The interaction between diet and density in influencing reproductive output in the echinoid Evechinuschloroticus (Val.)

Abstract

The effects of diet, density, and sex on the reproductive output and survival of Evechinus chloroticus (Val.) have been investigated by means of a field experiment. Densities used ranged between 2 and 20 per 0.5-m 2 cage. The urchins were given single species diets of Ecklonia radiata (C.Ag.) J.Ag., Carpophyllum maschalocarpum (Turn.) Grev., C. angustifolium J.Ag., and coralline algae. Diet and density interacted in influencing both the size of gonads and their histological condition. At low density those individuals fed Ecklonia had significantly larger gonads than did those fed less preferred algae. This effect disappeared at higher density and was contrasted with the relative constancy in size of gonad with increasing density for those animals fed other diets. Gonad volume was a useful predictor of reproductive output. In both winter and spring the size of gonad closely reflected differences in the representation of gametocytes within the gonad. The sex of individuals did not significantly affect the size of gonad; the low power of tests for this effect were, however, noted. The results presented are similar to those given in other studies of this nature. Difficulties in extrapolating these results to natural situations have been discussed and it is argued that knowledge that individual Evechinus display preferences among species of algae is of little use in predicting the field diet of this species.