a concentration of 1.25 µg/mL, while other diterpenoids known as Novaxenicins induced apoptosis in transformed mammalian cells at a similar concentration [238]. A diterpenoid, Blumiolide C, isolated from Xenia blumi (Formosan soft coral), demonstrated strong cytotoxicity against mouse lymphocytic leukemia cells (P-388) at a concentration of ED50 = 0.2 µg/mL (Effective Dose 50) and human colon adenocarcinoma cells (HT-29) at a concentration of ED50 = 0.5 µg/mL [237]. Cembranolide diterpene is also a therapeutic anticancer agent from Lobophytum cristagalli which has shown the inhibition of farnesyl protein transferase (FPT) with IC_50_ = 0.15 µM [180]. FPT is a crucial protein that mediates signal transduction and the regulation of cell differentiation and proliferation [240]. Klyxum simplex synthesizes diterpenes such as Simplexin E, which is found to significantly reduce the level of iNOS (Inducible nitric oxide synthase) and COX-2 (cyclooxygenase-2) proteins in lipopolysaccharide (LPS)-stimulated macrophage cells [178]. K. simplex also synthesizes two diterpene compounds, klysimplexins B and H, which show cytotoxic behavior towards human cancer cell lines.

In vitro studies revealed the cytotoxicity of klysimplexins (B and H) towards hepatocellular carcinoma (HepG2 and Hep3B), human breast carcinoma (MDA-MB-231 and MCF-7), human lung carcinoma (A549), and human gingival carcinoma (Ca9-22) cell lines [177]. Other genera of Alcyonacea (soft corals) which contain bioactive arsenal include Sinularia (Sinularia gibberosa, Sinularia querciformis, Sinularia grandilobata, Sinularia flexibilis, Clavularia koellikeri), Clavularia viridis, and Cespitularia hypotentaculata and exert diverse biological activities such as anti-inflammatory effects, antimicrobial activities, and cytotoxicity, even antifouling properties (e.g., antifouling metabolites), neurotrophic activity, and enzymes inhibiting behaviors [103]. Successively, Eskander et al. [220] studied the chemical defense strategy of the soft coral Sinularia polydactyla against biofilm-forming bacteria. They performed different chemical extraction procedures, i.e., by using methanol or hexane as solvent. Both methanol and hexane extracts inhibited the growth of the biofilm-forming bacteria after 4 h of treatment and affected the bacteria’s extracellular polymeric substance production (EPS). They also studied soft coral tissue damage, showing that this had lower antibiofilm activity. Finally, chemical analyses showed that extracts from intact coral were rich in sesquiterpenes, while coral damaged tissues were rich in cembranoids [220]. Ben-Ari et al. [227] studied reef-building corals, showing different nematocyst densities and hemolytic activities. For instance, the coral Stylophora pistillata, specifically the tips of the branches, had an increased hemolytic activity compared to the bases. In addition, nematocyst density and hemolytic activity were significantly reduced in species maintained for about one year in captivity compared to the corals sampled from the wild. The authors also identified Δ-Pocilopotoxin-Spi1 (Δ-PCTX-Spi1), a cysteine-containing actinoporin, in Stylophora [227]. However, they also noted that, during chromatography, the hemolytic activity was lost, suggesting that other compounds may contribute to that.

2.2.2. Gorgonian Corals

Gorgonian corals (known as sea fans, sea plumes, or sea whips) are prominent members of most tropical and subtropical marine habitats and flourish in the ocean, from the tideland to about 4 km deep in the ocean. These corals are grouped into 13 families comprising more than 6100 species, with the tropical western Atlantic (West Indian) and the Indo-Pacific regions being the two main areas abundant in gorgonian corals. The metabolites produced by these organisms have shown different biological activities such as antioxidant, antiviral, antiplasmodial, antituberculosis, and antitumor activities [241]. Three 8-hydroxybriarane diterpenoids isolated from Gorgonian corals Junceella juncea, junceols (A-C), and junceoal A showed the inhibitory effects on superoxide generation by human neutrophils with 45.64%, 159.60%, and 124.14%, respectively [175]. In a study by Chia-Cheng [173], juncin Z (obtained from the Gorgonian coral Junceella fragilis) proved to have anti-inflammatory properties by inhibiting the production of superoxide anions by human neutrophils at a concentration of 10 µM. A number of steroid skeletons were discovered in gorgonian coral Pinnigorgia sp., which were named as pinnigorgiols (A-E), pinnigorgiol A, compounds containing a rare tricylo [5,2,1,1] decane ring in their structures. Among them, pinnigorgiols D and E were documented to be 11-O-acetyl derivatives of pinnigorgiols A and B, respectively. In vitro investigation concluded that all the newly identified metabolites retained anti-inflammatory potential, with IC_50_ values of pinnigorgiols A-E in the superoxide anion production assay as 4.0, 2.5, 2.7, 3.5, and 3.9 μM, respectively, and inhibitory effects on the release of elastase as IC_50_ 5.3, 3.1, 2.7, 2.1, and 1.6 µM, respectively [197,242]. Another metabolite, known as Apo-9′-fucoxanthinone, isolated from a Pinnigorgia sp., displayed inhibitory effects on elastase release by human neutrophils, at a concentration of 5.75 µM [198,199].

