Sea Squirts — Subphylum Urochordata

An arctic Sea Squirt.

All urochordates, including the arctic ascidians, secrete a polysaccharide that forms a tunic-like covering over their body, which is responsible for their common name, "tunicate." Their chordate features occur in the larval stage, but are lost when they develop into adults.

Ascidians (askidion="bladder"), also known as sea squirts or tunicates, are typically marine animals that live on the ocean floor. They belong to the phylum Chordata because, as juveniles, they possess a notochord (a flexible,

rod-like structure that normally develops into a backbone). However, as juveniles develop into adults, the gene responsible for notochord formation is deactivated and the notochord disappears. Over 20 sea squirt species have been recorded from the Arctic Ocean.

Sea squirts range in size from 0.1 cm to 6 cm, and are often brightly coloured. Two openings, called siphons, are located near the top of the animal. One siphon is used to bring water into the mouth, while the other pushes water out of the body. These siphons are arranged at an angle to each other to avoid re-ingesting wastewater. Their flexible body wall consists of a layer of cells coated with a secreted covering or "tunic" of varying thickness. It ranges from smooth and slick to wrinkled and leathery. The tunic has an outer layer, which may be hardened with sand grains or calcium-rich spines, while the inner layer is often like jello. These features provide effective protection against water currents and abrasion.

The majority of arctic ascidians are large and solitary, such as Ciona intestinalis. They often inhabit shallow waters where they attach themselves to hard structures. Deepwater species that live on soft, muddy substrates are known to occur at depths exceeding 2000 m. Most ascidians are suspension feeders, using mucous nets to filter plankton and organic detritus from the seawater. They generate a current of water through their body using tiny hairs called cilia. Entering via one siphon, water passes into the mouth where a ring of fleshy tentacles prevents the entrance of large particles. There, water is then filtered through slits in an internal chamber and then flows out the other siphon.

Most sea squirts are hermaphrodites, possessing both male and female reproductive parts. Breeding seasons are influenced by environmental factors, such as temperature and light. Large numbers of eggs are released into the sea at the same time as sperm from other individuals. Fertilization is followed by the development of a free-swimming larva, which has a tail for locomotion. These larvae eventually settle on the substrate and develop into adults.