Ctenophore kairomones and modified aminosugar disaccharides alter the shadow response in a larval crab

Zoea larvae of the estuarine crab Rhithropanopeus harrisii (Gould) descend in the water column upon sudden decreases in irradiance, which serves as a shadow response for the avoidance of zooplankton predators such as ctenophores. This study tested the hypotheses that (i) kairomones from either the ctenophore Mnemiopsis leidyi or the fish Fundulus heteroclitus increase the shadow-response sensitivity by reducing the minimum decrease in irradiance necessary to evoke the response (i.e. threshold); and (ii) a similar increase in shadow-response sensitivity as occurs with ctenophore kairomones is also induced by the external mucus of ctenophores and modified aminosugar disaccharide hydrolysis products of acidic mucopolysaccharides. Light-adapted R. harrisii Stage I zoea were conditioned to chemical treatments for 3 h, and their photoresponses were tested in an apparatus that mimicked the underwater angular light distribution. In clean sea water, the shadow response was induced by a 50% reduction in irradiance (threshold). The threshold decreased to 30% in sea water with fish kairomones and 9% in sea water with ctenophore kairomones. A minimum concentration of 1.0 g wet weight ctenophore mucus l(-1) was required to reduce the shadow-response threshold. Chondroitin sulfate A disaccharides had an effect similar to kairomones and mucus in reducing the threshold, with the greatest effect at a concentration of 10(-6) m. Tests with other disaccharides indicated that only acetylamine-containing aminosugars were effective. The test hypotheses were supported; predator kairomones, ctenophore external mucus, and purified aminosugar disaccharides having acetylamine functionality reduced the threshold decrease in irradiance needed to evoke shadow responses in crab larvae.