Leathery Sea Squirt Styela plicata

Sea squirts, or ascidians, are small marine animals that belong to the subphylum group Tunicata. The Styela plicata species found in Sarasota Bay are composed of a double sac with a pair of siphons which are encased in a tough leathery coat or test. The siphons are used primarily for feeding and reproduction. The test is a protective mechanism.

Styela plicata are solitary, in the sense that they do not form centrally controlled colonies like some ascidians. However, they do tend to live in clusters composed of more Styela plicata, as well as countless other organisms. They are found throughout the warmer areas of the Atlantic, Pacific, and Indian oceans, as well as the Gulf of Mexico and the Caribbean.

An individual Styela may be anywhere from 3 to 14 centimeters in size. They feed by pumping seawater through one siphon into the filter-like inner sac, which is filled with cilia coated fibers, where detritus and microorganisms, such as plankton are trapped. The filtered water is then pumped out through the other siphon. This siphon, known as the atrium, is also used for disposing of waste and expelling sex cells. The siphons are able to pump water by use of tiny hair-like cells called cilia, which wave in a manner which causes water to flow.

The thick leathery defensive structure, known as the test, is of quite some scientific significance. It is composed of a material that is chemically, nearly identical to cellulose, the same material that forms the cell walls of plant cells. Tunicates are the only known group of animals which produce cellulose.

Sea squirts are hermaphrodites. They contain both eggs and sperm which are released into the water together for fertilization. From the eggs, spawn a larval stage resembling a tadpole. This form is quite extraordinary in that it possesses a notochord and a hollow dorsal nerve chord: characteristics of a primitive vertebrate. As such they are classified as Chordates, the phylum which contains all vertebrates such as fish, reptiles, and even humans. This larval form does not feed and after approximately two days, it attaches its head to a solid surface, and begins metamorphosing into the invertebrate adult form. The adult form loses its notochord, head, and tail, and develops the leathery test around its body. The adult sea squirt is completely sessile and will remain attached to its solid surface permanently unless forcefully removed by strong water movement or foraging animals.

Styela plicata live in shallow water, on rocks or on man-made structures such as boat hulls, dock pilings, or sea walls. They also form a substrate for other smaller marine animals to live on. Tiny anemones, crabs, shrimps, bivalves, sponges and even other forms of ascidians have been known to live on the surface of the sea squirt’s test.

The squirt’s main defense is the leathery, cellulose-infused test which can be up to ½ an inch thick. The Styela is also able to accumulate very high concentrations of the rare and toxic metal vanadium within their blood giving their blood a green color. Scientists are still unsure of why or how they are able to concentrate this metal in their body, but it may be used in the process of forming the cellulose fibers in the test. It is also possible that this is a defense mechanism, since vanadium would be toxic to predators.

Another interesting aspect of the anatomy of Styela is its heart. The heart is essentially controlled by two “pacemakers.” One will cause the heart to pump for about 30-100 beats in one direction, then the other takes over, allowing the first to recharge. Each time the switch is made, the flow of blood in the Styela circulatory system is reversed.

Styela are considered pests in that they foul dock pilings and the hulls of boats. They are also used as fishing bait in southern states. Despite this, their numbers are still prevalent.

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