Observations on the Breeding and Settlement ofMytilus Edulis(L.) in British Waters

Abstract

During 1946 and 1947, regular samples ofMytilus edulisfrom a number of localities ontheBritish coasts, including Conway, Brancaster and Liverpool, were examined for gonad conditionand spawning. For each sample, the mean stage of gonad development was computed. The criteria employed in distinguishing the stages of gonad development are described.Ripening of the gonads takes place within a few weeks of the onset of spawning, in general commencing when the sea temperature has risen above 7–0° C. There appears to be no correlation between nutritional condition and ripening of the gonads, or subsequent spawning.In all localities and in each year in which observations were made spawning occurred in late spring (mid-April to the end of May) and in most areas lasted for a short period only (2–4 weeks). At Brixham, in 1949 and 1950, the duration of the spawning period was longer (4-6 weeks). In most cases, 70–80% of the mature population spawned during the first 7–10 days of the breeding period. No evidence of periodic spawning was obtained.In all cases, spawning commenced in a period during which the mean temperature to which the mussels were exposed was rising from c. 9–5° C. to 11–12–5° C. In most cases, the onset of spawning was coincident with a period of spring tides, and of predominantly bright sunny weather. The initial rate of spawning appears to be directly related to the rate of increase in mean temperature to which the mussels are exposed.After spawning, mussels enter into a ‘neuter’ or ‘resting spent’ stage in which all traces of sexuality are lost.