In addition to anti-inflammatory properties, sea fans have a repertoire of compounds that induce oxidative stress in cancer cells. Four compounds, including perezone, were obtained from Caribbean gorgonian coral Pseudoterogorgia rigida using the bioassay-guided fractionation of the extract. All compounds were cytotoxic towards four human tumor cell lines, perezone being the most cytotoxic but not selective to tumor and non-tumor cell lines. Furthermore, perezone was assayed against HL-60 leukemia cells to examine the mechanisms of cytotoxicity. However, pre-treatment with an acute free radical scavenger (L-NAC) prior to cells’ exposure to perezone eliminated the production of intracellular reactive oxygen species (ROS) and lowered its higher cytotoxicity. These protective effects generated by L-NAC proved that the mechanism of perezone-induced cytotoxicity is partially due to ROS production and a subsequent induction of oxidative stress [211]. Two steroid compounds isolated from the Gorgonian Isis minorbrachyblasta, namely 22-epihippuristanol and hippuristanol, have been studied by Qi et al. [172] for their cytotoxicity. Hippuristanol exhibited moderate cytotoxicity against cancer cell lines (A549, HONE1, and HeLa); however, the epimer mixture of 22-epihippuristanol/hippuristanol (3:2 weight ratio) revealed a strong cytotoxic response against A549 and HONE1 cell lines with IC_50_ values of 4.2 and 4.8 µg/mL, indicating a possible synergistic effect on their cytotoxicity against A549 and HONE1 cell lines [172]. Gorgonians and soft corals are also rich in compounds with activity of enzyme inhibition, making these taxonomic groups an ideal target for bioprospecting marine natural products [243].

PTP1B (Protein tyrosine phosphatase 1B) inhibitors have been isolated from numerous species of soft corals including gorgonian corals which can be useful to inhibit PTP1B, known as the target enzyme of new therapeutic drugs (and evaluation of marine compounds as potent inhibitors) in the treatment of type 2 diabetes, obesity, and breast cancer [206]. Novel lipidyl pseudopteranoids, lipidyl pseudopteranes A-F, were isolated from the gorgonian Pseudopterogorgia acerosa; among them, lipidyl pseudopteranes A and D demonstrated modest but selective inhibitory activity against PTP1B, suggesting a promising drug target [206]. The enzyme acetylcholinesterase is considered a drug target for the treatment of neurodegenerative disorders, e.g., Alzheimer’s disease [244], for which several inhibitors have been approved by the FDA for clinical use in patients. The most powerful and well-characterized inhibitors to target this enzyme are membranes asperdiol and 14-acetoxycrassine, isolated from the soft coral Eunicea knighti and the gorgonian Pseudoplexaura porosa, respectively [150].

Similarly, Phospholipases A2 (PLA2) are esterases that cleave off phospholipids and liberate fatty acids and lysophospholipids and are considered promising targets for cancer treatment, as well as inflammation and atherosclerosis [245]. Numerous PLA2 inhibitors have been reported in gorgonian Euplexaura anastomosans, described as farnesylhydroquinone glycosides (Euplexides), and Euplexide A, B, and G [152] displayed inhibitory effects against PLA2, from 47 to 71%, at 50 µg/mL in addition to steroid compounds (Table 1) isolated from gorgonian Acabaria undulata [106]. IKKbeta is one of the two catalytic units that constitute the kinase complex IkB and is involved in nuclear factor-KB signaling, therefore, in the pathogenesis and progression of inflammatory diseases [246,247]. Folmer et al. [228] were successful in isolating carotenoid astaxanthin from gorgonian Subergorgia sp., and it showed inhibitory activity towards the IKKbeta kinase. However, another study revealed that astaxanthin is not synthesized by Subergorgia sp., but it is rather acquired from marine bacteria and algae [248], arguing that bioactive compounds can also be produced by bacteria or algae associated with gorgonians. Protein kinase C (PKC) is a family involved in cell signaling pathways that manage important events like cell proliferation and gene expression regulation [249], and this makes PKC a very crucial target for treating several cancers, neurological and cardiovascular disorders [250,251,252]. Gorgonian Pseudopterogorgia sp. produces three new 9,11-secosterols with inhibitory effects on PKC at the micromolar scale. Moreover, these three molecules were also evaluated in cell cultures due to the role of PKC in inflammatory and proliferative processes, and the compounds showed antiproliferative potential in cell cultures [154].

A number of 9,10 secosteroids (Astrogorgols A-N) isolated from the gorgonian Astrogorgia sp. were assessed against different human tumor-related protein kinases. The kinases evaluated were AKT1 (RAC-alpha serine/threonine-protein kinase), ALK (Anaplastic lymphoma kinase), ARK5 (AMPK-related protein kinase 5), Aurora-B, AXL (AXL receptor tyrosine kinase), FAK (Focal adhesion kinase), IGF-1R (Insulin-like growth factor 1 receptor tyrosine kinase), MEK1wt (MAP kinase 1), METwt (MET receptor Tyrosine Kinase), NEK2 (NIMA-related Kinase 2), NEK6 (NIMA-related Kinase 6), PIM1 (Serine/threonine-protein kinase PIM-1), PLK1 (Serine/threonine-protein kinase PLK1), PRK1 (Serine/threonine-protein kinase N1), SRC (Proto-oncogene tyrosine-protein kinase Src), and VEGF-R2 (VEGFR2 receptor tyrosine kinase). Among the compounds, calicoferol A and E, 24-exomethylenecalicoferol E, 9β-hydroxy-9,10-secosteroid astrogorgol F, and 9α-hydroxy-9,10-secosteroid astrogorgiadiol exerted significant inhibition of kinases LK, AXL, FAK, IGF1-R, METwt, SRC, and VEGFR2 [124]. On the contrary, 9,16 di-oxygenated molecules, including calicoferol I and B, exhibited low inhibition (IC_50_ > 100 µM) against these kinases, indicating oxygenation at C-16 as a cause for suppressed inhibitory activity [124]. The considerable inhibitory potential of 9,10-secosteriods against diverse sets of tumor-associated protein kinases and cytotoxic activities towards tumor cells implied that 9,10-secosteroids [243] may be used as protein kinase targeting inhibitors for the treatment of cancers. In spite of the fact that several enzyme inhibitors were identified in the last five decades from gorgonians and soft corals, not a single one of them possesses complete enzymological characterization. In many of the previous research works, such as that mentioned in a review by Córdova-Isaza et al. [243], only % inhibition at a fixed inhibitor concentration is calculated, and even the IC_50_ value is not described. This trend represents a lack of a uniform procedure for characterizing enzyme inhibitors and users’ unfamiliarity with enzymology [243].

