Immature Placuna placenta (Linnaeus) broodstock were given the micro algae Isochrysis galbana (T-ISO) Parke and Tetraselmis tetrahele (G.S. West) at different combinations and density levels and reared in tanks with or without mud substrate. The animals were also reared in the estuary and fed on available natural food. Monthly examination of gonad histology showed a significantly higher gonad index in broodstock reared in tanks with mud substrate than without (P < 0.05). In addition, those given a high-density algal diet (200,000 cells ml⁻¹, 3:1 mixture of I. galbana and T. tetrahele) had a significantly faster gonad development than the low-density diet (100,000 cells ml⁻¹, 1:1 mixture) fed broodstock reaching sexual maturity and gonad index of 330±14 after 1 month (P < 0.0001). P. placenta given the low-density algal diet attained gonad maturity after 3 months and estuary-reared animals after 2 months. Seawater irradiated by ultraviolet light (925–1395 mW h/l) induced spawning of all mature P. placenta. Estuary-reared animals had a higher fecundity and larger eggs and a higher percentage of fertilized eggs developed to straight-hinged larvae than those reared under other condition. Animals given a low-density algal diet released low number of small eggs which did not fertilize. Natural spawning also occurred in the estuary 2 months after stocking.