# An Annotated Checklist of Symbiotic Copepods of Mollusks in the Global Oceans: A Review of Diversity, Hosts and Geographical Distributions

## TL;DR

This paper compiles a detailed checklist of symbiotic copepods found in marine mollusks worldwide, highlighting their diversity, hosts, and geographic spread.

## Contribution

The paper provides a comprehensive annotated checklist of symbiotic copepods associated with mollusks, summarizing over 340 species and their relationships.

## Key findings

- 342 symbiotic copepod species are associated with over 435 mollusk species, mainly bivalves and gastropods.
- Cyclopoida is the most common order of these symbiotic copepods.
- Many species remain poorly studied, indicating a need for further research.

## Abstract

Symbiotic copepods usually live in the gills, mantle cavity and visceral mass of the marine mollusks, with hosts including polyplacophorans, bivalves, scaphopods, gastropods, and cephalopods. However, our understanding is limited regarding these copepods that are symbiotic with mollusks in the global oceans. To close this gap, we compiled a detailed list of copepods associated with mollusks found in the global oceans based on a review of the existing literature. This list includes information on species diversity, host associations, and geographical distributions. We report that 342 species of symbiotic copepods are associated with more than 435 species of mollusks, primarily bivalves and gastropods, with a few found in other molluscan classes. The most common order of the symbiotic copepods is Cyclopoida. However, many species have not been properly studied or collected, emphasizing the requirement for more research to completely understand the diversity of copepods symbiotic with mollusks in the global oceans.

Symbiotic copepods have a wide host group, including not only invertebrates but also vertebrates, with variable symbiotic sites and morphological characteristics. Even though symbiotic copepods exhibit remarkable diversity, our knowledge of them is still very limited, causing significant lacunae in our understanding of their taxonomic characteristics, host associations, and geographical distributions. To fill these knowledge gaps, we have compiled a comprehensive list of symbiotic copepods and their molluscan hosts in the global oceans based on an extensive literature review. The inventory provides a comprehensive synthesis of the diversity, hosts, and geographical distributions of the symbiotic copepods. This review summarizes information on copepods symbiotic with mollusks from 1863 to 2025. Our compilation records a total of 342 symbiotic copepod species associated with more than 435 species of mollusks. This total includes some copepod species for which no specific host has been identified. For each copepod species, we provide details on its hosts, geographical distributions and the original references.