Are the symbiont faunas of the venomous echinoids Toxopneustes pileolus and Tripneustes gratilla (Echinoidea, Toxopneustidae) similar?

The coexistence of taxonomically related hosts often leads to their infestation by the same symbiont species. This study tested the hypothesis that, despite the taxonomic proximity of the hosts, morphological differences have a predominant effect on the composition of their symbiont faunas. For this purpose, we studied the species composition, species richness and abundance of symbiont communities, associated with two tropical venomous echinoids Toxopneustes pileolus and Tripneustes gratilla (Toxopneustidae) and analyzed their symbiont specificity. Our results showed that 65.5% of T. gratilla and 91.0% of T. pileolus individuals were inhabited by symbionts. Both hosts harbored eleven species of obligate symbionts (polychaetes, gastropods, copepods, decapods and ophiuroids) all of them first recorded off the coast of Vietnam. Mean species richness was similar in both species, mean intensity were higher in T. gratilla than in T. pileolus, while the level of dominance was higher in T. pileolus. We find out that toxicity per se did not affect the composition of symbiont faunas; the symbiont fauna of T. gratilla is similar to that of taxonomically unrelated regular echinoids; whereas the symbiont fauna of T. pileolus is less diverse and more specific. We concluded that the depleted composition and specificity of of T. pileolus symbiont fauna are associated with its unique morphological feature – a cavity between the surface of the test and the canopy of pedicellariae, preventing the penetration of invaders onto the surface of the test and protecting their symbionts from predators.