PHENOTYPIC DIFFERENCES AMONG CLAWS OF EXTANT SIBLING SPECIES, THEIR HYBRIDS, AND FOSSILS OF THE STONE CRAB MENIPPE

Stone crabs (genus Menippe) are large, durophagous brachyuran crabs well known for their massive commercially harvested claws. Two sibling species of Menippe, distinguished by subtle color pattern and genetic differences, have discrete geographic ranges in the Gulf of Mexico and western Atlantic, despite a high potential for wide geographic dispersal by both species. Menippe mercenaria ranges from North Carolina, around the Florida peninsula, and through the Caribbean, while M. adina occurs in the Gulf of Mexico from northwestern Florida to Tamaulipas State, Mexico. Zones of hybridization between the two species have been reported in northwestern Florida and within the range of M. mercenaria along the coast of Georgia and northeastern Florida. Causes for the differentiation between western Atlantic and western Gulf populations of Menippe are unclear, but may involve a previous disruption of gene flow and/or adaptation to different habitats.

Claws from putatively pure Menippe mercenaria and M. adina populations, two major hybrid zone populations, and Plio-Pleistocene Menippe from southern Florida were examined. Preliminary results using landmark-based techniques show morphological differentiation among claws from populations of extant M. mercenaria, M. adina, and their hybrids. Greatest degrees of claw variability exist between two structurally different regions evident as proximally light and distally dark areas on the movable finger. Patterns displayed by the juncture of the light and dark regions allow for the discrimination of Menippe populations. The light and dark patterns on the fossil fingers from southern Florida are indistinguishable from those of M. adina.