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 afferent nerve activity. Fin ray nerves also respond to bending of the rays. These data demonstrate for the first time that membranous fins can function as passive mechanosensors and suggest that touch sensitive fins may be widespread in fishes that maintain a close association with the bottom substrate.
Pictus Swimming Kinematics
Excel file containing pectoral fin and tail angles relative to the body axis measured across non-consecutive bouts of free rhythmic swimming. For each fish (n = 5) and trial (5 trials per fish), angles were measured at each eighth of the tail-beat cycle, including the beginning and end of the cycle, for a total of nine time points per cycle. Data on the cycle duration for each trial analyzed are also included.
ppBend_Data
Excel file containing pectoral fin ray afferent responses to fin ray bending. Fin rays were exposed to a series of step-and-hold (1.2, 1.8, 2.4, 3.0, and 3.6 mm) and ramp-and-hold (0.8, 1.2, 2.4, 4.8, 9.6 mm/s; final displacement fixed at 2.4mm) bending stimuli. The hold period during fin ray bending trials was 5s. A burst of action potentials was classified as three or more spikes occurring within 50ms of each other. Each stimulus was divided into several temporal segments (i.e. bins) for analysis. Stimulus segment timing for step-and-hold trials: Prestart (0.5s prestimulus baseline), Start (stimulus onset to 1s afterwards), Hold (1.0s to 4.5s), and End (stimulus offset to 1s afterwards). Stimulus segment timing for ramp-and-hold trials: Prestart (0.5s prestimulus baseline), Start (duration of start ramp), Poststart (1s post-start ramp interval), Hold (1s hold interval), End (duration of the end ramp), and Postend (1s post-end ramp interval). Characteristics associated with a burst of action potentials during step-and-hold trials were analyzed only during Start and End stimulus segments. Bursts were not always recorded.
ppPush_Data
Excel file containing pectoral fin ray afferent responses to pressure. Fin rays were exposed to a series of step-and-hold (0.12, 0.3, 0.6, 1.2, 1.8mm) and ramp-and-hold (1.2, 2.4, 12, 24, 48mm/sec; final amplitude of 1.2mm) pressure stimuli. The hold period during fin ray bending trials was 3s. A burst of action potentials was classified as three or more spikes occurring within 50ms of each other. Each stimulus was divided into several temporal segments (i.e. bins) for analysis. Stimulus segment timing for step-and-hold trials Prestart (0.5s prestimulus baseline), Start (stimulus onset to 1s afterwards), Hold (1.0s to 2.0s), and End (stimulus offset to 1s afterwards). Stimulus segment timing for ramp-and-hold trials: Prestart (0.5s prestimulus baseline), Start (duration of start ramp), Poststart (1s post-start ramp interval), Hold (1s hold interval), End (duration of the end ramp), and Postend (1s post-end ramp interval). Characteristics associated with a burst of action potentials during step-and-hold trials were recorded only during Start and End stimulus segments. Bursts were not always recorded.
ppBrush_Data
Excel file containing pectoral fin ray afferent responses to surface brushing. Fin rays were exposed to a series of extensions at different distances (0.6, 1.2, 2.4, 3.6, 4.8mm at 2mm/sec) and velocities (0.6, 1.2, 2.4, 3.6, 4.8mm/sec; final brushing distance held constant at 2.4mm). Each stimulus was divided into several temporal segments (i.e. bins) for analysis. Stimulus segment timing: Prestart (0.5s prestimulus baseline), Start (duration of start ramp), Poststart (1s post-start ramp interval), Hold (1s hold interval), End (duration of the end ramp), and Postend (1s post-end ramp interval).
