There are about 1500 species of ascidians worldwide. Some species of these animals occur as solitary individuals, some in colonies.

They are classified as chordates, a group whose members are usually vertebrates. Whilst adult ascidians are simple, fixed, bottom dwellers, the larvae are free-swimming and supported by a notochord or primitive backbone.

Ascidians are filter feeders. Solitary ascidians typically have two open cylinders known as the inhalent and exhalent siphons. Water is drawn in through the inhalent siphon, strained through the sieve-like pharynx, from which plankton is removed and passed to the stomach. Oxygen is removed from the water and carbon dioxide is returned to it, then the water is expelled via the exhalent siphon. In many compound species there is a shared exhalent siphon.

Most ascidians are hermaphrodite, some release eggs into the water for external fertilisation, whilst others are fertilised internally by free swimming sperm. The ascidian larval stage last for only a few hours. The larvae don't feed, they just disperse. Ascidian eggs are easy to work with and are often used in studies of embryonic development.

Ascidian bodies have a tough cellulose-like covering known as a"tunic" , so ascidians are sometimes referred to as tunicates. When threatened hey may eject a jet of water, hence their other common name - sea squirts.

more information about ascidians see www.ascidians.com