Phenotypic variation across the range of the lined shore crab

Two isolated clades of the lined shore crab, Pachygrapsus crassipes, live on opposite sides of the North Pacific, presenting an interesting opportunity for studies of range limits and divergence, yet important gaps remain in our understanding of this species’ biology. Prior to this study, P. crassipes’ Asian range was unclear, and I confirmed that it is found throughout the main Japanese archipelago, though apparently not in the Ryukyu Archipelago. I then examined phenotypic variation of this species’ chela, which are conspicuously colored and larger in males. I found positive allometry for both sexes, which was stronger in males, suggesting that either selection acts on both sexes, but is stronger in males, or that a genetic correlation exists such that selection on males produces a response in females as well. I 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. I conclude that these clades are diverging phenotypically, but that these differences are not yet sufficient to warrant distinction as separate species.