AbstractThe asymmetric claws of Californian snapping shrimp (Alpheus californiensis) are comprised of a smaller pincer claw and a larger snapper claw. The present paper describes the morphology of the single closer muscle and the facilitation properties of an excitatory motor axon for both claws. In the snapper claw the closer muscle fibers are composed of uniformly long sarcomeres, measuring 10–15 μm in length. In the pincer the closer muscle fibers are of a smaller diameter and can be divided into two groups on the basis of sarcomere length. One group has short (2 and 3 μm) sarcomeres, while the other has intermediate (7 and 8 μm) sarcomeres. Microelectrode recordings from single closer muscle fibers showed that all fibers are supplied by the fast excitatory motor axon. The slow excitatory motor axon innervates only fibers on the dorsal and ventral surfaces of the closer muscle. Facilitation of excitatory junctional potentials (ejps), produced by stimulation of the fast axon with pairs of brief electrical shocks, revealed that at intervals above 100 ms no facilitation was recorded from either claw. At intervals below 100 ms, however, facilitation was recorded from pincer closer muscle fibers, but defacilitation or depression was recorded from the snapper. This was confirmed by making extracellular recordings of synaptic potentials from snapper closer muscle fi