Growth as a Strategy for Survival in two Marine Bivalves, Cerastoderma edule and Modiolus modiolus

Abstract

The growth of 2 marine bivalves in Strangford Lough, Northern Ireland, from stable conditions of subtidal reefs and from midtidal sand flats where environmental conditions are less predictable were studied. Cerastoderma grows rapidly in its 1st yr after settlement. It first reproduces during its 2nd yr and the resumption of rapid growth is generally delayed until spawning has been completed. In subsequent years, growth becomes progressively slower. In Modiolus reproduction is delayed and rapid growth can continue for several years. The growth strategies exhibited by these bivalves may reflect the essentially different patterns of mortality experienced by these 2 populations. In Cerastoderma, mortality is heavy in all age classes, and relatively few cockles survive beyond their 3rd yr. Early reproduction following a period of very rapid growth is probably the optimal strategy for this species. Modiolus experiences intense predation early in life but can escape further predation by growing too large to be eaten; only then is energy redirected towards reproductive development. These large but slower growing mussels are then able to live longer.