The extracellular hull of chiton eggs is often elaborated into cupules or spines that may be open or closed to the external environment. Scanning electron microscopy was used to examine the location of fertilizing sperm in eggs that had been exposed to a dilute sperm suspension to create natural fertilization or to a sperm concentrate to induce polyspermic egg penetration. The effect of cupules on sinking rates was tested in cupulous (free-spawning) and non-cupulous (brooding) species, by timing descent of eggs over a fixed distance in a large container of seawater. Densities of eggs were compared on Percoll gradients and found to be similar. It was found that hull cupules focus the sperm to specific regions of the egg surface in both brooding and free-spawning species. Furthermore, protruding cupules act as a parachute structures that can significantly reduce sinking rates.  

1. Fertilized egg of Mopalia ciliata, demonstrating open cupules. 
2. Naturally fertilized egg of Mopalia lignosa rolled on sticky tape to remove some cupules and demonstrate subdivision of each cupule into seven channels. Sperm can be seen penetrating the hull within some channels.
3. Egg of M. lignosa artificially fertilized with sperm concentrate to create polyspermy. Few sperm have penetrated hull between cupules, but numerous sperm can be seen penetrating within cupules.