Alteration of photoresponses involved in diel vertical migration of a crab larva by fish mucus and degradation products of mucopolysaccharides.

Photoresponses involved in the descent phase of nocturnal diel vertical migration (DVM) of larvae of the crab Rhithropanopeus harrisii were measured in a laboratory system that mimicked the underwater angular light distribution. The test hypothesis was that kairomones from fish that activate zooplankton photoresponses involved in DVM are derived from polysaccharides from the external mucus of fishes. Studies considered fish mucus from the mummichog (Fundulus heteroclitus) and disaccharides (originating from chondroitin sulfate A and heparin polysaccharides) that are likely constituents of fish mucus. R. harrisii larvae descend at sunrise with an isolume and remain near the isolume during the day. Since depth maintenance near the isolume depends upon a negative phototaxis, the lowest light intensity (threshold) that induces this response was used to quantify the effects of the test chemicals. It was predicted that exposure to fish kairomones would lower the photoresponse threshold, thereby resulting in larvae remaining deeper in the water column where light for visual predation was reduced. The photoresponse threshold declined as the concentration of fish mucus increased. Disaccharides originating from chondroitin sulfate A and heparin also decreased the photoresponse threshold as compared to responses in aged, filtered seawater. Collectively, the results support the hypothesis and indicate that disaccharide degradation products of predator mucus containing sulfated and acetylated amines can serve as kairomones.