Marine natural products (MNPs), including toxins, are very diverse in marine environments and exhibit a lot of biological activities of pharmacological interest; however, there are challenges associated with the bioavailability and stability of MNPs [253]. For instance, the low bioavailability of marine bioactive peptides (MBPs) represents a limitation due to their susceptibility to degradation by gastrointestinal digestion (of pancreatic, gastric enzymes), membrane enzymes of the small intestine, and under the influence of the acidic environment in the stomach [254,255]. Moreover, the high hygroscopicity of MBPs during storage compromises their stability, resulting in fast microbial and chemical degradation [256]. With recent technologies such as nanoencapsulation and Maillard reaction (MR), there have been promising results in preserving bioactivity, enhancing stability, and controlling the MBPs release with food and nutraceutical applications [253]. Tissue selectivity is a critical concern for any therapeutic agent, where compounds should be cytotoxic against tumor cells and not normal cells. Marine-derived compounds have strong cytotoxic activities against human cancer cell lines; however, they also indicate potential cytotoxicity to normal cells, which limits their therapeutic potential without further structural modification or targeted delivery systems [257], and most of the toxins or compounds in anthozoans display non-selective cytotoxicities.

A major bottleneck to drug development and ensuring sustainability from marine invertebrates is associated with the low biomass of most marine invertebrates, such as wild stocks, low ecosustainability of massive sampling, and, therefore, a very minute amount of pharmacologically active natural products is yielded. The production of pharmacologically active marine natural products can be attained, but in many cases, it lacks economic feasibility due to complex molecular structure and low yields [258]. Taxonomic identification also presents challenges, as many marine organisms cannot be cultured in the laboratory, making them not accessible for taxonomical identification. Such technical criticality can be addressed with recent advanced techniques (e.g., omics) and metagenomics which will alleviate the challenge of identifying and characterizing organisms without cultivation. The criteria for establishing a market demand for marine pharmaceuticals are restricted by long approval times, huge investments from discovery to market, and a high risk of failures due to toxicity and unsustainability [259]. The success of marine-derived drugs will improve with interdisciplinary collaborations, sharing information among stakeholders, collaboration between research institutions and industrial partners, and effective resource management.

3. Defensive Enzymes

3.1. Antioxidant Enzymes in Anthozoans

Antioxidant enzymes scavenge free radicals to protect cells against destructive oxy-radicals. In the case of increased levels, reactive radicals can result in a far-reaching combination of consequences which include lipid peroxidation, protein degradation, and DNA damage, ultimately inducing tissue damage and cell death [260]. Oxy-radicals have been linked to coral bleaching, and the activities of antioxidant enzymes in host and endosymbiotic algae have been documented. However, to locate the potential cellular targets of oxy-radicals in cnidarians, it is necessary to identify the tissues where these enzymes are active. Some of the antioxidant enzymes for scavenging and neutralizing oxy-radicals include superoxide dismutase/SOD (reduces superoxide to hydrogen peroxide (H_2_O_2_) and O_2_), catalase/CAT (converts H_2_O_2_ to 2H_2_O and O_2_), and glutathione peroxidase/GPX (converts reduced glutathione to oxidized glutathione and water) [261].

The antioxidant enzymes SOD, CAT, and GPX were localized in temperate sea anemone Anemonia viridis and tropical coral Goniopora stokesi by using immunocytochemical techniques. Further investigations involving the use of affinity-purified primary antibodies and transmission electron microscopy (TEM) showed that antioxidant enzymes were associated with granulated vesicles, accumulation bodies of endosymbiotic algae, and cnida. SOD and CAT gold-labeling were found in all forms of cnida; SOD was predominantly found on ruptured threads and shafts on b-mastigophore in A. virdis, likely suggesting that the b-mastigophore had endured autolysis and needed SOD to prevent damage to host cells. The presence of SOD and CAT in the accumulation body of endosymbiotic algae agrees with the presupposed role of these bodies in digestion and cell aging. CAT was also localized in isolated electron-rich bodies, often adjacent to microvillous borders in G. stokesi. Similar bodies were documented in A. viridis but composed of GPX instead of CAT, and GPX was also found in symbiotic algae, where it was associated with electron-rich bodies [261].

Corals are known for their fascinating appearance (coloration), largely due to fluorescent proteins (FPs). FPs are abundant and diverse in anthozoans with four basic color types: red (REP), green (GFP), cyan (CFP), and blue/purple non-fluorescent chromoprotein. However, their biological function in a symbiotic association is less understood and controversial. In a study, the presence of FPs was determined and quantified for seven Caribbean hard coral species (Montastraea annularis, Montastraea faveolata, Montastraea cavernosa, Diploria strigosa, Porites astreoides, Dichocoenia stokseii, and Sidastrea siderea) aided by the spectral emission analysis of tissue extracts. There was a positive correlation between FP concentration and H_2_O_2_ scavenging rates both in vivo (across multiple species) and in vitro (with purified proteins), showing antioxidant potential of tissue extracts [262]. Clarke et al. [263] demonstrated the antioxidant capacity of AnthoYFPs against oxidative stress of H_2_O_2_, UV light exposure, and thermal shock (37 °C), a subfamily of GFPs obtained from three species of intertidal sea anemones (Anthopleura elegantissima, Anthopleura sola, and Anthopleura xanthogrammica). The results revealed a higher frequency of dead cells in YFP-negative cells than in YFP-positive cells, with the strongest effects observed in H_2_O_2_ treatment. A similar effect was achieved by treatment with a different oxidizing agent, tert-butyl hydroperoxide, suggesting that AnthoYFP exerts an impartial protective effect, no matter what the source of reactive oxygen species (ROS) may be [263].

