Feeding responses of the bivalves Crassostrea gigas and Mytilus trossulus to chemical composition of fresh and aged kelp detritus

The chemical composition of kelps (e.g. poly- phenolics) deters grazing by herbivores, but kelp detritus is potentially a source of nutrition for suspension feed- ers. The effects of kelp detritus derived from two species (Agarum fimbriatum Harvey and Costaria costata (Turner) Saunders) on feedingof oysters, Crassostrea gigas Thunberg, and mussels, Mytilus trossulus Gould, were examined in feedingexperiments. Fresh and ag ed kelp particles were sequentially presented in combina- tion with the microalga Rhodomonas lens at an initial total concentration of 5·10 -4 ml -1 . Aging of kelp parti- cles for 4 days in seawater significantly reduced the concentration of polyphenolics without changing the total carbon or nitrogen content. Clearance rates of both mussels and oysters were significantly lower in the presence of fresh versus aged kelp particles, and clear- ance rates declined overall with decliningpolyphenolic concentrations. Video endoscopy was used to examine feedingselectivity at the level of the gill in oysters in the same food treatments used in the clearance rate experi- ments. Comparison of particle composition in the water versus the pseudofeces in both oysters and mussels was also used as a measure of feedingselectivity. When presented with R. lens in combination with fresh and aged kelp particles selectivity for R. lens tended to be greater against fresh than aged particles, and there was some indication that this was stronger for A. fimbriatum than for C. costata particles. The ability to select was lower at very high polyphenolic concentrations, which may reflect poisoningof sensory bindingsites. These data suggest that bivalves distinguish among particles of varyingchemical composition and respond by chang ing their clearance rates and their selectivity.