Saccular-specific, seasonal differences in hair cell density in a vocal fish with seasonal auditory plasticity.

Abstract

Plainfin midshipman fish (Porichthys notatus) rely on acoustic communication for courtship. Courting males build nests in rocky intertidal areas and emit low-frequency hums to attract reproductive females. There is surprising physiological plasticity in the female midshipman’s auditory system, with reproductive (summer) females exhibiting greater sensitivity to higher frequencies than non-reproductive (winter) females. Recent work demonstrates that this physiological plasticity occurs at the level of the saccule, which is the primary hearing organ in this species. This research examines potential morphological correlates of this physiological plasticity. Summer females have greater saccular hair bundle density than winter females and there is a corresponding increase in net cell addition as determined by quantification of dividing and dying cells and by counts of immature hair bundles. No seasonal differences are seen in the other inner ear end organs. Saccular hair cell number is not correlated with fish size, suggesting that the hair cell increase is not due to differences in fish size between sampling seasons. These data suggest that the hair cell increase in summer females may contribute to the greater auditory sensitivity seen in reproductive females. Future studies will examine morphological and molecular differences between saccular hair cells in winter and summer females.