The tunicates (or sea squirts) are a diverse marine group with more than 2000 living species. Constituting the subphylum Tunicata (or Urochordata), they represent the sister clade to the Euchordata, the subphylum including the vertebrates. Together, they are united as chordates. The close relationship to vertebrates is surprising at first glance, as tunicates sport a rather different appearance - most adults are basically bags of jelly. Their body is enclosed within a gelatinous tunic, an acellular membrane that consists of tunicin (which is very similar to cellulose) and contains calcium carbonate spicules. In the larval stage, however, tunicates look very different and much more like vertebrates. The mobile 'tadpole larvae' possess all chordate characteristics, such as a stiffening notochord that aids swimming, but lose them upon metamorphosis.

Traditionally, tunicates are divided into four classes: The Ascidiacea are sessile filter feeders that employ an enlarged pharynx with numerous slits. Water enters the body through the buccal siphon, food particles are entangled in mucus and then transported into the oesophagus by cilial movements, and water and waste are expelled through the atrial siphon. The mainly hermaphroditic ascidians typically form brightly coloured colonies characterised by high capacities for regeneration and budding. The pelagic Thaliacea, the floating colonies of which are transparent, use muscular contractions to generate a water current, which is employed for jet propulsion, filter feeding and ventilation. In the Larvacea, the larval form is retained, making them most similar to chordates. The smallest and most unusual group are the deep water Sorberacea. These carnivorous tunicates capture worms and other small prey with outstretched extensions of the buccal siphons. Recent molecular evidence, however, has suggested ascidian paraphyly, and an alternative taxonomy has been proposed. The fossil record of tunicates is sparse, and although there are reports from as far back as the Cambrian, these remain controversial. By and large the only reliable record is from the spicules.

Tunicates evidently evolved in shallow waters, but a doliolid thaliacean has migrated into deeper water. This pelagic species has come to strikingly resemble a craspedote hydromedusa (a cnidarian), adopting a body shape and mode of propulsion suited to a bathypelagic lifestyle. Thus it has abandoned filter feeding in favour of carnivory, trapping zooplankton with its elastic buccal siphon. This species is therefore different from other doliolids in terms of habitat, anatomy and feeding behaviour. Thaliacean tunicates are generally transparent, a trait that has evolved independently in numerous pelagic animals, including cnidarians, gastropod molluscs and alciopid annelids. The major advantage of transparency in the open water is that it functions effectively as camouflage at all depths and from all viewpoints. It can be achieved in different ways, but ultrastructural modifications seem to be of particular importance. Salpid thaliaceans provide another interesting example of convergence - here, different species with different morphologies show very similar filtration rates, resulting in optimised flow through the system and minimum energy expenditure.

Among ascidians, coloniality has evolved at least three times. The larvae of colonial species have converged on a very similar morphology that is characterised by a large trunk volume and a relatively short tail. It seems that these adaptations allow for more efficient swimming (which is relevant, as the larvae are the agents of dispersal) but entail a potential cost in terms of adult fecundity. One species of colonial ascidian, Ecteinascidia turbinata, is particularly noteworthy in terms of convergence, as it resembles a phlebobranch ascidian as an adult, but an aplousobranch ascidian in the larval stage. Based on an anatomical analysis, it was concluded that this species belongs to the Phlebobranchiata and that the larval similarities with aplousobranchs must be the result of convergent evolution. Some tropical ascidians in the family Didemnidae are unique among chordates in that they form a stable photosymbiosis with a prokaryotic alga. Hosts of this alga are found in four different didemnid genera, each of which also contains non-symbiotic species. All hosts transmit the symbiont vertically, but they employ different mechanisms, and a molecular phylogenetic analysis has provided evidence for multiple evolutionary origins of this symbiosis. Many ascidians show sperm chemotaxis, where the spermatozoa swim towards the egg, guided by a gradient of an attractive substance produced by the egg. This phenomenon, which increases the chances of finding an egg, is also observed in lower plants as well as in many animals, including mammals. In ascidians and hydrozoan cnidarians, many aspects of sperm behaviour are very similar, including details of the flagellar reorientations that the spermatozoa produce during chemotactic turning.