3.2. Enzymes in Symbionts

Most corals and sea anemones live in symbiosis with photosynthetic microorganisms (microalgae) known as zooxanthellae. The photosynthetic process of endosymbionts produces a hyperoxic state, which requires an efficient defense strategy in host cells against ROS [264,265]. To address this challenge, symbiotic cnidarians recruit diverse array of SOD isoforms, for instance, copper- and zinc-containing SODs (CuZnSODs) in a diploblastic organism such as anthozoan sea anemone Anemonia viridis [266]. To understand the mechanism of resistance of anthozoan hosts to hyperoxia, two CuZnSOD genes (named as AvCuZnSODa and AvCuZnSODb) were cloned into pGEM-Teasy vector (Promega), and molecular analysis revealed that the AvCuZnSODa transcript encodes an extracellular form of CuZnSOD, whereas the AvCuZnSODb transcript encodes an intracellular form. Upon in situ hybridization, both gene transcripts were documented to be expressed in endodermal and ectodermal cells of the sea anemone Anemonia viridis, not in zooxanthellae [267], representing a perfect example of effective defensive tools in the host when presented with harmful signals from endosymbionts. Proteins exhibiting resistance to hypoxia may be used for developing SOD mimics of biomedical and cosmetic interests [268,269]. Higuchi et al. [270] studied activities of SOD and CAT in a colony of corals Galaxea fascicularis with elevated concentrations of H_2_O_2_ in seawater using incubation chamber, and compared changes in enzyme activity to those induced by increased seawater temperature. It was found that CAT activities (in coral tissue and zooxanthellae) increased with elevated H_2_O_2_, but SOD activity remained relatively constant, suggesting that the spike of H_2_O_2_ in seawater affected coral cytol but did not trigger superoxide formation. On the contrary, increased seawater temperature led to elevation in both SOD and CAT activities in coral tissue and zooxanthellae during short term exposure (5-day period). This may be inferred that coral bleaching would likely not happen from short-term exposure to H_2_O_2_ concentrations in seawater [270].

Ramos and Garcia [271] investigated the cytochrome P450 monooxygenase (MFO) system and antioxidant enzyme responses in the scleractinian coral Montastraea faveolata when exposed to the organic contaminant, benzo(a)pyrene (B(a)P). Corals were subjected to 0.01 and 0.1 ppm B(a)P concentrations for 24 and 72 h, with enzymatic activities measured in host (polyp) and symbiotic zooxanthellae cells. Antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and glutathione S-transferase (GST) showed significant increases at the highest concentration and prolonged exposure time. Cytochrome P420 was present in all colonies, while cytochrome P450 content peaked in colonies exposed to the highest contaminant concentrations. NADPH cytochrome c reductase activity and pigment concentrations remained consistent across treatments. This research represents the first documented evidence of a detoxification mechanism induced in M. faveolata during acute organic contaminant exposure, highlighting the coral’s physiological response to environmental pollutants [271]. This study also acknowledges the induction of biotransformation and antioxidant enzymes in corals exposed to organic contaminants. Among other environmental stresses, Liñán-Cabello et al. [272] studied the short-term exposure of reef-building coral Pocillopora capitata to photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) over 32 h. Exposure to UVR resulted in lower carotenoid levels and antioxidant enzyme (SOD, CAT, GPx, and GST) activities compared to PAR, with reduced carotenoid-pigment-to-chlorophyll ratio. Despite rapid production of non-enzymatic antioxidants like mycosporine-like amino acids (MAAs) and carotenoid pigments, these mechanisms were insufficient to prevent reactive oxygen species (ROS) damage, leading to zooxanthellae expulsion at 33 times more than the rate observed in PAR treatments. The coral demonstrated short-term enzymatic adaptations to resist ROS propagation, potentially enabling survival in high UVR environments. However, additional environmental variables like turbidity, sediment, nutrients, temperature, and osmolarity could interact to cause irreversible damage, underscoring the need for a comprehensive management plan for Mexican Pacific coral reefs [272].

3.3. Species-Specific Enzymatic Response

Different species of corals exhibit species-specific susceptibility and tolerance under the same conditions. A study in 2021 examined shipping-induced stress in two octocoral species, Sinularia polydactyla and Sinularia asterolobata, transported from Indonesia to Europe, by assessing oxidative stress markers, energy reserves, and cellular damage upon arrival and after three months. S. polydactyla showed immediate detoxification efforts through increase in the second line of oxidative defense (increase in glutathione S-transferase (GST), total glutathione (tGSH) activities, and depleted CAT), but ultimately perished within 24 h of arrival. S. asterolobata activated antioxidative pathways (GST, CAT, and tGSH) post-shipping and demonstrated long-term adaptability, though experiencing significantly elevated lipid peroxidation levels after three months. The research underscores species-specific responses to shipping stress and the critical need for tailored transportation strategies to minimize biomass loss in coral trade and research contexts [273]. In Spain, the snakelocks anemone (Anemonia viridis) is a highly valued seafood product; Coll et al. [274] evaluated the physiological response of the snakelock anemone to different biotic and abiotic factors in an aquaculture system by assessing oxidative defense enzymes in tentacular and columnar tissues. The study evaluated multiple antioxidant enzymes including SOD, CAT, GPx, glutathione reductase (GR), glucose 6-phosphate dehydrogenase (G6PDH), glutathione S-transferase (GST), and DT-diaphorase, along with Trolox-equivalent antioxidant capacity (TEAC) and Malondialdehyde (MDA) for lipid peroxidation. Brackish water and integrated multitrophic aquaculture (IMTA) conditions triggered significant changes in glutathione-related enzymatic pathways, particularly in columnar tissue, while reduced light exposure did not compromise the species’ oxidative status despite its symbiotic relationship with photosynthetic organisms. Such studies stressed the improvement in environmental conditions in aquaculture and also the importance of the enzymatic machinery in anthozoan species [274].

