The reproductive cycle of the sand snail Umbonium (Suchium) giganteum was investigated on the basis of samples taken monthly during the period from December 1986 to June 1988. Seasonal changes in the gonadal index, size-frequency distribution of the ovarian egg, and the results of a spawning experiment in the laboratory were analyzed to determine the spawning season.
The gonadal index was low in August and September, but high during the period from December to June; the index values show a bimodal type of distribution with peaks in November-December and April. This is highly correlated to the seasonal changes in seawater temperature. As indicated from the period of maximum ripeness of gonad and the spawning observed in the laboratory, it is concluded that the spawning of U. (S.) giganteum occurs twice a year, i.e. in winter (December and January) and in late spring to early summer (April, May and June), and that the latter is the main spawning season.