Photograph by Roger Steene

Ascidians, more commonly known as tunicates, (Class Ascidiacea) abound in all the oceans of the world, on the edge of the sea between tides, and on the sea floor from the low tide mark to the great depths of oceanic trenches. They are members of the Subphylum Urochordata that are invertebrates that exhibit vertebrate features - they lack a backbone but posses four distinguishing chordate characteristics: at some point in the life cycle there can be found a notochord, a dorsal hollow nerve cord, gill slits and a postanal tail.

The best known ascidians are the sea squirts, see example illustrated above, found in large groups on rocky coasts. Many of them seem to be merely a leathery bag of water (hence their name from the Greek askos, meaning a leather wine flask). Although some species are solitary and large, the many colonial species have colonies comprising few to hundreds of tiny zooids. In the commen reef community they fulfil two important roles. Firstly they filter the water around the reef and help to keep it clear; and secondly, as they strain the minute dispersed plants and other organisms from the water for their own nutrition they are concentrating it into a form that makes it available to other animals.

Click here to view the 'Picture of the Week' archive.