**4. Discussion**

The use of genetic tools for investigating crustacean phylogenetics is widespread and both *cox*1 and H3 have proven suitable for resolving crab identities to species level [25,29,30]. By comparing *cox1* sequences of specimens with published available data for related taxa, we identified two clades: *T. corallina* and *T. bidentata*. The *cox1* mitochondrial gene, while overall highly conserved [31], contains a hyper-variable region that is useful for taxonomy. By performing the same procedure with the H3

gene, the same two clades were resolved: *T. corallina* and *T. bidentata*. The H3 nuclear gene evolves more slowly than mitochondrial genes [32], but nevertheless corroborated the *cox1* findings.

The delimitation of species is problematic for recently diverged lineages that are not yet morphologic distinct. Furthermore, phenotypic convergence can also lead to low morphologic divergence [33]. The integrative use of morphologic (traditional taxonomy) and genetic identification approaches employed herein could be extended to include behavioral and ecological characteristic [34,35] so as to serve as an even more holistic identification method. Using traditional taxonomic characterizations alone, *T. corallina*, *T. formosa* and *T. bidentata* were considered three different species. This conclusion was based on taxonomical parameters that were generally considered valid for discrimination between decapod species: the shape of the carapace, chelipeds and external coloration [24,25,36–39]. However, the molecular data support the conclusion that *T. formosa* is a morphotype of the species *T. bidentata* and thus should not be considered as a separate species.

Accurate identification of members of the *Trapezia* genus is of value not only for the future study of decapods, but also for study of their coral hosts. *Pocillopora* is considered the main reef-builder of the TEP region [40–42] and, unlike dominant corals in other parts of the Indo-Pacific, are markedly resilient to environmental change [43,44]. Given that adult healthy colonies seem always to be associated with at least one *Trapezia* sp. crab, it is possible that this high acclimatization capacity is linked to crab presence [40,45]. It is nevertheless unclear what implications future climate change-driven impacts will have on this mutualistic symbiosis and a more rigorous assessment of ecological implications of crab–coral mutualism in the TEP would be of value. In this case additional molecular markers and other traits (e.g., behavior or physiology of the crabs) could be incorporated into an investigation in order to gain new insights, including into the role of commensals in promoting resilience of the TEP's coral communities.

**Author Contributions:** H.M.C.-F., A.P.R.-T., A.L.C.-M. and E.B.-G. carried out the research. H.M.C.-F. and E.B.-G. analyzed the data. A.P.R.-T., E.B.-G. and A.L.C.-M. contributed reagents, laboratory equipment and analytical tools. H.M.C.-F., A.P.R.-T. and E.B.-G. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** HMCF was funded by a doctoral fellowship from the Consejo Nacional de Ciencia y Tecnologia (CONACYT-No. 632506). The present work was supported by the National Geographic Society (NGS-55349R-19 to APRT) and the project "Programa Integral de Fortalecimiento Institucional" of the Universidad de Guadalajara (P/PIFI-2010-14MSU0010Z-10 to ACM).

**Acknowledgments:** The authors thank Ismael Huerta for image editing and Peter Castro for advice on crab identification. A special thanks to Vladimir Pérez and Jeimy Santiago for their help in field and laboratory work. In addition, we thank the authorities of the Islas Marietas National Park (CONANP) for their assistance with field operations. Finally, we thank to Anderson Mayfield and Rupert Ormond for the English proofreading of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.
