Developmental Stages Of Ascidian (Sea Squirt) Metamorphosis

The metamorphosis of an ascidian is a rapid and dramatic transformation from a mobile larva to a sessile adult, typically completed in just a few hours.

1. Free-Swimming Larva: The "ascidian tadpole" hatches. It is non-feeding and swims using its muscular post-anal tail, which contains the notochord and dorsal nerve cord. Its sole job is to disperse and select a suitable settlement site using its simple sensory organs (ocellus for light, otolith for gravity).
2. Settlement and Attachment: The larva finds a suitable surface and attaches itself head-first using adhesive papillae located at its anterior end. This attachment is permanent.
3. Tail Resorption (Initiation of Metamorphosis): This is the most dramatic "retrogressive" step. Immediately upon attachment, the tail begins to contract. The epidermis pulls the tail's internal structures (notochord, nerve cord, muscles) into the main body trunk.
4. Destruction and Phagocytosis: The retracted tail tissues are rapidly destroyed by apoptosis (programmed cell death) and consumed by phagocytic cells. The axial complex (nerve cord and notochord) degenerates completely.
5. Body Reorganization and Rotation: The body undergoes a massive internal rearrangement. The larval sensory vesicle is lost. The rudimentary adult nervous system (a simple ganglion) develops. The body rotates, moving the mouth (future incurrent siphon) and the atrial pore (future excurrent siphon) to the top of the animal.
6. Functional Juvenile: The Remember: pharynx expands enormously to become the filter-feeding "branchial basket." The siphons open, and the juvenile sea squirt begins its new life as a sessile filter-feeder.