Effects of temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis

To assess the effects of both temperature and food ration on gonad growth and oogenesis of the green sea urchin, Strongylocentrotus droebachiensis (O.F. Muller), individuals collected December 1996 (winter experiment) and June 1997 (summer experiment) were maintained for 3 months in one of four experimental treatments: (1) 3 degreesC and fed ad libitum (high ration), (2) 3 degreesC and fed one-seventh of the maximum ration (low ration), and (3) 12 degreesC and fed the high ration; (4) 12 degreesC and fed the low ration. All individuals were fed an artificial diet and exposed to only 1 h of light every day. At the end of both experiments, mean gonad indices of sea urchins fed the high ration had increased significantly (11-24% and 6-19% in the winter and summer experiments, respectively), while the gonad indices of individuals fed the low ration did not change. At the high ration (both experiments), the increase in gonad index of sea urchins occurred primarily as the result of a significant increase in the mass of nutritive phagocytes, as revealed by histological analyses. Primary oocytes were significantly larger in individuals held at 3 OC than at 12 degreesC throughout the winter experiment, regardless of food ration; during the summer experiment, primary oocytes were significantly larger in individuals receiving the high ration, regardless of the temperature at which they were held. These results suggest that: (1) food availability is the most important factor regulating energy storage and the relative size of gonads throughout the year, (2) temperature affects the rate of growth and maturation of primary oocytes during the later stages of oogenesis, and (3) once gametogenesis has been initiated, mature ova will be produced, even under conditions of low food availability. Conditions of high food availability in summer and low temperature in winter would thus favor reproductive output in sea urchin populations.