Phenotypic variation across the range of the lined shore crab Pachygrapsus crassipes Randall, 1840 (Decapoda, Grapsidae)

Two isolated clades of the lined shore crab, Pachygrapsus crassipes, live on opposite sides of the northern Pacific, presenting an interesting opportunity for studies of range limits and divergence. Prior to this study, P. crassipes’ Asian range was unclear; we confirmed that it is found throughout the main Japanese Archipelago, though sporadic or absent from the Ryukyu Archipelago. We examined phenotypic variation of this species’ chelae, which are conspicuously colored and larger in males, and found positive allometry for both sexes, which was stronger in males, a common feature of sexually selected ornaments and weapons. We also found that Asian and North American clades differ significantly in chela reflectance — in contrast to previous studies, which stated that these clades were phenotypically identical. We conclude that these clades are diverging phenotypically, but that these differences are not yet sufficient to warrant distinction as separate species.