Fatty acid profiles in the gonads of the sea urchin Strongylocentrotus droebachiensis on natural algal diets

We examined fatty acid (FA) compositions of gonads of the green sea urchin Strongylocentrotus droebachiensis collected from a grazing aggregation (front) at the edge of a kelp bed and from barrens dominated by coralline algae, and those of urchins fed single algal diets in the laboratory. We compared these gonad FAs with those of the algal diets, which represented known urchin food sources in rocky subtidal habitats. Gonads of urchins collected from both wild habitats, and of urchins fed kelp Saccharina longicruris in the laboratory, contained more lipid than did urchins fed single diets of barrens macroalgae (Agarum clathratum, coralline red algae or Desmarestia viridis). Substantial biosynthesis of non-methylene interrupted dienes and other FAs by urchins markedly affected their overall FA signatures. Although the FA compositions of gonads of laboratory-fed urchins did not clearly correspond with those of their diets, 3 clusters of urchins were distinguished in multivariate space using multidimensional scaling (PADS): (1) urchins fed single diets of barrens macroalgae in the laboratory; (2) urchins fed S. longicruris in the laboratory or collected from the grazing front; and (3) urchins from the barrens. Characteristics of FA signatures found in urchins from the barrens suggested the occurrence of benthic diatoms in their diet. Our results indicate that, while the FA signatures of urchin gonads are affected by diet and can be used to differentiate feeding groups of urchins in the laboratory and field, significant de novo biosynthesis and/or modification of FAs precludes correspondence of urchin FAs to those of their algal diets.