COHEN, J.H.*; FORWARD, JR., R.B.: Alteration of the shadow response in a larval crab by ctenophore kairomones

Zoeal larvae of the estuarine crab Rhithropanopeus harrisii display negative phototaxis upon sudden decreases in light intensity. This photobehavior participates in a predator avoidance shadow response which functions in the avoidance of non-visually orienting surface predators such as ctenophores. This study tested the hypotheses (1) that chemical cues (i.e., kairomones) from ctenophores activate the shadow response and (2) that ctenophore mucus, as well as aminosugar hydrolysis products of polysaccharides thought to be present in mucus, serve as kairomones. Light-adapted R. harrisii stage I zoea were exposed for 3 hours to one of the following treatments: (a) 100 kdalton filtered seawater, (b) mucus collected from the ctenophore Mnemiopsis leidyi, (c) a concentration series of aminosugar disaccharides derived from chondroitin sulfate polysaccharides, or (d) aminosugar disaccharides derived from hyaluronic acid polysaccharides. Characteristics of larval shadow responses were measured in an apparatus that simulated the underwater angular light distribution. It was determined that the minimum change in light intensity necessary to evoke a shadow response decreased when zoea were exposed to either ctenophore mucus or to specific aminosugar disaccharides. These results support the test hypotheses that ctenophore derived kairomones, most likely associated with their external mucus, activate the shadow response in R. harrisii zoea.