Morphology and Behavior of an Unusually Flexible Thoracic Limb in the Snapping Shrimp, Alpheus heterochelis

The second thoracic limb in the snapping shrimp, Alpheus heterochelis, is much thinner, more elongated and flexible, and has a larger ganglion than its serial homologs. The greater length and flexibility is largely due to one of the limb segments--viz., the carpus--which consists of five separate segments, rather than the single segment typical of the other limbs. Externally, the multi-segmented carpus is relatively free of cuticular projections except for scattered simple setae. The adjoining segments--the merus and the propus--are also smooth except for clusters of long simple setae on the pollex and dactyl. Internally, each of the carpal segments has three muscles--a bender, stretcher and rotator--all restricted to the distal half of the segment. In keeping with the sensillum-free exterior of the multisegmented carpus, only about 1000 axon profiles originate in the carpus out of a total of 6000 counted at the base of the ganglion. This total number is roughly half that found in the first thoracic limbs. Conversely, the number of axon profiles in the longitudinal connectives to the second thoracic ganglion is about 25% greater than that to the first thoracic ganglion and may partly account for the size difference between these two ganglia. In terms of their behavior, the second thoracic limbs are almost constantly active, mostly probing the substrate, and occasionally grooming various body parts. Part of the probing behavior consists of food foraging and retrieval, especially from concealed and hard-to-reach locations. Because of their flexibility, these limbs are particularly adept at such movements.