Cryptic diversity: Who are those coral guard crabs?

Researchers at the University of Guam Marine Laboratory have integrated novel sequencing technologies to reveal hidden diversity in coral reef ecosystems.

Coral reefs provide the structure for an astounding array of diversity, with many examples of symbiotic relationships. Certain species of coral provide food and protection for coral guard crabs (Trapezia), which live safely ensconced in the branches of the corals and defend them from predators like the crown-of-thorns starfish.

“As coral ecosystems around the world decline, it is vital that we uncover the restricted distributions of reef-associated species so that we may better understand how to protect them,” said Kenzie Pollard, co-lead on the study.

Applying the new sequencing strategy, GRAS-Di, which is optimal for the application of degraded DNA, the scientists incorporated the archival DNA of coral guard crabs, thereby vastly increasing the number of localities in the study and yielding high-quality genomic data. “This strategy allowed our team to create a phylogenic tree of the Trapezia species from across much of the taxa’s distribution range in the Indo-Pacific,” explained co-lead Sarah Lemer.

Fresh specimens were collected from reefs in Guam and the Northern Mariana Islands, with samples taken from collections from the Florida Museum of Natural History and the Paris Museum National d'Histoire Naturelle.

Although morphologically similar, this study revealed that the species known as Trapezia bidentata is comprised of three putative species (found in the Pacific Ocean, Indian Ocean, and Marquesas Islands). The study also clarified species relationships of 12 out of 22 currently recognized species in the Trapezia genus and reported the first observation of T. tigrina on Guam reefs.

“While not the first study of its kind, the use of archival DNA as a resource for NextGen sequencing to clarify species boundaries within a broadly dispersed species was previously difficult to do. This is primarily due to the limitations of sequencing technologies that utilize restriction enzymes with degraded DNA. We hope to encourage further studies of this kind to continue clarifying species distributions and revealing cryptic diversity,” stated Pollard.

Their work, Genomic Insights From Natural History Collections Reveal Cryptic Speciation in Coral Guard Crabs (Family: Trapeziidae), was published Feb. 19, 2025, in Ecology and Evolution.

Pollard and Lemer were joined in the study by Carlos Leiva and Heloise Rouzé.

Future investigations of this team include clarifying the entire genus phylogeny of Trapezia and collaborating with crustacean taxonomists to describe the species presented in this study to understand at what level each species differs.

Their results highlight the need to 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.

Genomic Insights From Natural History Collections Reveal Cryptic Speciation in Coral Guard Crabs (Family: Trapeziidae) Pollard, K., Leiva, C., Rouzé, H. and Lemer, S., 2025. Ecology and Evolution, 15(2), p.e70960.