**1. Introduction**

Coral communities are highly productive ecosystems that harbor a high biodiversity and biomass of small crustaceans [1], among which decapods constitute nearly one-third of the total species [2]. Their relevance is associated not only with the high biomass they represent for the ecosystem, since Crustaceans can affect and/or modify their communities through herbivory, predation, consumption of detritus and nonorganic components and oxygenation of the sediment through burrow construction [3]. Furthermore, crustaceans form numerous, complex trophic relationships that are prone to change in response to both natural and anthropogenic stressors that can threaten the integrity of the coral reef ecosystem, including of crustacean assemblages [4].

Members of the genus *Trapezia* (Decapoda, Brachyura, Trapeziidae) are ecologically important crustaceans that form obligate symbioses with hermatypic corals of the most widely distributed and abundant genus of the Tropical Eastern Pacific (TEP): *Pocillopora* [5]. *Trapezia* crabs live their entire lives, sheltered among the coral branches, providing to the whole colony a defense system against conspecifics and against predators that threaten the coral, such as the corallivorous crown-of-thorns sea star (*Acanthaster* sp.) [6]. The crabs not only defend the coral, but they remove detritus and promote branch elongation [7]. In exchange, the crabs benefit from the coral-provided shelter, as well as nourishment in the form of the detritus that accumulates in the coral branch mucus [8]. In terms of within-colony *Trapezia* distribution, there are typically size-based hierarchies in which mature couples dominate and are located among the central branches. Smaller-sized individuals (new recruits, juveniles and even adults) mainly live at the base of the coral colony [8], promoting a hierarchy in the use of resources according to the size of the organisms. Due the complex ecological role of the

*Oceans* **2020**, *1*

carbs, the *Trapezia–Pocillopora* relationship is now considered as mutualism rather than an obligate commensalism [9,10].

The distribution of *Trapezia* predominantly mirrors that of their coral hosts [11]. Despite the TEP region having been historically characterized as a region with suboptimal conditions for coral growth/development, there are important coral communities present. However, they do face seasonal upwelling, internal waves, high turbidity, eutrophic conditions, high interannual variation in seawater quality and frequent and intense ENSO events [12,13], all of which can negatively affect corals, decreasing their cover and therefore the available habitat and food supply for the crabs. Nevertheless, the TEP harbors well-developed, shallow coral communities, and the northern TEP is considered especially biodiverse [14,15].

To date four *Trapezia* species have been recorded in the TEP: *Trapezia digitalis* and *Trapezia ferruginea*, which are widely distributed across the Indian and Pacific oceans and the endemic *T. corallina* and *T. formosa* [16]. Nevertheless, due to the almost imperceptible morphologic differences among species, some taxonomists have considered there to be only two [16]: *T. corallina* (synonymized with *T. digitalis*) and *T. formosa* (synonymized with both *T. bidentata* and *T. ferruginea*). Therefore, despite the existence of historical records of *Trapezia* within pocilloporid corals, their taxonomic classification remains unclear. Given (1) their importance to coral and coral reef health, (2) the inability to distinguish between putatively different species and (3) the need to understand whether certain crab species demonstrate specificity to (or a preference for) certain coral species, we sought to use morphometric and molecular taxonomic tools to better understand the ecology of the crab–coral symbiosis in the TEP.
