Distributional Consequences of Adhesive Eggs and Anural Development in the Ascidian Molgula pacifica (Huntsman, 1912)

Molgula pacifica (Huntsman) is a recently rediscovered ascidian that occupies shallow subtidal rocks on wave-swept coasts of British Columbia. Individuals occur most abundantly at sites with intermediate exposure at or near 4 m depth. On a scale of centimeters, they are highly aggregated. Molgula pacifica is hermaphroditic, self-fertile, and oviparous. Embryos develop on the bottom without passing through a typical tadpole stage. Each of the egg follicle cells contains a single large adhesive vacuole that occupies most of the cell volume. Shortly after spawning these vacuoles rupture, causing the follicle cells to secrete a sticky mucus coat that adheres the egg to the substratum. Juveniles hatch and move away from the chorion using epidermal ampullae, as reported for other anural molgulids. Adhesive eggs may be an adaptation that permits anural development in high-energy hard-bottom habitats. Egg adhesion may also explain the small-scale distribution of the species.