Variability of carbon and nitrogen composition in body compartments of strongylocentrotus nudus and hemicentrotus puicherrimus (echinodermata: Echinoidea) associated with food availability

Little is known about the differences in elemental composition in body compartments of consumers in rocky subtidal bottom ecosystems. We conducted a series of feeding experiments to investigate differences in the carbon and nitrogen content in the body compartments of edible sea urchins, Strongylocentrotus nudus and Hemicentrotus pulcherrimus from three different macrophyte beds: the kelp Eisenia bicyclis; fucoid Sargassum yezoense, and crustose coralline algae. We also investigated various differences between urchins feeding on Saccharinajaponica kelp ad libitum and those kept in starvation conditions. We found that the gonad index of S. nudus in kelp beds was higher than that of those in the fticoid bed. No significant difference was found in the gonad index of H. pulcherrimus from kelp and fucoid beds. The lantern indices of S. nudus in kelp beds and those fed with kelp were significantly lower than those in fucoid and coralline algal beds, and the starved ones and the carbon content of lantern in the kelp bed was also significantly lower than those in fucoid bed. Both species of sea urchins fed on kelp had increases in the gut index, carbon content and the C:N ratio of gut. The C:N ratios of the predominant algae in each bed were 1.2-6.0 times higher than those of the whole body of both species of sea urchin. There were no significant differences in the C:N ratios in the gonads among animals receiving the same treatment, and the C:N ratios in S. nudus were higher than those in H. pulcherrimus. These results suggest carbon and nitrogen compositions changed in the Aristotleos lantern of S. nudus and gut of S. nudus and H. puicherrimus in an imbalanced manner, while the composition of the gonads during the growth stage is stable and may be homeostatic for each species.