**Hazel M. Canizales-Flores, Alma P. Rodríguez-Troncoso \*, Eric Bautista-Guerrero and Amílcar L. Cupul-Magaña**

Laboratorio de Ecología Marina, Centro Universitario de la Costa, Universidad de Guadalajara, Av. Universidad No. 203. Puerto Vallarta, Jalisco 48280, Mexico; Hazel\_MariaCF@hotmail.com (H.M.C.-F.); ericbguerrero@gmail.com (E.B.-G.); amilcar.cupul@gmail.com (A.L.C.-M.)

**\*** Correspondence: pao.rodriguezt@gmail.com; Tel.: +52-322-226-2319

Received: 15 July 2020; Accepted: 24 August 2020; Published: 26 August 2020

**Abstract:** To date, *Trapezia* spp. crabs have been considered obligate symbionts of pocilloporid corals. They protect their coral hosts from predators and are essential for the health of certain coral species. However, the basic details of this group of crustaceans are lacking, and there is a need for species-level molecular markers. The Tropical Eastern Pacific (TEP) region harbors important coral communities mainly built by corals of the genus *Pocillopora*, with three known *Trapezia* species known to associate with them: *Trapezia bidentata*, *T. formosa* and *T. corallina*. Both taxonomic and molecular analyses were carried out with samples of all three crab species collected from *Pocillopora* spp. in the Central Mexican Pacific. Analysis of both a mitochondrial and a nuclear gene revealed only two species, *T. corallina* and *T. bidentata*. *T. formosa* however appears to be a morphotype of *T. bidentata.* The use of integrative taxonomy for this group has increased the knowledge of the biodiversity not only of the study area, but of the whole TEP and will enhance the future study of the *Trapezia–Pocillopora* symbiosis.

**Keywords:** coral reefs; cox1; H3; crustacea; molecular systematics; morphotypes
