# Genomic Insights From Natural History Collections Reveal Cryptic Speciation in Coral Guard Crabs (Family: Trapeziidae)

## TL;DR

This study uses genomic data from museum and fresh samples to uncover hidden species diversity in coral crabs, revealing complex evolutionary patterns and the need for further research.

## Contribution

The study introduces a novel method for genomic analysis of degraded DNA from museum samples, uncovering cryptic speciation in coral crabs.

## Key findings

- A strongly supported phylogeny of the Trapezia genus was recovered, suggesting division into two genera.
- Four distinct genetic clusters were identified in T. bidentata, indicating cryptic speciation in the Indian Ocean and Marquesas Islands.
- Central and West Pacific populations showed signs of admixture, suggesting long pelagic dispersal and gene flow.

## Abstract

Mutualistic relationships such as the one between Trapezia crabs and coral colonies are common in reef organisms and play a crucial role in coral resilience and resistance to climate‐induced stressor, yet very little is known about the taxonomic diversity and evolutionary history of the species involved. Despite being essential actors of coral reefs and threatened by the ongoing degradation of their habitat, little genetic information is available for Trapezia crabs, including the exact number of species and their relationships. To overcome this limitation, we sampled Natural History Collections, an important and underutilized source of genomic data. We used a novel approach optimized for degraded DNA to generate high‐quality genomic data from a combination of 166 museum tissues and freshly collected samples and recovered a strongly supported phylogeny of the Trapezia genus, clarifying species relationships of a majority of taxa and suggesting the potential division of Trapezia into two genera. We then focused on the most widespread species
T. bidentata
and identified four distinct genetic clusters, suggesting high divergence and cryptic speciation in the Indian Ocean and the Marquesas Islands. Populations of the Central and West Pacific showed signs of admixture across a heterogeneous seascape, attributing to a potentially long pelagic dispersal phase and expansive gene pool. Our results highlight the need to further explore the genetic diversity within other Trapezia species and other coral‐associated organisms, as they are likely to exhibit more complex genetic patterns than previously understood.

We explored the evolutionary history of Trapezia crabs, key coral mutualists, using genomic data from museum and freshly collected samples. We recovered a well‐supported phylogeny of the Trapezia genus, clarifying species relationships and supporting the division of Trapezia into two genera. Detailed analyses of the widespread species T. bidentata identified four distinct genetic clusters and two cryptic species in the Indian Ocean and the Marquesas Islands, while populations in the Central and West Pacific showed signs of admixture, indicating extensive gene flow. These findings underscore the need for further investigation into the genetic diversity of Trapezia and other coral‐associated species, as they likely exhibit more complex patterns than previously known.