ARE ATLANTIC AND INDO-PACIFIC POPULATIONS OF THE RAFTING CRAB, PLAGUSIA DEPRESSA (FABRICIUS), DISTINCT? NEW EVIDENCE FROM LARVAL MORPHOLOGY AND mtDNA

Crabs of the genus Plagusia Latreille, 1804 (Crustacea: Brachyura: Plagusiidae) are well known for their habit of clinging to driftwood or ship hulls and are therefore prone to trans-oceanic transport. One of the consequences of this large dispersal potential seems to be the circumtropical distribution of one species, Plagusia depressa (Fabricius, 1775). This species comprises two subspecies, the Atlantic P. d. depressa and the Indo-Pacific P. d. squamosa (Herbst, 1790) (= P. d. tuberculata Lamarck, 1818). There are only subtle differences in adult morphology between these subspecies. For further comparison of the Atlantic and Indo-Pacific populations, we describe the morphology of the first zoeal stage of P. d. squamosa and the megalopa of P. d. depressa and present sequence data of the mitochondrial 16S rRNA gene. The comparisons of zoeae, megalopae, and mtDNA all provide evidence that the two subspecies of Plagusia depressa are clearly distinct. We therefore propose that Plagusia squamosa deserves species status, that the adult morphology is conserved, and that the major continents represent barriers for dispersal of this rafting tropical crab. '