Tunicate - Life Cycle

Life Cycle

Most tunicates are hermaphrodites. The eggs are kept inside their body until they hatch, while sperm is released into the water where it fertilizes other individuals when brought in with incoming water.

Some larval forms appear very much like primitive chordates with a notochord (stiffening rod). Superficially, the larva resemble small tadpoles. They swim with a tail, and may have a simple eye, or ocellus, and balancing organ, or statolith. Some forms have a calcareous spicule that may be preserved as a fossil. They have appeared from the Jurassic to the present, with one proposed Neoproterozoic form, Yarnemia.

The larval form ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding, though it may have a digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.

In growing to adulthood, tunicates develop a thick protective covering, called a tunic, or test, around their barrel-shaped bodies.

During embryonic development, tunicates exhibit "determinate cleavage", where the fate of the cells is set early on with reduced cell numbers and genomes that are rapidly evolving. In contrast, the amphioxus and vertebrates show cell determination relatively late in development and cell cleavage is indeterminate. The genome evolution of amphioxus and vertebrates is also relatively slow.