ACTIVATION OF BRINE SHRIMP NAUPLII PHOTORESPONSES INVOLVED IN DIEL VERTICAL MIGRATION BY CHEMICAL CUES FROM VISUAL AND NONVISUAL PLANKTIVORES

The effects of exposure to visual and non-visual planktivores on the p hotoresponses involved in the descent phase of nocturnal diel vertical migration (DVM) of brine shrimp (Artemia franciscana) naupliar larvae were measured in a laboratory system that mimicked the underwater ang ular light distribution. This species was used as a model for testing the general effects of different planktivores because a previous study demonstrated that naupliar photoresponses were activated by exposure to one fish species that was a visual planktivore, but did not co-exis t with brine shrimp. The present study tested other fish and non-visua l planktivores (ctenophores, chaetognaths, blue crab postlarvae). Phot oresponses were activated by 1 day exposure to: (i) three species of f ish (Atlantic menhaden larvae, mummichog and pinfish) and (ii) water t hat had previously contained the fish or ctenophores. Thus, chemical c ues from both visually and non-visually hunting planktivores activated photoresponses, contrary to the hypothesis that nocturnal DVM functio ns for avoidance of visual planktivores. Activation occurred within 5 min, indicating that brine shrimp nauplii have a phenotypic response t o zooplankton planktivores. Photoresponse sensitivity decreased with d ecreasing concentration of chemical cue, indicating that activation of DVM should vary with planktivore abundance. In contrast, photorespons e activation was very weak after exposure to the physical presence of two non-visual, vertically migrating planktivores (blue crab postlarva e and chaetognaths). The results support the predictions that zooplank ton DVM pattern should vary with exposure to different planktivore typ es and that migration amplitude should increase with increasing plankt ivore abundance.