Ultrastructure and physiology of the outer row statolith sensilla of the blue crab Callinectes sapidus

The outer crescent row of statolith sensilla of Callinectes sapidus was examined morphologically using scanning and transmission electron microscopy. The number and diameters of axons within the nerve bundle supplying the sensilla were determined. Intracellular injections of Lucifer Yellow were used to examine the central projections of single sensory neurons from the statolith sensilla. In addition, responses of the sensory neurons to displacements of the sensilla were analyzed using electrophysiological techniques. The outer crescent row consists of 7-9 cuticular sensory sensilla. Each has long filamentous extensions projecting from a main shaft. These sensilla ((x) over bar = 73 mu m in length) are innervated by bipolar sensory neurons with large diameter axons ((x) over bar = 29 mu m) passing in the antennular nerve. The afferent fibers from the outer sensilla row appear to be identical and project into median antennular neuropils bilaterally. Extracellular recordings during displacement of single sensilla support the hypothesis that these sensilla are acceleration-sensitive mechanoreceptors. Stimulation of the nerve bundle innervating the outer row statolith sensilla produces short latency, large amplitude depolarizing potentials in antennular withdrawal interneurons, suggesting that deflection of outer row sensilla is capable of initiating antennular withdrawal.