Asymmetry in Male Fiddler Crabs is Related to the Basic Pattern of Claw-waving Display

Morphological asymmetry was correlated with the pattern of claw-waving display in males from five species of fiddler crabs: three vertical wavers (Uca urvillei, U. dussumieri, U. vocans), a lateral waver (U. annulipes), and an intermediate waver (U. tetragonon). On the first, second and third ambulatory legs of male lateral waver crabs, the distance between the inner edge of the basis and the outer edge of the merus was larger on the side bearing the major cheliped than it was on the side with the minor cheliped. A similar asymmetry was observed in male intermediate waver crabs, but only the first ambulatory leg was involved. This morphological asymmetry is clearly related to the style of waving adopted by these crabs. When lateral wavers display, the weight of the major cheliped (which forms about one-third of the total body weight) is carried largely by the anterior ambulatory legs on the same side of the body, but the imbalance of weight during display is less in the intermediate waver. In the vertical waver crab horizontal motion of the major cheliped occurs relatively rarely; thus there is hardly any additional load on the ambulatory legs, which showed no asymmetry. However, the total length of the five sterna bearing thoracic legs tended to be larger on vertical waver males than on the female crabs. Thus the sterna of male crabs bulge outwards more than those of female crabs, and the angle between the sternum bearing the cheliped and the ground surface is larger in male crabs than in females. This may be an adaption enabling the cheliped of the male to be raised higher during the waving display.