Tunicates - Urochordates

Australia: The Land Where Time Began

Tunicates - Urochordates

Usually called sea squirts because when picked up they squirt water, their main means of defense. They are filter feeders, water passing through the mouth and out through the gill slits where it is filtered for the minute food particles it contains. The tunicate name refers to the tough outer cellulose "tunic", the cellulose that also gives plants their internal support of wood. Ascidians is another name they are known by, derived from the Latin word meaning wineskin.

Though the adults don't resemble fish their larvae resemble closely primitive fish, with a long muscular tail like a tadpole. They have a notochord starting behind the head and a spinal nerve cord. There are a number of sensory structures on the head end enabling them to detect light direction and gravity. There are 3 sticky structures like hairs on the head, papillae, that attach to the substrate where the larva settles. Once settled it metamorphoses into the adult animal, remaining immobile it resorbs the long tail metabolising the recovered tissue. They are hermaphroditic and have the ability to become either sex as they mature, and can also reproduce by asexual budding.

Fossils that are believed to be tunicates have been found as early as the beginning of the Palaeozoic, though not all agree, some doubting these fossils are actually tunicates, preferring to allocate them to new groups. Ausia fenestrata, from the Nama Group in Namibia, of Ediacaran age, and a closely related form, an Ausia-like genus from the Onega Peninsula, northern Russia, are thought to possibly be the oldest known urochordates. According to Long it has been shown that these Ediacaran age forms may have affinities with the ascidians, as shown by research carried out by Pat Vickers-Rich in 2007.

A couple of forms of Early Cambrian age have been found in China, Shankouclava shankouense, a Lower Cambrian species from a deposit in the vicinity of Anning, near Kunming, and Cheunggongella from the Chengjiang deposits. The latter genus is very similar to Styla, an extant tunicate. Palaeobotryllus, an Upper Cambrian species from Nevada has a bubble-like form that closely resembles the colonies of Botryllus, an extant tunicate. Microscopic platelets from unknown animals have been found that have been compared with spicules present in the tunicates of the present. This has led to speculation that the animals that produced the platelets may be ancient tunicates.

Fenghsiangia, a tube-shaped form from the Early Ordovician of China, that was described by Long and Clive Burrett in 1989, has a tubular exoskeleton of phosphatic material with tubercles on the inside of the tube formed by large blisters. They thought it was probably allied to the first vertebrates, as vertebrate bone is the only tissue known to form such tubercles. As the tubercles are on the inner surface of the tube instead of the external surface it differs from all known vertebrates.

When 146 nuclear genes from a number of vertebrate and invertebrate species were analysed by Frederick Delsuc et al. of the University of Montreal it was, according to Long, conclusively demonstrated that the tunicates are the group most closely related to vertebrates. This has led to the group name for vertebrates and tunicates combined, Olfactores, first recognised by Dick Jeffries in 1991, being reinstated. Another discovery, that the embryos of urochordates have migratory cells from which pigment cells develop suggest the presence of some form or primitive neural crest.. Neural crest cells are the only cells that can achieve this in vertebrates, leading to the conclusion that primitive precursor neural crest cells must be present in urochordates.

The midbrain-hindbrain boundary has been shown by developmental studies using lancelets to be a significant organiser region in defining vertebrates, because there are several organiser genes that are expressed there (Holland & Holland, 2001).

Sources & Further reading