Degradation state of algal diets affects fatty acid composition but not size of red urchin gonads

Kelps in temperate marine ecosystems produce substantial detrital biomass that provides a carbon subsidy to consumers. After detachment, detrital kelp degrades and potentially changes in nutritional quality. Red sea urchins Mesocentrotus franciscanus are an abundant consumer of drift algae, but little is known about the effects of kelp degradation on feeding preferences or nutritional value to urchins. We compared the response of gonad index (GI), carbon and nitrogen stable isotope (SI) content, and fatty acid (FA) composition in the gonads of red urchins fed 2 species of fresh or degraded kelp for 17 wk. We found significant effects of kelp species but not kelp degradation state on urchin GI and SI, but also significant interactions between kelp species and degradation state. Urchins with greater gonad growth had δ 13 C values more similar to those of their diet than did urchins with low gonad growth. Multivariate FA composition of the kelp diets (both in terms of species and degradation state) and the gonads of urchins fed those diets differed significantly. Several polyunsaturated essential FA (including SDA, EPA, and ARA) and FA summary classes were important for differentiating urchin gonads among treatments, suggesting kelp species-dependent effects of diet degradation. The total concentration of ω-3 and ω-6 FA declined with degradation state in both diets but did not differ in the gonads of urchins fed these diets. Thus, even if diets are depleted in certain FA, urchins are able to maintain concentrations of these FA in their gonads.