Classification
Tunicates are more closely related to craniates (including hagfish, lampreys, and jawed vertebrates) than to lancelets, echinoderms, hemichordates, Xenoturbella or other invertebrates.
The clade comprising Tunicates and Vertebrates is called Olfactores.
The Tunicata contains about 3,000 species, usually divided into the following classes:
- Ascidiacea (Aplousobranchia, Phlebobranchia, and Stolidobranchia)
- Thaliacea (Pyrosomida, Doliolida, and Salpida)
- Appendicularia (Larvacea)
- Sorberacea
Although the traditional classification is followed for now, newer evidence suggests that the Ascidiacea is an artificial group of paraphyletic status.
The new classification would be:
- Stolidobranchia
- Aplousobranchia, Phlebobranchia and Thaliacea
- Appendicularia
- Sorberacea would belong somewhere in Ascidiacea, or be in a taxon on its own
The species Ciona intestinalis and Ciona savignyi have attracted interest in biology for developmental studies. Both species' mitochondrial and nuclear genomes have been sequenced. Moreover, the nuclear genome of the appendicularian Oikopleura dioica appears to be one of the smallest among metazoans.
Sea squirts have become a testing ground in the controversy about the extent to which cross-species gene transfer and hybridization have influenced animal evolution. In 1990, Donald I. Williamson of the University of Liverpool (U.K.) fertilised sea squirt (Ascidia mentula) eggs with sea urchin (Echinus esculentus) sperm resulting in fertile adults that resembled urchins, but Michael W. Hart of Simon Fraser University failed to find sea-squirt DNA in tissue samples from the supposed hybrids. Williamson claims to have repeated the experiment with sea urchin eggs and sea squirt sperm, producing sea urchin larvae which developed into squirt-like juveniles. On the other hand, Syvanen and Ducore of the University of California have suggested that sea squirts descended from a hybrid between a chordate and a likely extinct protostome ancestor at a time before the diversification of round worms and arthropods. This study also examined whether there was evidence of a sea urchin/tunicate hybridization event that could possibly explain the distribution of genes in modern sea squirts—none could be seen.
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