Morphological convergence from independent crab lineages contributes to the flourishing symbiotic diversity between crabs and branching corals

Symbiosis is one of the most important evolutionary processes shaping biodiversity. Phylogenetic and morphological analyses can enhance our understanding of the origin and evolution of symbiosis. The symbiotic relationship between Trapezioidea crabs and branching corals, with high species diversity and flexible symbiosis, is iconic coral/crustacea symbiosis in the marine ecosystem. In this study, the evolutionary history of Trapezioidea crabs was investigated by constructing a phylogenetic relationship based on the whole mitogenome sequences of 66 Brachyura species, including 17 newly sequenced Trapezioidea species. Trapezioidea crabs were split at least into two phylogenetically distant lineages originated at different times. The two lineages, both including facultative and obligate symbionts, convergently evolved smooth and flattened trapezoidal carapaces favorable for specific coral-living. The significantly larger cheliped in obligate species indicated a deeper degree of coral dependence compared with facultative and other associate species.