Eggs with larger accessory structures are more likely to be fertilized in both low and high sperm concentrations in Styela plicata (Ascidiaceae)

Abstract

The evolution of egg size has been intensively studied due to its influence on both fecundity and offspring performance. In marine broadcast spawners, egg size influences the probability of sperm–egg collision, and therefore, egg size can also influence fertilization success, depending on the local concentration of sperm. Many broadcast-spawning species have egg accessory structures that are thought to be a cheap means of altering egg size, but their influence on fertilization remains controversial. To determine the relative influences of ovicell size and follicle cell size on fertilization success in the ascidian Styela plicata, the size distribution of eggs that were not successfully fertilized in both high and low sperm concentrations was compared to that of unfertilized controls. At high sperm concentrations, a greater proportion of eggs with smaller ovicells were fertilized, resulting in smaller larvae hatching from this treatment. Eggs with a large follicle cell area relative to ovicell area were preferentially fertilized in both high and low sperm concentration treatments. Hence, follicle cells do not eliminate selection on ovicell size at fertilization in S. plicata. Furthermore, follicle cells appear to increase fertilization success across a range of sperm concentrations by performing different functions in each environment—increasing the target size of eggs in low-sperm concentrations and presumably reducing polyspermy in high sperm concentrations.