Dimorphism of Chelipeds and Systematic Status of a Fiddler Crab, Uca marionis (DESMAREST)

Abstract

In some fiddler crabs of the genus Uca the toothless type of the chela of the male large cheliped has been dealt with as full species or its subspecies distinct from the toothed type. However, HAGEN (1962) and YAMAGUCIII (1973) experimentally showed in U. tangeri and U. lactea that the toothless type is differentiated by the regeneration from the toothed type. The present populational study at Ryukyu Islands also revealed that in U. marionis (DESMAREST) the toothless type is the regenerative type and it is unsuitable to treat the toothed and toothless type individuals as different species or subspecies respectively. In this species, both toothed and toothless type males live together in the same place and no difference was observed between the males of two types in the behaviors and activities. The characteristic toothed chela with two deep notches was found in the individuals larger than 6.5 mm in carapace width, but the toothless type was observed in the individuals larger than 9 mm and it was always fewer than the toothed type as shown in Fig. 2 and Table 1. The percentage of toothless type increased according to the increasing of size as shown in Fig. 2. On the other hand, an enormously large male with a small and toothless chela soon after the regeneration (shown in Fig. 3) was found among the specimens examined. It is reasonable to conclude that the differentiated large chela is toothed at the beginning and that when the large cheliped is lost by the fighting or other activities the large one with toothless chela is regenerated.