The investigation of immune responses in two Argentinian sea anemone species, Aulactinia marplatensis and Bunodosoma zamponii, by examining phenoloxidase and peroxidase activities across their ectoderm, endoderm, and tentacles. While both enzymes were detected throughout all tissues, B. zamponii demonstrated notably higher phenoloxidase production, suggesting enhanced disease resistance and stress tolerance compared to A. marplatensis [275]. Palmer et al. [276] revealed three melanin-synthesis pathway components, mono-phenoloxidase, ortho-diphenoloxidase (tyrosinase-type pathway), and para-diphenoloxidase (laccase-type pathway), in their active form known as phenoloxidase (PO) and inactive form known as prophenoloxidase (PPO), in 22 diverse species of Indo-Pacific anthozoans, including 18 hard corals, 3 soft corals, and a zoanthid. Melanin synthesis enzymatic activities varied among taxa, and inactive tyrosinase-type activity (PPO) and active laccase-type activity (PO) correlated with taxonomic patterns in diseases resistance, whereas the negative relationship between bleaching susceptibility and stored enzymes of less cytotoxic laccase pathways at the family level suggested that melanin production from this pathway may increase bleaching resistance, possibly via protection against light-induced damage without extreme cytotoxicity of the tyrosinase-type pathway [276].

To investigate specific inflammatory responses of Anemonia sulcata when exposed to pathogenic threats, Trapani et al. [277] studied enzymatic activity following bacterial injections of Escherichia coli and Vibrio alginolyticus. By analyzing the enzymatic activity of protease, phosphatase, and esterase, it was revealed that the injection of different bacterial strains alters the expression of these enzymes, implying a correlation between the appearance of an inflammatory reaction and modification of enzymatic activities. Apart from enzymes, enzyme inhibitors have been isolated from sea anemones. For instance, a peptide inhibitor of mammalian α-amylases, Magnificamide, has been isolated from the sea anemone Heteractis magnidica, which has the potential application in controlling postprandial hyperglycemia in diabetes mellitus [278] by inhibiting α-amylases and resolving the challenges of high immunogenicity associated with bacterial-derived polypeptide inhibitors.

4. Molecular Resources Available: Genomes and Transcriptomes

The appearance of the genomic era has stipulated crucial and astonishing insights into the genetic composition of the common ancestor of cnidarians and bilaterians. This has advanced our understanding of how metazoan genomes evolved and when important gene families arose and diverged in animal evolution. By sequencing several cnidarians’ genomes, it has been revealed that cnidarians have a great repertoire of genes which show genome synteny with vertebrates, with fewer gene losses in the anthozoan cnidarian lineage than ecdysozoans (Drosophila melanogaster or Caenorhabditis elegans) [43]. Cnidarian genomes also possess a rich repertoire of transcription factors, including those that in bilaterian model organisms regulate the development of key bilaterian traits, for instance, mesoderm, nervous system, and bilaterality. Overall, the genomics and transcriptomics analysis in anthozoan cnidarians suggest that the most conserved genes in our genomes and mechanisms guiding their expression have evolved prior to the divergence of cnidarians and bilaterians about millions of years ago [43].

Looking on the public database GenBank/NBCI (https://www.ncbi.nlm.nih.gov/genbank/; accessed on 7 April 2025) and searching for anthozoans, a total of 282 genomes were found. However, the genomes, which were annotated either by NCBI RefSeq or by the GenBank submitter, were considered. The search revealed that there are 24 genomes (or 28, where some of the genomes are annotated by both methods) available for Anthozoa. From the NCBI database, it was possible to retrieve significant features such as assembly accession number, organism name, assembly release date, sequencing technology, total genes (annotated), and protein-coding genes (annotated) (as reported in Table 2).

Several authors also combined genomic and transcriptomic responses in anthozoans in order to better understand the species’ response to specific abiotic and biotic stressors. Some examples of habitats/conditions associated with genetic pathways are described in the sections below.

4.1. Genomes and Transcriptomes in the Deep Sea

Deep-sea hydrothermal vents and cold seeps are characterized by darkness, extreme hydrostatic pressure, and the presence of reducing chemicals such as hydrogen sulfide and methane that serve as energy sources to fuel chemosynthesis. They represent a different ecological niche for those organisms that depend on photosynthesis for production and, as such, provide us with very distinctive ecological and evolutionary systems from those commonly studied. The genome (including 30164 protein-coding genes and 14806 tRNA genes) of deep-sea anemone Actinernus sp. was reported to contain a mega-array of ANTP-class homeobox genes. The analysis of homeobox genes disclosed that the longest chromosome hosts a diverse array of Hox clusters, Hox-linked (HoxL) homeobox genes, NK clusters, and NK-linked (NKL) homeobox genes, and the presence of these genes may suggest an ancient ancestral state for these key developmental control genes responsible for molecular adaptations to deep-sea habitats [279]. In addition, the poorly understood tissue (tentacle) regeneration in cnidarians due to non-coding RNAs such as microRNAs (miRNAs) has been investigated using transcriptomics [280]. The sequencing and assembling genome of the sea anemone, Exaiptasia pallida, were carried out after tentacular excision at nine time points, from 0 h to 8 days. The study demonstrated that, in addition to Wnt signaling pathway and ANTP-class of homeobox genes that are previously reported to be involved in tissue regeneration in other cnidarians, GLWamide neuropeptide (out of 4 annotated neuropeptides) and sesquiterpenoid pathways genes may contribute to the late phase of cnidarian tissue regeneration. The expression profiles of genes involved in sesquiterpenoid biosynthetic pathways indicated the down-regulation of Acetyl-CoA acetyltransferase (ACAT) and the isoprenylation pathway genes. During the nine time points, 127 mRNAs and 141 miRNAs were up-regulated, and 58 mRNAs and 4 miRNAs were down-regulated [280].

