Saturday, September 25, 2010
September 25, 2010 : Sea Squirt
Tunicates, also known as urochordates, are members of the subphylum Tunicata or Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor and are commonly known as sea squirts and sea pork,others – such as salps, doliolids and pyrosomes – live above in the pelagic zone as adults.
Most tunicates feed by filtering sea water through pharyngeal slits, but some are sub-marine predators such as the Megalodicopia hians. Like other chordates, tunicates have a notochord during their early development, but lack myomeric segmentation throughout the body and tail as adults. Tunicates lack the kidney-like metanephridial organs, and the original coelom body-cavity develops into a pericardial cavity and gonads. Except for the pharynx, heart and gonads, the organs are enclosed in a membrane called an epicardium, which is surrounded by the jelly-like mesenchyme. Tunicates begin life in a mobile larval stage that resembles a tadpole, later developing into a barrel-like and usually sedentary adult form.
Tunicates apparently evolved in the early Cambrian period, beginning c 540 million years ago. Despite their simple appearance, tunicates are closely related to vertebrates, which include fish and all land animals with bones.
Most tunicates are hermaphrodites. The eggs are kept inside their body until they hatch, while sperm is released into the water where it fertilizes other individuals when brought in with incoming water.
Some larval forms appear very much like primitive chordates with a notochord (stiffening rod). Superficially, the larva resemble small tadpoles. They swim with a tail, and may have a simple eye, or Ocellus, and balancing organ, or Statolith. Some forms have a calcereous spicule that may be preserved as a fossil. They have appeared from the Jurassic to the present, with one proposed Neoproterozoic form, Yarnemia.
The larval stage ends when the tunicate finds a suitable rock to affix to and cements itself in place. The larval form is not capable of feeding though it may have a digestive system, and is only a dispersal mechanism. Many physical changes occur to the tunicate's body, one of the most interesting being the digestion of the cerebral ganglion, which controls movement and is the equivalent of the human brain. From this comes the common saying that the sea squirt "eats its own brain". In some classes, the adults remain pelagic (swimming or drifting in the open sea), although their larvae undergo similar metamorphoses to a higher or lower degree.
Once grown, adults can develop a thick covering, called a tunic, to protect their barrel-shaped bodies from enemies.
Tunicates are suspension feeders. They have two openings in their body cavity: an in-current and an ex-current siphon. The in-current siphon is used to intake food and water, and the ex-current siphon expels waste and water. The tunicate's primary food source is plankton. Plankton gets entangled in the mucus secreted from the endostyle. The tunicate's pharynx is covered by miniature hairs called ciliate cells which allow the consumed plankton to pass down through to the esophagus. Their guts are U-shaped, and their anuses empty directly to the outside environment. Tunicates are also the only animals able to create cellulose.
Tunicate blood is particularly interesting. It contains high concentrations of the transition metal vanadium and vanadium-associated proteins as well as higher than usual levels of lithium. Some tunicates can concentrate vanadium up to a level one million times that of the surrounding seawater. Specialized cells can concentrate heavy metals, which are then deposited in the tunic.