# Beauty in the beast – Placozoan biodiversity explored through molluscan predator genomics

## TL;DR

This study reveals new placozoan species in seafloor sediments through DNA analysis of predatory sea slugs, expanding their known biodiversity and habitat.

## Contribution

The discovery of three new placozoan lineages and mitochondrial chromosomes in sea slug metagenomic data expands placozoan biodiversity and ecological understanding.

## Key findings

- Three previously unknown placozoan lineages were identified in sea slug metagenomic samples.
- Four placozoan genera were confirmed through mitochondrial chromosome assembly.
- Placozoans were found in seafloor sediment interstices, challenging previous habitat assumptions.

## Abstract

The marine animal phylum Placozoa is characterized by a poorly explored cryptic biodiversity combined with very limited knowledge of their ecology. While placozoans are typically found as part of the epibenthos of coastal waters, known placozoan predators, namely small, shell‐less sea slugs belonging to the family Rhodopidae (Mollusca: Gastropoda: Heterobranchia), inhabit the interstitium of seafloor sediment. In order to gain further insights into this predator–prey relationship and to expand our understanding of placozoan ecological niches, we screened publicly available whole‐body metagenomic data from two rhodopid specimens collected from coastal sediments. Our analysis not only revealed the signatures of three previously unknown placozoan lineages in these sea slug samples but also enabled the assembly of three complete and two partial mitochondrial chromosomes belonging to four previously described placozoan genera, substantially extending the picture of placozoan biodiversity. Our findings further refine the molecular phylogeny of the Placozoa, corroborate the recently established taxonomic ranks in this phylum, and provide molecular support that known placozoan clades should be referred to as genera. We finally discuss the main finding of our study – the presence of placozoans in the sea floor sediment interstitium – in the context of their ecological, biological, and natural history implications.

Previous assumptions about the primary habitat of Placozoa are challenged, and their broader diversity beyond disc‐shaped grazers in coastal waters is revealed. The discovery of placozoan‐predatory sea slugs prompts consideration of a new ecological niche: seafloor sediment interstices. Metagenomic analysis of these sea slugs confirms the presence of placozoan DNA, expanding our understanding of their biodiversity and ecological significance.