The Thaliacea comprise a class of marine animals within the subphylum Tunicata. Unlike their bottom-dwelling relatives the ascidians, thaliaceans are free-floating for their entire lifespan. The group includes both solitary and colonial species.
All of the thaliaceans are filter feeders. They have a transparent barrel-shaped body through which they pump water, propelling them through the sea and from which they extract food. The bulk of the body consists of the large pharynx. Water enters the pharynx through the large buccal siphon at the front end of the animal, and is forced through a number of slits in the pharyngeal wall into an atrium lying just behind it. From here, the water is expelled through an atrial siphon at the posterior end. The pharynx is both a respiratory organ and a digestive one, filtering food from the water with the aid of a net of mucus slowly pulled across the slits by cilia.
There are three orders of Thaliacea. Pyrosomes are colonial animals, with multiple tiny zooids arranged in a cylinder closed at one end. All of the atrial siphons point inwards, emptying into a single, common cloaca in the centre of the cylinder. As the water exhaled by the zooids exits through a common opening, the water movement slowly propels the pyrosome through the sea.
Doliolids and salpids alternate between solitary and colonial life stages. Salp colonies can be several meters in length. Doliolids and salps rely on muscular action to propel themselves through surrounding sea water.
Thaliceans have a complex life cycle. Eggs hatch into an individual called a tadpole larva, which is the common larval stage for all Urochordates. This then develops into an oozoid, which reproduces asexually by budding to produce a number of blastozoids. The individual blastozoids then reproduce sexually to produce the eggs and the next generation of tadpole larvae, which develop into oozoids.
Thaliaceans are 30% carbon by mass. Therefore their dense bodies sink to the bottom of the oceans when they die and this may be a major part of the worldwide carbon cycle.1
The class is a relatively small one, and is divided into three orders, each containing a single family:
- Order Pyrosomida
- Order Doliolida
- Order Salpida
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 1042–1043. ISBN 0-03-056747-5.
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