Fiddler crab claw shape variation: A geometric morphometric analysis across the genus Uca (Crustacea: Brachyura: Ocypodidae)

For decades, the extreme asymmetric chelipeds of male fiddler crabs (genus Uca) have been used as a model system for the description and discussion of allometry. Almost all previous studies, however, have concentrated on intraspecific variation and have not examined claw variation among species. In this study, modern methods of describing shape and size, geometric morphometrics, are used to study claw variation across the genus. These analyses are also performed in a phylogenetic context using independent contrasts. Within and among species, major claws show allometric trends in both shape and size. Minor claw growth is isometric within species; across species, these claws are isometric with respect to size, but allometric with respect to shape. Although the variation is much greater in major claws, the allometric pattern of shape change for both majors and minors can be characterized by a general increase in the length of the pollex relative to the manus and the size of the propodus relative to the carpus. There is some phylogenetic clustering of claw shape and size, but there does not appear to be a significant level of phylogenetic dependence because no conclusions are changed when independent contrasts are used. Regenerated major claws have stronger allometric patterns than unregenerated claws, causing them to have shapes associated with relatively larger claws. Minor claw shape shows a strong correlation with habitat type.