Swimming behavior of the sand crab, Emerita analoga (Crustacea, Anomura)

Summary

• 1. Sand crabs of the genus Emerita swim by rapidly beating their uropods. The morphology and physiology of the uropod muscles and their innervation in E. analoga have been described. A total of 11 muscles, 7 axial and 4 in the protopodite, under the command of about 50 motoneurons, control the cyclical beating of the uropods.

• 2. Of the 7 axial muscles, 2 lie within the 6th abdominal segment and 5 occupy the telson. Two of the latter, the power-stroke muscle and the return-stroke muscle, perform the basic cycle of movement of the uropod; both are simply innervated (1 excitor, 1 inhibitor; 2 excitors, 1 inhibitor). The power-stroke muscle is unusual in that it shows small, facilitating excitatory junctional potentials rather than the large muscle potentials more commonly found in phasic crustacean muscles.

• 3. Peripheral inhibition in the power muscles is post-synaptic and of long duration (∼1 sec). Its effectiveness varies considerably between individuals, and the behavioral role of the inhibition is not clear.

• 4. The group of uropod muscles considered as a whole shows a range of physiological characteristics which distinguishes them from decapod limb muscles, on the one hand, and abdominal muscles, on the other. They show a broad range of properties and, except for the two power muscles, rich innervation.