The Tunicata (=Urochordata) comprise a subphylum of the phylum Chordata, the group that includes vertebrates. Although tunicates are close relatives of the vertebrates, this relationship is not superficially obvious. The basic tunicate body form includes a body covered by a “tunic” made of a cellulose-like polysaccharide, a notochord that is restricted to the tail and usually present only in larval stages (with the exception of adult appendicularian tunicates), a dorsal nerve cord present in larval stages, and a U-shaped gut. The pharynx (branchial chamber) typically has numerous mucus-covered gill slits. Water flows into the mouth and pharynx via an incurrent siphon and exits via an excurrent siphon; the anus empties into the excurrent flow just as it leaves the body. Nearly all tunicates are marine suspension feeders, but this is otherwise a very diverse group. Although the larvaceans are entirely sexual, many tunicates also reproduce asexually. Most tunicate species are hermaphroditic. Tunicate development is reviewed by Jeffery and Swalla (1997).
Ascidean tunicates (sea squirts, Ascidiacea) include both solitary (although individuals are sometimes clumped together) and truly compound colonial species, with many small individuals living together in a common gelatinous matrix. Individuals range in size from 1 mm to 60 cm and some colonies may measure several meters across. Water enters the body through an incurrent siphon and exits through an excurrent siphon. Both siphons are pointed upward. Ascideans have a global distribution and are found from shallow waters to the deep sea. They may attach to nearly any substratum. They are most abundant and diverse along rock shorelines and in deep sea mud.
Salps (pelagic tunicates, Thaliacea) float singly or in cylindrical or chainlike colonies that may reach several meters in length. The incurrent and excurrent siphons of salps are positioned at opposite ends, providing thrust for locomotion. Most salps are gelatinous and transparent. Salps are known from throughout the oceans, but are especially abundant in tropical and subtropical waters. They occur from the ocean surface down to around 1500 meters.
The larvaceans (appendicularians, Appendicularia=Larvacea) are solitary, luminescent zooplankton, rarely exceeding 5 mm. The name “larvacean” is a reference to their resemblance to the larval stages of some other tunicates. Larvaceans live in a gelatinous casing, or “house”, that is secreted around the body and is involved in their complex feeding. Larvaceans feed mainly on tiny phytoplankton and bacteria—as small as 0.1 µm. The anus of larvaceans opens directly to the outside into the path of water flowing out of the excurrent siphon.
The Sorberacea are deep sea ascidian-like tunicates that retain a dorsal nerve cord in the adult stage. They are carnivorous and lack a perforated branchial sac.
The feces of salps and larvaceans, and the abandoned gelatinous casings of larvaceans (used in feeding), represent an important source of food and particulate organic carbon in the open sea.
(Brusca and Brusca 2003)
The precise relationships among the chordate subphyla remains somewhat controversial. Cameron et al. (2000) used an 18S ribosomal DNA data set to investigate relationships among the deuterostomes, including chordates. They concluded that the cephalochordates, not the tunicates, are sister to the vertebrates. Based on their analysis, in combination with morphological and life history data, the authors concluded that Tunicata should be treated as a phylum, rather than a subphylum within Chordata, that is sister to the Chordata (Vertebrata + Cephalochordata). This grouping of vertebrates with cephalochordates is consistent with the traditional view of deuterostome relationships, a view that has been challenged by a number of more recent studies, which recover vertebrates and tunicates as sister taxa (e.g., see review by Stach 2008 and references therein; Swalla and Smith (2008) note that mitochondrial and ribosomal evidence places cephalochordates as sister group to the vertebrates, whereas genomic evidence places tunicates as the sister group to the vertebrates).
Lambert (2005a,b) reviewed the history of research on hemichordates, cephalochordates, and tunicates as well as diverse aspects of their biology.