Effects of temporary food limitation on development and mortality of Macoma balthica larvae

Long-term observations (1973 to 2001) of populations of the intertidal bivalve Macoma balthica, in the western Wadden Sea, have suggested that larval mortality is strongly density-dependent. In addition, density-independent factors may affect larval mortality. One possible source for such an effect is food limitation. In laboratory experiments, M. balthica larvae were reared under food limiting conditions, both quantitatively (high or low food level) and temporally (starvation during the first, second or third week). The results indicated that larvae offered high food levels grew significantly faster (6.9 mu m d super(-1)), and metamorphosed earlier (16.5 d) and at greater length (264 mu m) than larvae subjected to low food level (4.4 mu m d super(-1), 19.3 d and 244 mu m, respectively). For both food levels, starvation in the first week resulted in a late metamorphosis at a large size, starvation in the second week resulted in an early metamorphosis at a small size, and starvation in the third week yielded intermediate results. Larval mortality was always lower in the low food condition, but the timing of starvation had no impact on mortality. This suggests that larvae, in vivo, do not die directly from food limitation alone. The results are discussed with reference to models of metamorphosis.