Gametogenesis and breeding of Ostrea edulis on the west coast of Ireland

Gametogenesis, spawning and larval production of the European flat oyster Ostrea edulis L. were studied in sublittoral beds in Ballinakill Harbour and Kilkieran Bay, Co. Galway, from April 1977 to November 1978 and from November 1978 to May 1980 respectively. Histol ogical preparations of gonads and percentages of oysters brooding larvae were used to monitor breeding.Maturation began in March/April with ripe gametes appearing in some oysters in mid-May. Incubating oysters were observed from mid-June to August/September. Spent oysters were numerous in September. Gametogenesis was at a minimum from October to February depending on the year of study. Ripeness as measured by four different parameters was observed to peak around 29 June 1977 and 4 July 1978 in Ballinakill Harbour, and on 10 July 1979 in Kilkieran Bay. The onset of maximum ripeness in both populations could be predicted using the formula:

$\text{D=}\text{∑}\text{i=1}\text{N}\text{(t}{}_{\mathrm{i}}\text{? t}{}_0$

where D is the thermal constant in degree-days, n = the number of days required to reach ripeness, t_i = temperature to which the oysters were exposed daily (°C) and t₀ is the developmental zero for gametogenesis. The mean thermal constant as calculated from the gametogenesis study was 554.52 ± 44.52 degree-days. Redevelopment of gametes began during late winter at mean seawater temperatures above approximately 7.0°C.