Functional differences between the major and minor claws of fiddler crabs ( Uca, family Ocypodidae, Order Decapoda, Subphylum Crustacea): A result of selection or developmental constraint?

Abstract

We studied the functional differentiation between the major and minor claws of fiddler crabs ( Uca, family Ocypodidae) by means of a strain gauge apPARATUS and by morphometric estimates of claw function. In Uca pugnax, (Smith 1870) closing force is an approximately log-linear function of claw length, but the slope for minor claws is greater than that for the major claws, and the y-intercept for major claws is displaced upwards from that of the minor claw. The pattern of closure also differs: major claws impart rhythmic short pulls, whereas minor claws squeeze far more irregularly. The short pulls of the major claw correspond to rapid squeezes observed in videotapes of combat, whereas the irregular pulls of the minor claw match expectations for gathering particles and transferring them to the mouth parts. The mechanical advantage declines steadily with increasing major claw length. These results suggest that the evolution of form of the extremely large major claws in fiddler crabs involved functional shifts, as opposed to a developmentally constrained extrapolation in size of the minor claw.