EFFECTS OF FEEDING AND PREDATOR EXPOSURE ON PHOTORESPONSES DURING DIEL VERTICAL MIGRATION OF BRINE SHRIMP LARVAE

Effects of feeding-starvation and exposure to a planktivorous fish (Atlantic on photoresponses involved in nocturnal diel vertical migration (DVM) of brine shrimp (Artemia) naupliar larvae were measured in a laboratory system that mimics the distribution of underwater angular light. Exposure to predators activated photoresponses involved in the descent response to an increase in light intensity (as at sunrise) within 1 d. Activation was quickly reversed by exposure to clean water and was not affected by feeding or starvation. Thus brine shrimp nauplii have a phenotypic response to a zooplankton predator. Starvation for 1 d activated photoresponses involved in the ascent response to a decrease in light intensity (as at sunset). Feeding on phytoplankton quickly reversed activation, which was not affected by exposure to predators. Both photoresponses were controlled by levels of light adaptation and initiated by rates of change in intensity such as occur at twilight. Light serves as a directional cue for the ascent response (positive phototaxis). These results provide behavioral evidence to support the hypotheses that descent at sunrise during DVM functions for predator avoidance and ascent at sunset functions for feeding.