Sexually dimorphic patterns of neural organization in the feeding appendages of fiddler crabs

The organization of sensory nerves and sensilla was examined in the feeding claw of two species of fiddler crabs, Uca pugnax and U. pugilator, using neuroanatomical and behavioral techniques. Surveys of the populations of axons indicate that claws of adult crabs contain 25000-40000 neurons. Approximately 85% of the population consists of axons with diameters less than 1 microm, suggesting they may represent chemosensory neurons. Females show an enhanced population of these small (putative chemosensory) axons relative to males, providing a mechanism to explain previously observed sexual differences in behavioral chemosensitivity to feeding stimulants. Surveys of the claw surface show a variety of external structures that could contain either chemo- or mechanosensory receptor neurons. There are hair-like sensilla of several types, some of which are more abundant in females than in males. In addition, claws show previously undescribed pit sensilla reminiscent of known bimodal chemo- and mechanosensory sensilla found in certain decapod crustaceans. Morphological properties of hair-like sensilla, as well as their small number in relation to the large population of presumptive chemosensory axons, suggest that they have a limited role in chemosensation. Most of the chemosensory axons probably originate in the pit sensilla.