CONSTRAINTS IMPOSED BY THE MAJOR CHELIPED ON BURROW CONSTRUCTION IN THE MALE FIDDLER CRAB, UCA ANNULIPES (H. MILNE EDWARDS, 1837)

The burrows excavated by male Uca annulipes at a lagoonal shore on Pulau Hantu Besar, an offshore island of Singapore, are generally less acutely bent, i.e., have significantly smaller mid-angles, than the burrows of females. In an ex situ study, burrows dug by males with the major cheliped removed, and by males with intact major cheliped, were cast with wax. Seven non-angular burrow morphological parameters (burrow diameter, total burrow depth, burrow neck height, curved burrow length, horizontal length, chamber diameter and burrow volume) and two angular variables (mid-angle and ground-angle) were compared to determine if sexual dimorphism in burrow curvature was due to the effect of the major cheliped on burrow excavation. A similar experiment using females with two intact chelipeds and one cheliped removed was conducted to determine if male crabs without the major cheliped dug burrows with dimensions similar to those of females with one cheliped removed. Mean mid-angle of burrows (± SE) was 24.9 ± 1.3° and 16.0 ± 1.0° for males whose major chelipeds were removed and with major intact chelipeds, respectively. Of all the burrow morphological parameters, only the mean mid-angle differed significantly between the two groups of crabs. Female crabs (with and without cheliped removal) did not excavate significantly different-shaped burrows (mid-angles: 20.7 ± 2.1° and 18.2 db 1.5°, respectively) suggesting that removal of cheliped did not affect burrow architecture. The curvature of the burrows of males that had the major cheliped removed was not significantly different from those of females with one cheliped removed (mid-angles: 24.9 ± 1.3° and 20.7 ± 2.1°, respectively). Results of the study showed that the major cheliped in a male U. annulipes is most likely to be responsible for the sexual dimorphism in curvature of the burrows.