Depth regulation of larval marine decapod crustaceans: test of an hypothesis

This study tested the hypothesis that the dimensions and symmetry of the depth regulatory window of crustacean larvae are controlled by the level of light adaptation. Responses of first and last zoeal stages of the crab Rhithropanopeus harrisii (Gould) to different rates of pressure change were analyzed with a video system. Crabs were collected from the Neuse River estuary (North Carolina, USA) from May to September 1988. Responses were measured when larvae were adapted to light having an angular light distribution similar to that underwater at intensities ranging from one log unit above the lower phototaxis threshold to four log units higher. For both zoeal stages in darkness and at 10^-6 W m^-2, the distance larvae descend before responding to a pressure increase was much shorter than the distance they would ascend before responding to a pressure decrease. When adapted to a light level of 10^-4 W m^-2 both zoeal stages descended and ascended approximately equal distances before responding to an increase or decrease in pressure, respectively. Finally at the highest test light intensity (10^-2 W m^-2), the ascent distance was much shorter than the descent distance. These results support the hypothesis. The depth regulatory window dimensions predict an ascent in the water column upon adaptation to low light intensities and descent at high light levels. Thus Sulkin's negative feedback model provides the general mechanism of depth regulation. The effects of light adaptation on the limits of the depth regulatory window provide an additional component that negates the requirement for depth regulation at an absolute depth. The composite model can be termed the light-dependent negative feedback model of depth regulation.