The adaptations in sea anemones to hydrothermal vents have been studied through the lens of comparative transcriptomics of deep-sea anemones and shallow water sea anemones. Xu et al. [281] sequenced the transcriptome of the hydrothermal vent sea anemone Alvinactis sp. to elucidate the mechanism of adaptation to vent conditions and compared it to another deep-sea anemone (Paraphelliactis xishaensis) and five shallow water sea anemones. There was a total of 117 positively selected genes and 46 significantly expanded gene families reported in Alvinactis sp., which may contribute to vent-specific adaptations, providing the first transcriptome of sea anemones that are inhabitants of hydrothermal vents and an extreme environment. Sea anemone venom is a marine drug resource, not only for the defense system in the organism, but also has value in pharmacology and biotechnology. In a study by Fu and colleagues [282], a transcriptomic approach was used to sequence venom components of different developmental stages of the sea anemone Exaiptasia diaphana, and 533 putative proteins, as well as peptide toxin sequences, were found. The 533 identified transcripts were classified into 75 known superfamilies based on predicted functions, 72.98% proteins and 27.02% peptides. Protein constituents primarily corresponded to metalloproteases, chymotrypsinogen-like, pancreatic lipase-related protein-like (PLRP-like), G-protein-coupled receptor and collagen, while peptide sequences correspond to the ShK domain, thrombin, Kunitz-type, and insulin-like peptide and defensin [282].

Other sequencing projects on early-diverging metazoans such as cnidarians have focused on the innate immunity gene repertoire. An example is the study by Goldstone [283] in 2008 which, by using a genome and transcriptome search, looked for sequences coding proteins involved in the chemical defensome of the starlet sea anemone Nematostella vectensis. The study showed the presence of several sequences related to receptors and signal transduction, efflux transporter proteins, oxidative, reductive, and conjugative biotransformation enzymes, as well as antioxidants, heat shock proteins, and metal detoxification enzymes. Interestingly, the absence of specific proteins was reported, for example, the absence of metallothionein genes, suggesting gene loss. With this comparative analysis, the presence of 266 genes belonging to the sea anemone defensome was reported, compared to 218 in humans, 270 in tunicates, and 423 in sea urchins [283].

There is little information about immunity-associated gene regulation in the host’s early response against bacterial infections in marine environments. Seneca et al. [284] used RNA-seq symbiotic sea anemone Exaiptasia pallida strain CC7 as a model species to illustrate innate immune response to Vibrio parahaemolyticus strain infection and lipopolysaccharides (a Gram-negative specific endotoxin) exposure. Analysis focused on three main objectives: genes differentially expressed in infected anemones, gene expression variation over the onset of infection, and comparison between responses in both types of exposures. Gene expression and functional analysis documented hundreds to thousands of genes responsive to bacterial infection at different exposure periods. The results indicated that non-canonical cytoplasmic pattern recognition receptors (PRRs) such as NOD-like and RIG-I-like receptor homologs take part in the molecular immune response in E. pallida. Moreover, several members of lectin-complement pathways were over-expressed in parallel with novel transmembrane and Ig (immunoglobulin) containing ficolins (CniFLs), suggesting a potent defense against pathogens. Interestingly, the sea anemone lacked typical Toll-like receptors (TLRs), while a TLR-like pathway, including up-regulated MyD88, TRAF6, NF-κB, and AP-1 genes, was activated in the organisms, which were not induced by lipopolysaccharide exposure, proposing an alternative ligand-to-PRR activation. The study further revealed the activation of cytokine-dependent signaling pathways as part of the innate immune response following vibrio exposure, two of which (involving tumor necrosis factor rectors (TNFRs) and several downstream signaling genes) could induce an inflammatory response and/or apoptosis [284]. Venom in the sea anemones that have co-evolved with clownfish via a comparison of transcriptomes was studied in a clownfish-hosting anemone representing each of three major clades of sea anemone hosts: Entacmaea, Stichodactylina, and Heteractina. By investigating transcriptomic data to identify key differences and similarities in venom profiles, in 1121 transcript-matching-verified toxins across all species, hemolytic and hemorrhagic toxins were most dominant [285].

Genomic efforts towards corals have been substantially extended in recent years; genomic and transcriptomic data now in existence for at least 20 coral species, along with comparative molecular studies in corals, have identified genes responsible for biomineralization, symbiosis, and environmental responses [286] and can help us understand the evolution of specific immune gene repertoires in corals. The genome of scleractinian coral Pocillopora damicornis was sequenced and annotated by Cunning et al. [287] to find answers to three critical questions about (1) genes that are either unique to or show diversification within the Scleractinia lineage, (2) genes that are specific to or diversified within individual scleractinian coral species, and (3) features that distinguish the P. damicornis genome from other corals’ genomes. The results reported that 46.6% of genes had orthologs in all other scleractinians, signifying the presence of basic housekeeping genes in the coral’s core genome. Among these core genes, 3.7% were specific to scleractinians with immune functionality, which may translate into an important role in immune processes in coral evolution. Genes only unique to P. damicornis were enriched in cellular signaling and stress response pathways, and such immune-associated gene family expansions were found in each coral species, which emphasizes immune system diversification at various taxonomic levels. Most of the P. damicornis-specific genes were unannotatable in the study; however, protein domain homology disclosed significant enrichment for 11 GO terms, which included the GPCR signaling pathway, bioluminescence, activation of NF-κB inducing kinase, and positively regulated Jun N-terminal cascade (JNK) cascade [287].

