Planktivore chemical cues mediate zooplankton diel vertical migration

Most zooplankton undergo daily patterns of vertical movement termed diel vertical migration (DVM). The most common pattern involves an ascent near sunset and descent near sunrise. The most widely accepted functional explanations are that animals rise to feed at night and descend to avoid visually-oriented predators during the day. Recent studies indicate the presence and absence of planktivores can alter DVM through rapid alterations of zooplankton photoresponses. Since light is the most important cue for DVM, alterations in migration by cues from predators should involve changes in photoresponses. Thus, the research objectives were (1) to determine whether photoresponses involved in DVM are altered by the presence or absence of different planktivores, (2) to determine whether chemical cues from planktivores induce alterations in zooplankton photoresponses, (3) to characterize the planktivore chemical cues.;Larvae of the crab Rhithropanopeus harrisii and brine shrimp Artemia franciscana were studied as representative zooplankters. The three general types of test planktivores were (1) fish as representative predators that feed by sight and do not undergo DVM, (2) ctenophores, as sightless predators that do not undergo DVM, and (3) blue crab postlarvae and chaetognaths, which also do not sight their prey but do undergo DVM. Photoresponses were tested in an apparatus that simulated the natural underwater angular light distribution and behavior recorded with a video system. For both R. harrisii zoea and brine shrimp nauplii, planktivore chemical cues rapidly and reversibly induce photoresponses mediating DVM and these photoresponses varied with planktivore type and cue concentration.;Brine shrimp nauplii photoresponses involved in DVM were used as a bioassay to determine the chemical nature of the cue in fish odor that alters photoresponses. The first set of experiments addressed the biological, physical and chemical stability of the chemical cue in fish odor. The second set of experiments investigated the hypothesis that the active chemical cues in fish odor are degradation products of the large proteoglycan molecules that comprise fish mucus. The results demonstrated that the active chemical cue in fish odor is stable at temperature and pH extremes, but degrades at room temperature within 2 days. The chemical cue is small (...