Directional selectivities of near-field filiform hair mechanoreceptors on the crayfish tailfan (Crustacea: Decapoda)

Directional selectivities of mechanoreceptors that innervate filiform hairs on the crayfish tailfan were investigated with unidirectional, sinusoidal, water-motion stimuli. These recordings provide the first representative sample from filiform hair sensilla on the entire tailfan. The filiform hair receptors exhibit unimodal directional selectivity patterns that were well fitted by a cardioid function with a half-width of 122 degrees. The preferred directions correspond to the major axis of hair motion, and are perpendicular to the orientation of lateral branches on the main hair shaft. Pooled plots of preferred directions demonstrate quadrimodal patterns on the telson and endopods which are associated with hair location, and a bimodal pattern on the exopods. For each appendage, the combination of the overall pattern of preferred directions with "coarse coding" of direction by individual receptors provides sensitivity to a full 0-366 degrees range of water motion and the potential to discriminate the direction of water motion throughout this range. The results demonstrate several similarities to the wind-sensitive cercal receptor system in orthopteroid insects, and suggest that crustacean filiform hair receptors provide a sufficient sensory basis for behavioral orientation to water currents and shorelines.