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Ascidiacea World Database

By Noa Shenkar, Arjan Gittenberger, Gretchen Lambert, Marc Rius, Rosana Moreira da Rocha, Billie J Swalla, Xavier Turon

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Ascidians (Phylum: Chordata, Class: Ascidiacea), or sea squirts, are the largest and most diverse class of the sub-phylum Tunicata (also known as Urochordata). They comprise approximately 3000 described species found in all marine habitats from shallow water to the deep sea. There are no freshwater species, and most cannot tolerate salinities below about 20‰. Recent phylogenomic studies suggest that they are actually the sister group to the vertebrates (Bourlat et al., 2006), although this conflicts with rRNA and mitochondrial data (Swalla and Smith, 2008). Adult ascidians bear little resemblance to typical chordates, though their short-lived non-feeding tadpole larvae clearly exhibit the four fundamental characteristics of the phylum: a dorsal tubular nerve cord, notochord, rudimentary pharyngeal gill slits and a post-anal tail. Another important character is the presence of the endostyle in the pharynx that will evolve as the thyroid gland in vertebrates. Following settlement, the lecithotrophic larvae undergo metamorphosis during which they lose all of these characteristics except for the endostyle and the gill slit rudiments in the pharynx (Millar, 1971), which become functional and multiply to form the branchial sac. The name "tunicate" (sub-phylum Tunicata) comes from the polysaccharide-containing tunic that envelops the animal and forms a somewhat flexible skeleton (Swalla and Cameron, 2010). Various proteins and blood cells occur in the tunic, and spicules in a few species (Monniot et al., 1991; Hirose 2009). Ascidians filter their food from the water-column via an oral siphon that brings water into the branchial sac where food items such as microalgae are filtered onto a mucus net; water, feces and gametes are expelled through an excurrent siphon. Particles suspended in the current are trapped along the wall of the branchial sac in a mucus net produced by the endostyle. The net pores range from 0.1 to 1 µm, allowing ascidians to filter even very small particulate matter, primarily in the range of 0.5 to 10 µm diameter (Bak et al., 1998; Bone et al., 2003). Several solitary species are cultured for food in Japan, Korea, France and Chile (Lambert, 2005) or extracted straight from rocky shores for human consumption or bait (Branch et al. 2010).

During the past two decades enormous progress has been achieved in the fields of development, evolution, immunology, natural products and ecology of ascidians. Their small genome, small cell number and (usually) short life-cycle make them an attractive model system for developmental biologists (Dehal et al., 2001; Nishida and Sawada 2001). Investigating the phylogenetic position of the subphylum Tunicata in relation to the other subphyla in the phylum Chordata is crucial to the understanding of possible mechanisms of chordate evolution (Swalla et al., 2000; Zeng and Swalla, 2005). The study of self/non-self recognition in ascidians provides important information regarding the evolutionary origin of the vertebrate immune system (Khalturin and Bosch, 2007). In addition, ascidians provide a fertile ground for studies in the field of natural products (Wang et al., 2007 for review), and play an important role in marine bioinvasions across the globe (Locke and Carman, 2009).


Kowalevsky discovered the chordate nature of the ascidian tadpole larva in 1866; the Ascidiacea were then reclassified as chordates rather than as molluscs (Raff and Love, 2004). Following Lahille (1886), the class Ascidiacea is now divided into three orders based on the structure of the adult branchial sac: Aplousobranchia (colonial), Phlebobranchia and Stolidobranchia. This is the current designation used by most ascidian taxonomists. All ascidians are hermaphrodites, having both male and female gonads, though many are not self-fertile. There are both solitary and colonial species; numerous excellent anatomical illustrations can be found in Monniot et al. (1991).

