Periodic, moderate water flow reversibly increases hair bundle density and size in Nematostella vectensis

Abstract

Animals employ hair bundles on hair cells to detect flow, vibrations, and gravity. Hair bundles on sea anemone tentacles detect nearby vibrations in the water column produced by prey movements and then regulate discharge of cnidae to capture prey. This study investigates (i): the progressive effects of periodic water flow on hair bundle morphology and density of hair bundles and cnidae in sea anemones, (ii): the reversibility of the flow response and (iii): the ability of the response to be expedited with increased flow duration. Linear density of hair bundles along tentacles and each hair bundle's dimensions was measured in anemones exposed to flow and in the absence of flow. With increasing days of flow, hair bundles in anemones exposed to flow for one hour every week day for twenty days increased in density and grew longer and wider at bases and middles whereas controls did not. Time courses fit to a linear function exhibited significantly larger positive slopes from animals exposed to flow compared to controls. Hair bundles in anemones exposed to flow for three hours each day increased in linear density, length, base-width and middle-width after ten days of flow and returned to control levels after ten days following cessation of flow. An apparent increase in density of cnidae with flow barely missed statistical significance. Therefore, anemone hair bundles are dynamically and reversibly modified by periodic, moderate flow to become more abundant and robust. These findings may have relevance to hair cells in acoustico-lateralis systems of higher animals.