GRAVINESE, P.M.*; TANKERSLEY, R.A.; Florida Institute of Technology, Melbourne; Florida Institute of Technology, Melbourne: Behavioral Basis of Depth Regulation in Larvae of the Florida Stone Crab Menippe mercenaria: Responses to Hydrostatic Pressure

Behavioral responses of many crustacean larvae to changes in hydrostatic pressure have been hypothesized to form the basis of a negative feedback system for depth regulation. Increases in pressure experienced by larvae during a descent evoke a corrective ascent response by an increase in swimming speed (high barokinesis) or negative geotaxis. Similarly, decreases in pressure during an ascent induce a corrective descent response due to passive sinking or a decrease in swimming speed (low barokinesis). We examined and compared the responses of Stage I, III, and V zoeae of the Florida stone crab, Menippe mercenaria, to gravity and hydrostatic pressure to determine if (1) responses to pressure underlie depth regulation and maintenance and (2) larval responses change during development. All three larval stages were negatively buoyant with mean passive sinking rates ranging from 0.32 cm sec-1 for Stage I zoeae to 1.50 cm sec-1 for Stage V. Responses to gravity varied throughout development, with early stage larvae (I, III) exhibiting a negative geotaxis and late stage zoeae (V) exhibiting a positive geotaxis. When exposed to gradual changes in hydrostatic pressure designed to simulate rates experienced during vertical swimming or sinking, increases in pressure evoked an ascent response in all larval stages. Similarly, larvae descended in response to decreases in pressure. However, rates of change in pressure necessary to evoke an appropriate corrective response (i.e., threshold) varied with larval stage, suggesting that the precision of depth regulation changes during development.