Molecular phylogeny of grapsoid and ocypodoid crabs with special reference to the genera Metaplax and Macrophthalmus

Some species of the genus Metaplax belonging to the family Grapsidae have occasionally been reported to perform waving display that is a characteristic behavior in the family Ocypodidae but uncommon in grapsids. The morphology and life styles of species of Metaplax are also quite similar to those of the ocypodid genus Macrophthalmus. To examine whether ecological and morphological similarities between Metaplax and Macrophthalmus are based on convergent evolution or their common evolutionary history, 841-bp nucleotide sequences from the 16S mitochondrial ribosomal RNA gene of 19 grapsids, 10 ocypodids, and 3 camptandriids, including four species of Metaplax and four species of Macrophthalmus, were analyzed. The resultant phylogenetic tree revealed that both families Grapsidae and Ocypodidae are polyphyletic. Macrophthalmus was distinct from any other ocypodid genera studied, forming a sister group relationship with grapsid species of the subfamily Varuninae, and Metaplax, Cyclograpsus, and Helice of the subfamily Sesarminae. Metaplax, Cyclograpsus, and Helice were found to be more closely related to Varuninae than to other Sesarminae species, indicating that the subfamily Sesarminae is polyphyletic. These relationships were in agreement with the distribution pattern of a tRNA(Val) gene rearrangement on the inferred tree. This molecular phylogenetic analysis suggests that the behavioral and morphological similarities observed between Metaplax and Macrophthalmus species are probably due to convergent evolution, despite a close phylogenetic relationship. The waving display in intertidal crabs of the families Grapsidae and Ocypodidae may have evolved several times in their lineages, associated with exposed semi-terrestrial habitat of the intertidal environment.