Abstract
1. The main period of gametogenesis in both sexes of Cyprina islandica occurs in late fall and early winter. In December or January the gonads of some clams present a morphologically ripe appearance, such individuals constituting approximately 20 per cent by March 15. Gametogenesis is slowed down but not entirely arrested during late winter and is resumed at a rapid rate with the spring increase in temperature.2. Spawning begins near the end of June or early in July when the water temperature is approximately 13.5° C.3. The dates when 50 per cent of the clam population should be expected to enter the stage of being ripe but not having begun to spawn, the stage of partial spawning, and the stage of completed spawning should fall within ± 3 days of June 4, August 13 and October 6 respectively.4. In adult C. islandica the sexes are separate and well defined even immediately after spawning, thus indicating that sex reversal is uncommon.5. The eggs of C. islandica cannot be artificially fertilized.
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