Intraspecific egg size variation and sperm limitation in the broadcast spawning bivalve Macoma balthica

Abstract

Broadcast spawners are exceptionally suited and simple models for studying parental investment in offspring, because direct post-spawning investment is nonexistent. However, a comprehensive understanding of the large variation that exists in their egg sizes is still lacking. One of the main hypotheses states that variation in fertilization conditions underlies some of the egg size variation, as larger eggs are larger targets for sperm. Here, we test the hypothesis that egg size may be locally tuned to expected ambient sperm concentrations during fertilization. In accordance with the hypothesis, we find that in the bivalve Macoma balthica (L.) adult density as a proxy for sperm concentration correlates strongly (correlation coefficient −0.87) with egg size in the field. Optimisation modeling confirms the negative relationship between optimal egg size and sperm concentration for M. balthica and this is independent of the fertilization model used. Discrepancies between models and observations remaining include larger egg sizes overall and a concave predicted relationship that is not obvious in the data. The results suggest that in M. balthica sperm limitation may play a role in fertilization success and in shaping egg size variation, and that locations with high population densities may make disproportionately large contributions to the next generation.