Mouse-over to preview, click for species


Go directly to a list of taxonomic groups

Sea Squirts: Ascidian Tunicates

Halocynthia aurantium

Ascidians are common mostly sessile marine invertebrates with solitary and colonial forms. Most species are found nearshore and typically on hard bottoms, but they do also occur down to the deep sea where they are typically found on soft bottoms. There are 56 species recognized as living in the Arctic.

Ascidians the benthic class within the tunicates, a term referring to the special cuticular covering, the tunic, which can be thick like cartilage or transparent gelatinous. This ‘skin’ is composed of sugars, proteins and water but also of a substance called tunicin, somewhat similar to cellulose that gives the tunic strength.

Ascidians are filter-feeders that take in water through a siphon. The water flows through mucus-covered gill slits into an atrium and exits through a second siphon. An organ called an endostyle, an early forerunner of the vertebrate thyroid gland, produces special mucus that is moved over the pharynx and traps zooplankton food particles that end up in a stomach and intestine for digestion. The common name sea-squirt referes to their habit of contracting when disturbed, resulting in water being squirted out of the siphon.

An interesting physiological characteristic is that the heart of ascidians alternates the flow direction of the blood every few minutes. A morphological curiosity is that the tadpole-like larvae have a tail that contains a hollow dorsal nerve tube and a notochord which place the ascidians in the group ‘urochordata’, although adults have little resemblance to other chordates.

Page Author: Bodil Bluhm
Created: Sept 24, 2010

Total view statistics