Evolution of ascidians

Evolution of extant ascidians appears to have proceeded in two streams from some large solitary ancestor like Ciona intestinalis, a solitary aplousobranch ascidian. Endodermal tissue of paired epicardial sacs, outgrowths of the gut at the posterior end of the pharynx, is the regenerative tissue in Ciona. This appears to be homologous with its role in most related aplouso-branch families where replicates are formed by horizontal division of the abdomen or posterior abdomen involving epicardial tissue. The other main evolutionary pathway appears to be associated with the use of pericardial sacs for an excretory purpose in the largely solitary phlebobranch and stolidobranch ascidians.

Apart from Ciona, Aplousobranchia are almost exclusively colonial. Colonial taxa, however, have evolved in otherwise primarily solitary Suborders Phlebobranchia and Stolidobranchia by stolonial or pallial budding, involving only ectodermal tissue. These colonies are convergent with those of Aplouso-branchia, suggesting that similar selective pressures are operative. Colonial zooids of all suborders are small and simplified, with small branchial sacs and without digestive diverticulae of the gut. Ovaries are small and testes are large, a condition possibly associated with internal fertilisation and intracolonial incubation of embryos. In colonial species special organs for excretion have been detected only in colonial Styel-idae where they persist as relicts of their stolidobranch ancestors.

Solitary phlebobranch and stolidobranch species display similar evidence of convergent evolution in general adaptations of organs to serve larger individuals. For instance, conspicuous in solitary taxa are large branchial sacs and increases in filtering area (e.g. the formation of folds in Stolidobranchia and stigmata spiralling around conical prominences in the pharyngeal wall in both Phlebobranchia and Stolidobranchia). The gut is large with complex digestive organs branching off it. Most species have special excretory cells derived from the epicardium, and in the stolidobranch family Molgulidae a large kidney is formed from epicardial tissue. Also, with few exceptions, male and female go-nads are large, species are oviparous, fertilised externally and seldom adaptated to retain developing embryos in the large pharynx and surrounding peri-branchial sac.

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