Geographic variation in the reproductive cycle of the saucer scallop, Amusium balloti, (Bernardi, 1861) (Mollusca:Pectinidae), along the Western Australian coast

Abstract

The reproductive cycle of the saucer scallop Amusium balloti was examined at three sites spanning a large part of the north-south range of the species on the western coast of Western Australia. Variations in gonad dry weight were highly cyclical, with minimum gonad weights in Shark Bay (25�S) occurring between January and March and maximum gonad weights occurring around July or August. The pattern of variation in gonad weight at the Abrolhos Islands (29�S) and Fremantle (32�S) was similarly cyclical, but the times of high and low gonad weights occurred at similar times at both sites and approximately two months later than in Shark Bay. The variations in gonad weight were associated with large variations in meat weight and condition, which would need to be considered in any yield analyses of fisheries for this species. Gonad dry weight increased rapidly following the end of the resting period, and histological examination showed that gonads contained mature gametes within one to two months of the end of the resting period. Although gonad weights continued generally to increase over a period of three to four months following the end of the resting period, variations in gonad weight during this period indicated that partial spawning may be occurring before the time of peak gonad weight. Residual gametes were observed in the ciliated ducts of scallop gonads within one to two months of the start of the rapid increase in gonad weight until the onset of the resting period, indicating that partial spawning was occurring throughout this time. Estimation of the settlement dates of recruits supported the conclusion that spawning commenced shortly after gonads began their rapid weight increase and suggested that spawning may follow a lunar cycle. Partial spawning provides multiple opportunities for larvae to develop in environments conducive to high larval survival and may be an important factor in determining recruitment success.