Gonad fatty acids and trophic interactions of the echinoid Psammechinus miliaris

Variation in gonadal somatic indices (GSI) and fatty acid signatures of 2 contrasting Scottish west coast populations of the echinoid Psammechinus miliaris were examined. P. miliaris was sampled from both the shallow subtidal and at the upper limit of its distribution in the intertidal. The intertidal population had a significantly higher GSI than the subtidal at both locations. Multivariate analysis (ANOSIM) of the fatty acid signature showed significant variation between the 2 depths as well as between sex and location. SIMPER analysis indicated a complex pattern of variation between location and depths. Both 20:4(n-6) and 20:5(n-6), associated with brown algae, were found in higher levels in subtidal populations, while levels of 22:6(n-3) were higher in the intertidal at one site, and 18:4(n-3) higher at the other. Of these acids, 22:6(n-3) is associated with filter feeding invertebrates, and 18:4(n-3) with green algae. Omnivorous diets in urchins have been associated with increased gonadal growth and the results from this study strongly suggest that the observed differences in GSI between intertidal and subtidal P. miliaris result from a higher quality diet in the intertidal, consisting of invertebrates and green algae.