Spatial and temporal variations of the reproduction cycle and energy allocation of the bivalve Macoma balthica (L.) on a tidal flat

Abstract

In a tidal flat of the St Lawrence estuary, spawning of Macoma balthica (L.) occurred in the late spring (May–June) and lasted ≈ 1.5 months. At the upper (3 m above the lower-low water) and lower (1.5 m above the lower-low water) intertidal levels, individuals began spawning at the same moment but the spawning period began 2wk earlier in 1986 than in 1985. At the lower level only, the weight of sexual products increased linearly with shell length. The shell length at sexual maturity was the same at both intertidal levels but individuals at the lower level became sexually mature earlier due to their higher growth rate. At the upper level, a high proportion of individuals of ⩾9.5 mm contained no sexual products during the spawning period. Spatio-temporal variation of sexual-product and soft-tissue dry weights showed that individuals at both intertidal levels used an opportunistic reproductive strategy. This strategy allows investment of any surplus energy in gamete production when more favorable environmental conditions are encountered. Spatial variations in total biomass showed that gamete production for the whole population is largely assured by individuals from the lower level which live in a more stable environment. The important contribution of lower-level individuals to total fecundity and weak annual variations of the fecundity in the lower-intertidal zone allow regulation of recruitement for the whole population year after year.