This phylum is actually divided into three subphyla, however, only one is of interest to marine hobbyists. And that is the subphylum "Tunicata" which contains the tunicates and ascidians, or sea squirts as they are often called.
There are over 1500 species in this subphylum, and the Class Ascidiacea contains two major divisions; Enterogona and Pleurogona that interest hobbyists. These contain colonial and solitary 'sea squirts.' Enterogona contains the Orders Aplousobranchia and Phlebobranchia. Pleurogona contains the Order Stolidobranchia.
Most sea squirts are found on reef drop-offs and/or under overhangs, with some found at the base of corals. Occasionally, they are found attached to live rock or corals entering the aquarium. They are called 'sea squirts' because a jet of water is expelled when they contract. Very few of these beautiful creatures are sustainable for more than a few months. And in fact, their general life span is generally no longer than a year.
Many 'tunicates' have male and female reproductive organs, therefore produce both sperm and eggs. Following a free swimming or planktonic larval stage the adults live a sessile lifestyle. Their bodies are encased in a protective tunic, hence the name 'tunicate.' This supportive enclosure may be smooth or leathery, and is usually attached to firm substrate, e.g., rock, seaweed, or even other animals.
There are two openings in the tunic, one is often larger than the other. The larger one, called the buccal siphon is where beating cilia draw in water. It enters a chamber called the pharyngeal basket, which filters the water for oxygen and foodstuffs. The filtered water then is expelled through the smaller opening called the atrial siphon. Food particles are mixed with mucus and drawn into the oesophagus and digested. Resulting waste products also leave through the smaller opening.
Unfortunately they require heavy concentrations of suspended food particles and/or bacteria laden waters. And as noted above, have a very short life span, about one year and sometimes far less. However, there are some species occasionally seen in the trade that are extremely colorful and attract hobbyists. Only those willing to provide for their demanding level of care should attempt keeping them in closed systems. Actually, 99.9% should be left in the wild.
For those that want to maintain them in aquaria, these creatures require numerous feedings per day of live and/or preserved commercial phytoplankton products or that of animal and plant powders that produce suspended products in the bulk water.
The following are mostly shown only to depict their beauty. Please think twice about removing these beautiful creatures from the wild! And for more information about these pretty creatures, read "Reef Invertebrates, An Essential Guide to Selection, Care and Compatibility" by Anthony Calfo & Bob Fenner, ISBN 0-9672630-3-4, and checkout its 'review' posted in the Product & Book Review section of this website. I should also note that the Taxonomy encompassing these animals is somewhat confusing for a non-scientist such as myself. Therefore, I've used a Checklist for Ascidiacea published by the Australian Government Department of the Environment and Heritage to help properly place the following species. If you have corrections, and/or comments or species to add, please contact me. (saltcorner@cs.com)
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