Density, size distribution, energy allocation and seasonal variations in shell and soft tissue growth at two tidal levels of a Macoma balthica (L.) population

Abstract

A 2-yr field study of growth in the bivalve Macoma balthica (L.) at two tidal levels in the St Lawrence Estuary, Canada, showed that in spring there is a time-lag of 2 wk in all growth parameters between the upper and lower tidal levels. The growth cycle could be divided in three phases, occurring in spring, summer and autumn, respectively. The zone of highest growth rate of both shell and soft tissue shifted from the upper tidal level during the spring phase to the lower tidal level during the summer phase. This resulted in individuals living at the lower level being longer and heavier than those from the upper level. The autumn phase was a period of nearly stagnant shell and soft tissue growth except for the older individuals from the upper level which showed a weight loss of the soft tissues. The first and the second phases would be linked to the temperature whereas the third one would be related to the food availability. Comparison of shell and soft tissue growth trajectories showed that at the two tidal levels tissue weight gains did not exactly coincide with increased shell growth rate. This study shows that any life cycle population study should include samples from a wide range of environmental conditions, covering the entire area of distribution of any local population.