4.2. Genomes and Transcriptomes in Response to Abiotic and Biotic Stressors

Climate change factors such as increased sea surface temperatures and ocean acidification can disrupt the symbiotic relationship between reef-building corals and their algal symbionts in the event of coral bleaching. Coral bleaching further increases the chances for disease outbreaks which can permanently modify reef ecosystems. Whole (meta)transcriptome analysis was used by Pinzón et al. [288] to assess the effects of a natural bleaching event on genes involved in the innate immune system of Caribbean coral Orbicella faveolata. The findings revealed that each portion of the holobiont (O. faveolate, algal symbiont symbiodinium spp., and other eukaryotes (e.g., endolithic algae, fungi, ciliates, etc.)) has distinguished responses to bleaching and recovery from bleaching, where the coral host response appeared to be masked by responses of the associated organisms. Coral bleaching changed the expression of genes associated with innate immunity, and these effects lasted (at least one year), even after the recovery of symbiotic populations [288]. In addition to vulnerability to thermal stress in the extensive regime of climate change, O. faveolate is also a victim of disease outbreaks in marine ecosystems. Colonies of O. faveolata were exposed to lipopolysaccharides (LPS), bacterial pathogen-associated molecular patterns (PAMPs), and changes in profiles of gene expression and protein activity were examined by Fuess et al. [289]. Differential expression analysis identified 17 immune-related transcripts that could be classified into one of the three processes of immunity: recognition, signaling, and effector response. Network analyses demonstrated several groups of transcripts correlated to immune protein activity; several transcripts annotated as positive regulators of the immunity were included in these groups, and some were down-regulated after LPS exposure [289]. The reported pattern of dysfunctional gene expression and protein behavior may explicate the processes responsible for disease susceptibility in coral species.

The effects of thermal stress on coral immunity against Vibrio coralliilyticus were investigated through whole-genome transcriptomic analysis in the primary polyp of the Coral Acropora digitifera. The authors reported that bacterial invasion suppressed gene expression concerning innate immune response, mainly down-regulating toll-like receptors (TLRs), nucleotide-binding oligomerization domain-containing proteins (NODs), myeloid differentiation primary response protein (MYD88), and NOD-like receptors (NLRs) under thermal stress. Additionally, to neutralize the infected pathogens, the coral employed complex changes such as altered mitochondrial metabolism and protein metabolism, exosomal intercellular communication for delivery of biochemical cues (e.g., microRNA, proteins, and lipids), and extracellular matrix (ECM) remodeling [290]. In another study by Libro et al. [291], next-generation RNA-seq was used to generate a transcriptome-wide profile of the immune response of the Staghorn coral Acropora cervicornis to White Band Disease (WBD) by comparing healthy (asymptomatic) and infected coral tissues. Differentially expressed transcripts were documented in coral and non-coral datasets to identify gene sequences that are up- and down-regulated due to infection. Their findings revealed that infected coral exhibited substantial changes in gene expression across 4% of coral transcriptome, and the transcripts (of infected coral) were involved in responses such as macrophage-mediated pathogen recognition and ROS production, phagocytosis (Macrophage receptor multiple epidermal growth factor-like domains protein 10 (MEGF10) and actin-22 (act22)) and key mediators of apoptosis (up-regulated tumor necrosis factor receptor superfamily member 1A (TNFRSF1A) and caspase 3 (CASP-3), and up-regulated calcium homeostasis. Furthermore, an enzyme known as allene oxide synthase-lipoxygenase was also up-regulated, suggesting its role in allene oxide pathways in coral immunity. Surprisingly, none of the three primary innate immune pathways—Toll-like receptors (TLRs), Complement and prophenoloxydase pathways—were strongly involved with response of A. cervicornis to infection, and the 52 putative Symbiodinium or algal transcripts had no contribution to coral functions, proposing that immune response is mediated by coral host, not by its symbiont [291].

4.3. Genomes and Transcriptomes to Study Toxins and Bleaching Events

The transcriptome of zoanthid Protopalythoa variabilis was investigated for the presence of peptide toxin-related components in its tissues, and several predicted polypeptides with canonical venom protein features were identified. These polypeptides consist of putative proteins belonging to diverse toxin families, including neurotoxic peptides, hemostatic and hemorrhagic toxins, membrane-active (pore-forming) proteins, protease inhibitors, mixed-function venom enzymes, and venom auxiliary proteins. The functional analysis of two predicted toxin products, Shk/Aurelin-Like Peptide and Anthozoan neurotoxin-like peptide, demonstrated in vivo neurotoxicity that impaired swimming in larval zebrafish. The complex array of venom-related transcripts that are identified in P. variabilis provides insight into toxin distribution among soft corals and can help in comprehending the evolution of venom polypeptides in toxiferous organisms [292]. Diterpenes are major defensive small molecules that help soft corals to survive without a tough exterior skeleton. The discovery of coral defensive biosynthetic genes would prove that marine animals, in addition to their symbionts, can produce defensive molecules, which pinpoint a key biochemical event that has emerged in the soft-bodied corals and also provide information for bioprospecting marine drugs. Scesa et al. [293] described the discovery of terpene biosynthetic gene clusters (BGCs) by using genomic and transcriptomic sequencing of eleutherobin producer Erythropodium caribaeorum, which led to heterologous expression and in vitro characterization of two diterpene-producing enzymes. These coral-encoded terpene cyclase genes synthesize the eunicellane precursor of eleutherobin and cembrene, representative precursors for more than 2500 terpenes found in octocorals, implying that these terpene cyclases mediate an ancient evolutionary role in coral defense.

