COORDINATION BETWEEN THE LEGS AND TAIL DURING DIGGING AND SWIMMING INSAND CRABS

Rhythmic leg movements and tailflipping are mutually exclusive behavio urs in most decapod crustaceans, but sand crabs (Anomura: Hippoidea) c ombine leg movements with simultaneous tailflipping or uropod beating for both digging and swimming. We examined the coordination between th e legs and tail (abdomen and tailfan) of Blepharipoda occidentalis, Le pidopa californica (Albuneidae), and Emerita analoga (Hippidae). When either albuneid swims, the tail cycles at a higher frequency than the legs, and the two rhythms are not coupled. When albuneids begin diggin g, the tail's frequency drops to that of the legs, and its rhythm beco mes phase coupled to the legs. In E. analoga the legs seldom move duri ng swimming by uropod beating. During digging the frequency of the uro pods and fourth legs starts at about double that of the second and thi rd legs, but drops to that of the second and third legs as digging pro gresses. The fourth legs in E. analoga are coupled with the uropods; t heir outward movement (= power stroke) is concurrent with the uropod r eturn stroke. The familial differences in leg coordination and in the coordination of the legs and tail account for the smooth descent of E. analoga beneath sand compared to the stepwise descent of the albuneid s.