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

►Egg size decreases with adult density (sperm concentration proxy) in the field in a marine bivalve. ►This is interpreted as evidence for a sperm limitation effect on egg size. ►Optimisation modeling confirms that optimal egg size increases with sperm limitation. ►High density areas may make disproportionately large contributions to the next generation.