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Coral guard crabs in the genus Trapezia are well-documented defenders of their pocilloporid coral hosts against coral predators such as the Crown-of-Thorns seastar (Acanthaster planci complex). The objectives of this study were to examine the protective services of six species of Trapezia against corallivory, and the extent of functional diversity among these Trapezia species. Studies conducted in Mo'orea, French Polynesia showed the Trapezia – coral mutualism protected the host corals from multiple predators through functional diversity in the assemblage of crab symbionts. Species differed in their defensive efficacy, but species within similar size classes shared similar abilities. Smaller Trapezia species, which were previously thought to be ineffective guards, play important defensive roles against small corallivores. We also measured the benefits of this mutualism to nearby coral species in the midst of an Acanthaster outbreak that reduced the live coral cover on the fore reef to less than 4%. The mutualism positively affects the reef coral demography and potential for recovery during adverse predation events through shelter of multiple species of small corals near the host coral. Our results show that some Trapezia species may be functionally equivalent within the same size class, decreasing the threat of gaps in coral protection caused by absence or replacement of any single Trapezia species.
Percentage of coral tissue volume proxy consumed by Drupellacornus in corals hosting the small size-class of Trapezia serenei and Trapezia punctimanus. Letters indicate post-hoc statistically significant differences between groups.
Percentage of coral tissue volume proxy consumed by Culcita novaeguineae in corals hosting the large size-class of Trapezia bidentata and Trapezia serenei. Letters indicate post-hoc statistically significant differences between groups.