Touch sensation by pectoral fins of the catfish Pimelodus pictus

Mechanosensation is fundamental to many tetrapod limb functions yet it remains largely uninvestigated in the paired fins of fishes, the limb homologs. Here we examine whether membranous fins may function as passive structures for touch sensation in the absence of extensive fin ray movement. We investigate the pectoral fins of the pictus catfish (Pimelodus pictus), a species that lives in close association with the benthic substrate and whose fins are positioned near its ventral margin. Kinematic analysis shows that the pectoral fins are held at a constant angle of partial protraction during routine forward swimming and do not appear to generate propulsive force. Immunohistochemistry reveals that the fins are highly innervated and we observe putative mechanoreceptors at nerve fibre endings. To test for the ability to sense mechanical perturbations, activity of fin ray nerve fibres was recorded in response to touch and bend stimulation. Both pressure and light surface brushing generated a...