Remarkable bilaterally asymmetrical gonopores and gonopods in a new genus and species of brachyuran crab from the Ryukyu Islands, Japan (Decapoda: Brachyura: Xanthidae)

A new genus and new species of xanthid crab, Mabui calculusn. gen., n. sp., are described from Okinawa Island, Ryukyu Islands, Japan. The new taxon is unique for Brachyura by having bilaterally asymmetrical male gonopods and female gonopores, with the particular reproductive structure always distinctly more developed on one side than in the other. This remarkable condition, so far unique among brachyurans, was observed in all specimens examined (N = 53), except for an undeveloped subadult female. The right chelae of all intact males and large females are almost always larger than the left chela, and each right chela has a large and modified cutting tooth. Other than in the asymmetrical external reproductive structures, the new taxon is morphologically closest to the xanthines Megametope ogaensis Sakai, 1974 and Jacforus cavatus (Rathbun, 1907), but can easily be distinguished by the characters of the carapace, third maxilliped, ambulatory legs, and male pleon.