Behavioral responses of larvae of the crab Neopanope sayi to hydrostatic pressure

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Abstract
Responses of Stages I and IV zoeae of the crab Neopanope sayi (Smith) to changes in hydrostatic pressure were studied using a closed circuit video system to describe the behavior underlying depth regulation. This species occurs as an adult in estuaries but larvae develop in lower estuarine and coastal areas. Larval responses changed little throughout zoeal development. Step increases in pressure evoked an ascent response (thresholds = 3 to 8 mbar) through negative geotaxis and an increase in swimming speed (high barokinesis). Larvae descended, probably by passive sinking, upon step decreases in pressure (threshold = 8 mbar). An ascent upon a pressure decrease was not observed. During high barokinesis larvae responded to the absolute amount of pressure change, not to the rate of change. Similar directional responses occurred at different rates of change in pressure. Larvae can ascend and descend fast enough under some conditions to induce rates of pressure that evoke corrective behavioral responses for depth regulation. These results support Sulkin''s (1984) negative feedback model. The depth regulatory window, however, was symmetrical and approximately equal at both zoeal stages upon adaptation to different light levels. Thus the light-dependent negative feedback model of depth regulation is not supported, which indicates the depth regulatory demands in upper estuarine and coastal areas are different.