grows to quite large size with predominant siphons. Interestingly, in still water the inhalent or branchial siphon tends to point more sideways, while in high-current regions it tends to grow more back on itself. When an individual is forcibly bent over in the current, this means that water enters the branchial siphon mostly by current force alone. The exhalent or atrial siphon tends to point away from the body, which means that water is drawn out by viscous entrainment. Both are energy-savings adaptations. If you can recall your high-school or first-year college physics about Bernouli's Principle, then this will be clearer; otherwise, go to A SNAIL'S ODYSSEY
for a description of this interesting bit of functional morphology.
There are 50-or so species of sea squirts or tunicates (Class Ascidiacea) along the British Columbian coast, but probably less than 20 of these are common. However, the relatively large size of some of the solitary forms, their overall diversity of shapes and colours, their unusual life styles, and their evolutionary relationships to vertebrates, make them invertebrates of great interest. They come in two main forms, solitary and colonial, with the latter being separated into social and compound types. Solitary forms are the most visually obvious and these make up most of the species presently represented on E-Fauna BC. They are attached by one end, often from a stalk, and have a body encased in a protective outer covering or tunic drawn into two prominent siphons. The tunic is non-living and made up of a cellulose-like substance that is often extremely tough in texture. Compound colonial forms grow in flat, plating colonies that can be easily differentiated from sponges by their usually smooth, slippery texture. The two colonial types, social and compound, are comprised of small units or zooids, each functionally equivalent to what has been described for solitary forms, but differing in the extent to which the zooids are fused together.
The evolutionary link to vertebrates and other chordates comes in the possession of pharyngeal gill slits, notochord, and hollow dorsal-tubular nerve cord in the "tadpole" larva. All but the gill slits are lost during metamorphosis of the larva to adult form. The familiar expression, "For Man was once a leather bottel", derives from these evolutionary characters.
Whether solitary or colonial, the pattern of water circulation for feeding and respiratory purposes is similar. Seawater enters via branchial siphons and exits via atrial siphons. Within the body the water is sieved through a barrel-like structure known as the branchial basket, where food particles are removed and consumed. As the waste water exits, it carries feces and other wastes and, in season, reproductive products. Foods eaten include particulate organic matter and a variety of small planktonic organisms, including many invertebrate larvae. Any junk material taken in can be forcibly expelled or back-flushed via the branchial siphon by rapid muscle contraction; hence the name "sea squirt".
For information on the different types of tunicates with details on feeding, reproduction, and defenses see A Snail's Odyssey..
Note Author: Tom Carefoot, Department of Zoology, University of British Columbia