The blue coral Heliopora coerulea inhabits shallow water habitats and demonstrates the optimal growth rate at a temperature which is very close to the temperature threshold causing bleaching in scleractinian corals. Guzman et al. [294] attempted to understand the molecular mechanisms involved in the biology and ecology of H. coerulea by generating a reference genome of this coral by next-generation sequencing. Metatranscriptome assembly contained transcript sequences from both the coral host and its symbiont, a thermotolerant C3-Gulf ITS2 type Symbiodinium. The transcriptome of the blue coral displayed several gene families involved in stress response, including heat shock proteins and antioxidants that may be associated with maintaining cellular homeostasis, and genes associated with signal transduction and stimulus response [294]. The sea fan coral, Gorgonia ventalina, has suffered large-scale declines in the Caribbean in the 1990s due to an infection known as Aspergillosis caused by a fungal pathogen Aspergillus sydowii. A sea fan pathogen, an Aplanochytrium spp., which is a marine stramenopile protist also damaged the host, mainly through the longitudinal tearing of the host gorgonin (skeleton) and degradation of the host’s polyps. Burge et al. [295] used short-read sequencing (Illumina GAIIx) to generate a transcriptome of the sea fan coral and to characterize the sea fan host response to Aplanochytrium spp. using RNA-seq analysis. The analysis revealed the presence of 210 differentially expressed genes (DEGs) in sea fans exposed to the Aplanochytrium parasite. Several DEGs had putative immune functions such as the role in pathogen recognition (e.g., Tachylectin-5A, Protein G7c, and Neuronal pentraxin-2), genes involved in wound healing (Matrix metalloproteinase or peroxidasin), and antimicrobial peptides (e.g., arenicin-2 and royalisin). Functional enrichment analysis identified that the majority of enriched genes encoded ribosomal proteins involved in protein translation and energy production, and all these genes were up-regulated in exposed sea fans [295].

Finally, Shinzato et al. [296] focused on a genome analysis approach to investigate the defense system of the coral Acropora digitifera. The identified genes included some xenobiotic receptors, transcription factors, antioxidants, metal-related enzymes, and heat shock proteins. Also, in this case, the authors did not find any metallothionein, but found various multicopper oxidases and one phytochelatin synthase [296] Such research on the key aspects of sea fan immunology will enable further studies targeting environmental drivers of disease and host immunity, and also reveal important genes in invertebrate innate immune pathways. These efforts to explore the genomes or transcriptomes of different classes of anthozoans provide a broad understanding of the mechanisms of resistance, evolution, or adaptation in marine organisms in response to different stresses, including defense against predation, infections, and bleaching events.

5. Conclusions

A number of different compounds and toxins were identified in diverse members of the subphylum Anthozoa, and their biological activities, along with their structures (when available/possible), were reported in the current review. The species diversity in Anthozoa also translates into chemical diversity, which makes it quite challenging for cross-comparison across scientific studies. In addition to the richness of toxins/compounds, the diversity of the research methodologies used also makes it difficult to perform comparisons between anthozoan studies.

Some of the compounds and toxins showed bioactivities as potential drug candidates which can improve the current state of pharmaceuticals for human treatments. These compounds were produced in response to the danger of predations, climate change (e.g., thermal stress), and as part of symbiotic associations, which represent the defensive strategies that have evolved over a period of millions of years. Among them, numerous compounds have demonstrated biological activities which are useful for human health, for instance, inhibiting microbial growth, mediating inflammations, producing anticancer or antiproliferative effects against an array of cancer cell lines, and HIV-inhibitory activity. Additionally, some of the compounds or their extracts can help in alleviating the multi-drug resistance not only in humans but also can be used in biotechnological applications in aquaculture. Anthozoan molecules were able to alter various cascade pathways in human cells, such as those related to NF-κB and Jun N-terminal cascade (JNK), and considering that these pathways are known to be involved in various inflammatory and cancerous pathologies, they represent a valuable source for new drug candidates. The active concentrations of the compounds and/or extracts ranged from an MIC value of 2.5 µg/mL (tirandamycin A and tirandamycin B) to 5 µg/mL (sotirandamycin) as potential bacteriostatic agents, cytotoxicity of Blumiolide C ranged from an ED value of 0.2 µg/mL (against mouse lymphocytic leukemia cells (P-388)) to 0.5 µg/mL (against human colon adenocarcinoma cells (HT-29)), and the anti-inflammatory potency of junicin Z at a concentration of 10 µM by inhabiting superoxide anions produced by human neutrophils to promising results of Apo-9′-fucoxanthinone at 5.75 µM of inhibitory effects on elastase secretion by human neutrophils. Experiments have been performed in vitro and in vivo using models such as zebrafish and brine shrimps, which further strengthens the promising results of using anthozoan-derived compounds for possible future drugs from the sea.

This review provides an overview of known chemical defense strategies within Anthozoa and acknowledges the limitations in taxonomic coverage due to the group’s vast diversity and the uneven availability of biochemical data. Several lineages remain underexplored, and their potential for unique defensive metabolites is yet to be uncovered. Future research should prioritize broadening taxonomic sampling and integrating chemical, ecological, and phylogenetic approaches to fully understand the diversity and evolution of chemical defenses across Anthozoa.