Response properties and biological function of the skate electrosensory system during ontogeny.

This study examined the response properties of skate electrosensory primary afferent neurons of pre-hatch embryo (8-11 weeks), post-hatch juvenile (1-8 months), and adult (> 2 year) clearnose skates (Raja eglanteria) to determine whether encoding of electrosensory information changes with age, and if the electrosense is adapted to encode natural bioelectric stimuli across life history stages. During ontogeny, electrosensory primary afferents increase resting discharge rate, spike regularity, and sensitivity at best frequency. Best frequency was at 1-2 Hz for embryos, showed an upwards shift to 5 Hz in juveniles, and a downward shift to 2-3 HZ in adults. Encapsulated embryos exhibit ventilatory movements that are interrupted by a "freeze response" when presented with weak uniform fields at 0.5 and 1 Hz. This phasic electric stimulus contains spectral information found in potentials produced by natural fish predators, and therefore indicates that the embryo electrosense can efficiently mediate predator detection and avoidance. In contrast, reproductively active adult clearnose skates discharge their electric organs at rates near the peak frequency sensitivity of the adult electrosensory system, which; facilitates electric communication during social behavior. We suggest that life-history-dependent functions such as these may shape the evolution of the low-frequency response properties for the elasmobranch electrosensory system.