Reproduction and life cycle

With rare exceptions, tunicates are hermaphrodites, but reproduction may be by sexual or asexual (budding) means. In general, hermaphroditic animals do not self-fertilize (i.e., provide both the male and female gametes) if they can avoid doing so, a rule that seems also to be true of tunicates. In primitive forms the eggs are fertilized, and development takes place, in the surrounding water, but often embryos are retained in the female’s atrium or elsewhere until the larva is developed.

The larval stage is brief; the larva does not feed, but concentrates on finding an appropriate place for the adult to live. In keeping with this motile phase, the muscular tail comprises two-thirds of the larval body; it is supported by a notochord and contains a nerve cord. Gravity- and light-sensitive sensory vesicles along the dorsal surface of the larval body orient the animal as it swims. After a period of up to a few days, the larva will settle and attach itself to a surface using three anterior adhesive papillae. As the larva metamorphoses into an adult, the tail resorbs, providing food reserves for the developing animal. Free-swimming tunicates metamorphose without attachment.

Colonies are formed by asexual reproduction, with zooids usually being formed by budding. In thaliaceans, two groups (dolioloids and salps) have a complex system of alternating phases; the first phase reproduces by budding, and the resulting individuals may release sperm and eggs.