When under competition for fertilisations, males are thought to increase their reproductive success by releasing as many sperm as possible into the reproductive arena and in many species, this prediction holds. For marine invertebrates, which utilise the ancestral strategy of broadcast spawning eggs and sperm, however, it appears that males tend to release their sperm more slowly than females release their eggs. Marine invertebrate eggs typically have a relatively slow permanent block to polyspermy (whereby eggs become impermeable to further sperm attachment), and for several minutes after fertilisation, sperm can continue to attach to a fertilised egg. We hypothesised that releasing sperm slowly minimises the wastage of sperm on already fertilised eggs. We simulated different sperm release rates in a flume using the broadcast spawning polychaete, Galeolaria caespitosa. Sperm release rates strongly affected overall fertilisation success: higher release rates resulted in lower fertilisation rates. Laboratory studies confirmed that the permanent block to polyspermy in G. caespitosa took less than a minute to form but this lag was sufficient to result in some sperm wastage. Thus upstream, fertilised eggs that have not formed a permanent block to polyspermy can remove sperm from the pool that would otherwise fertilise downstream sibling eggs. We suggest that while electrical blocks to polyspermy evolved in response to excess sperm, permanent blocks to polyspermy could have evolved in response to sperm limitation (insufficient sperm).