GEOGRAPHIC-VARIATION IN THE REPRODUCTIVE-CYCLE OF THE SAUCER SCALLOP,AMUSIUM-BALLOTI (BERNARDI, 1861) (MOLLUSCA, PECTINIDAE), ALONG THE WESTERN-AUSTRALIAN COAST

The reproductive cycle of the saucer scallop Amusium balloti was exami ned at three sites spanning a large part of the north-south range of t he species on the western coast of Western Australia. Variations in go nad dry weight were highly cyclical, with minimum gonad weights in Sha rk Bay (25 degrees S) occurring between January and March and maximum gonad weights occurring around July or August. The pattern of variatio n in gonad weight at the Abrolhos Islands (29 degrees S) and Fremantle (32 degrees S) was similarly cyclical, but the times of high and low gonad weights occurred at similar times at both sites and approximatel y two months later than in Shark Bay. The variations in gonad weight w ere associated with large variations in meat weight and condition, whi ch 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 co ntained mature gametes within one to two months of the end of the rest ing period. Although gonad weights continued generally to increase ove r a period of three to four months following the end of the resting pe riod, variations in gonad weight during this period indicated that par tial spawning may be occurring before the time of peak gonad weight. R esidual gametes were observed in the ciliated ducts of scallop gonads within one to two months of the start of the rapid increase in gonad w eight until the onset of the resting period, indicating that partial s pawning was occurring throughout this time. Estimation of the settleme nt dates of recruits supported the conclusion that spawning commenced shortly after gonads began their rapid weight increase and suggested t hat spawning may follow a lunar cycle. Partial spawning provides multi ple opportunities for larvae to develop in environments conducive to h igh larval survival and may be an important factor in determining recr uitment success.