Occurrences of Abnormal Males in a Fiddler Crab, Uca marionis (DESMAREST), with Notes on Asymmetry of Chelipeds

Abstract

As shown in Table 1, the examination of the male fiddler crabs referable to Uca marionis (DESMAREST) from the Ryukyu Islands revealed that this species is usually right-handed and quite different from the cases of U. pugilator and U. pugnax studied by YERKES (1901) and U. lactea by YAMAGUCHI (1973) in which the ratio of the right-and left-handed large cheliped is recognized to be 1: 1. In the field observation it may be briefly pointed out that the right-handed large cheliped is possibly effective for the typical fighting without injury as shown in Fig. 2. Some important contributions to the differentiation of the large cheliped were made by MORGAN (1923, 24) and YAMAGUCIII (op. cit.) on the immature males of U. pugilator or U. pugnax and U. lactea, respectively. In the American species dealt with by MORGAN the immature males pass through a stage in which two large chelipeds are present and the loss of one or the other of the chelipeds is the determining factor leading to the fixed asymmetry of the later stages. In U. lactea, on the other hand, both chelipeds of the immature males are small. Before the differentiation of the large cheliped most of the immature males lose the cheliped of one side, but it seems to be not the indispensable process for differentiation. Fig. 1 shows the result of the present survey based on 71 immature crabs of U. marionis smaller than 6.0 mm in carapace width. The large cheliped is differentiated in the male larger than 4.25 mm, but there is no individual bearing two large chelipeds. The differentiation of the large cheliped in U. marionis seems to be similar to U. lactea at least in having no stage with two large chelipeds in an immature male. During the course of the present populational observation we came across two males only with the small chelipeds from Yaeyama Group in the Ryukyu Islands, which are 13.15mm and 18.05mm in carapace width represented in Fig. 4. Such abnormal males only with the small chelipeds may be caused by the fact that in the critical stage of differentiation of the large cheliped the male loses the power to develop the normal asymmetry regenerating only the small chelipeds by the simultaneous loss of both chelipeds experimentally shown by MORGAN (1924). Among the crab specimens from the Palau Islands is otherwise an abnormal male with carapace width 13.45 mm bearing the symmetrical large chelipeds shown in Fig. 3. Up till now only two examples of such double-clawed male are recorded in U. pugilator and U. pugnax by MORGAN (1920) who described that, if some designation is called for, it might be said to be a super-male, or at least an over-clawed male. As he rightly mentioned, it is apparent that such double-clawed male is functionally normal and could not have occurred through the parasitism or disease, and also highly probable that it may be due to some accident in the development determining the asymmetry of normal male. A reasonable interpretation of occurrence of such double-clawed male may be naturally introduced from the type of U. pugilator or U. pugnax with two large chelipeds in the immature male rather than that of U. lactea with two small ones. The occurrence of a double-clawed male in U. marionis in which the immature crab probably' bears no stage with two large chelipeds is noteworthy, even if by the unknown cause.