EFFECTS OF FOOD RATION AND FEEDING REGIME ON GROWTH AND REPRODUCTION OF THE SEA-URCHIN STRONGYLOCENTROTUS-DROEBACHIENSIS

To determine the effects food ration and feeding regime on growth and reproduction of Strong-ylocentrotus droebachiensis (Muller), sea urchi ns in laboratory aquaria were fed kelp (Laminaria longicruris) supplie d at either a high (H, ad libidum daily) or a low (L, ad libidum 1 d w k(-l)) ration in two successive 12-wk intervals during the reproductiv e period. After 24 wk, urchins fed the high ration continuously (HH) o r for the last 12. wk only (LH) had a significantly greater mean gonad index [(gonad weight/total body weight) x 100] and body weight than u rchins fed the low ration continuously (LL) or for the last 12 wk only (HL). Urchins in the HL treatment had a significantly greater gonad i ndex than those in the LL treatment; there was no significant differen ce in gonad index between the LH and HH treatments. Females had a grea ter gonad index than males in the low ration (LL and HL) treatments at the end of the experiment; there was no significant difference betwee n sexes in the high ration (LH, HH) treatments. Gametogenesis proceede d to maturation in all treatments and some individuals spawned at the end of the experiment. Females in the high ration (HH and LH) treatmen ts had a greater proportion of nutritive phagocytes in their ovaries t han females in the low ration treatments, but there was no effect of f eeding treatment on oocyte or ovum size. Feeding treatment had no effe ct on the relative abundance of nutritive phagocytes in the testes, al though the proportion of spermatocytes was higher (and that of spermat ozoa lower) in the high ration than in the low ration treatments. Urch ins in the high ration treatments had a lower mean jaw height index [( jaw height/test diameter) x 100] and greater mean test diameter than t hose in the low ration treatments at the end of the experiment, althou gh these differences were not statistically significant. Feeding rate on kelp at the end of the experiment was significantly greater for urc hins in the low ration than in the high ration treatments. Our experim ental results show that even relatively low rations of kelp support so matic and gonadal growth in S. droebachiensis. Increasing the supply o f kelp, particularly during the period of active gametogenesis, result s in maximal rates of growth and reproduction. These results suggest t hat populations of S. droebachiensis in barrens may derive a substanti al proportion of their nutrition from drift kelp, which may contribute to their persistence in these habitats. They also explain the large b ody size, high reproductive effort and fecundity of urchins grazing on kelp beds. These findings have important implications for understandi ng the dynamics of natural populations of S. droebachiensis and for de velopment of effective aquacultural practices.