Ascidian systematics is the domain of specialists, but a keen observer can differentiate the orders and most families, and recognize the well-described common species. Although you “cannot judge an ascidian by its cover”, underwater photographs can provide much information and assist in distinguishing one species from another (Monniot et al., 1991) especially if the fauna of the given region has already been studied. The invaluable monographs published by Van Name (1945), C. and F. Monniot (Monniot et al., 1991; Monniot and Monniot, 1996, 2001 and many others) and Kott (1985, 1990, 1992, 2001 and others) greatly assist in identifying the ascidian fauna worldwide to species level. Tabular keys to the families and genera of the world can be found in Monniot et al. (1991). A comprehensive listing of nearly all the publications on ascidians since 1995 can be found by clicking on the various newsletters listed at

Literature cited

  • Bak RPM, Joenje M, de Jong I, Lambrechts DYM, Nieuwland G (1998) Bacterial suspension feeding by coral reef benthic organisms. Mar Ecol Prog Ser 175: 285–288
  • Bone Q, Carre C, Chang P (2003) Tunicate feeding filters. J Mar Biol Ass UK 83: 907-919
  • Khalturin K, Bosch TCG (2007) Self/nonself discrimination at the basis of chordate evolution: limits on molecular conservation. Curr Opin Immunol 19: 4-9
  • Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR & Telford MJ (2006) Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature 444: 85-88
  • Branch GM, Griffiths CL, Branch ML, Beckley LE (2010) Two Oceans: A guide to the marine life of southern Africa. Struik Publishers, Cape Town, 456 pp.
  • Dehal P et al. (+ 86 additional authors) (2002) The draft genome of Ciona intestinalis: Insights into chordate and vertebrate origins. Science 298: 2157-2167
  • Hirose E (2009) Ascidian tunic cells: morphology and functional diversity of free cells outside the epidermis. Invertebr Biol 128: 83–96
  • Kott P (1985) The Australian Ascidiacea. Part 1: Phlobranchia and Stolidobranchia. Memoirs of the Queensland Museum 23: 1-440
  • Kott P (1990) The Australian Ascidiacea. Part 2: Aplousobranchia (1). Memoirs of the Queensland Museum 29 (1): 1-226
  • Kott P (1992) The Australian Ascidiacea. Part 3: Aplousobranchia (2). Memoirs of the Queensland Museum 32 (2): 375-620
  • Kott P (2001) The Australian Ascidiacea. Part 4: Aplousobranchia (3), Didemnidae. Memoirs of the Queensland Museum 47 (1): 1-407
  • Lahille MF (1886) Sur la classification des Tuniciers. C. R. Acad. Sci. Paris 102: 1573–1575
  • Lambert G (2005) Ecology and natural history of the protochordates. Can J Zool 83: 34-50
  • Locke A, Carman M (2009) An overview of the 2nd International Invasive Sea Squirt Conference: What we learned. Aquat Inv 4: 1-4
  • Millar RH (1971) The biology of ascidians. Adv Mar Biol 9: 1-100
  • Monniot C, Monniot F, Laboute P (1991) Coral reef ascidians of New Caledonia. ORSTOM, Paris
  • Monniot C, Monniot F (1996) New collections of ascidians from the western Pacific and Southeastern Asia. Micronesia 29 (2): 133-279
  • Monniot C, Monniot F (2001) Ascidians from the tropical western Pacific. Zoosystema 23 (2): 201-383
  • Nishida H, Sawada K (2001) Macho-1 encodes a localized mRNA in ascidian eggs that specifies muscle fate during embryogenesis. Nature 409 : 724–729
  • Perrier E (1898) Note sur la Classification des Tuniciers. CR Acad Sci Paris 126: 1–5.
  • Raff RA, Love AC (2004) Kowalevsky, Comparative Evolutionary Embryology, and the Intellectual Lineage of Evo-Devo. J Exp Zool (Mol Dev Evol) 302B: 19-34
  • Swalla BJ, Cameron CJ (2010) “Tunicata” in Phylocode. Tunicata” in Phylocode: A Phylogenetic Code of Biological Nomenclature. Version 2b P. D. Cantino and K. de Queiroz, eds.
  • Swalla BJ, Cameron CB, Corley LS, Garey JR (2000) Urochordates are monophyletic within the deuterostomes. Syst Biol 49: 52-64
  • Swalla BJ, Smith AB (2008) Deciphering Deuterostome Phylogeny: Molecular, morphological and palaeontological perspectives. Phil Trans R Soc B 363: 1557-1568
  • Van Name WG (1945) The North and South American ascidians. Bull Am Mus Nat Hist 84: 1–476
  • Wang WF, Namikoshi M (2007) Bioactive nitrogenous metabolites from ascidians. Heterocycles 74: 53-88
  • Zeng L, Swalla BJ (2005) Molecular phylogeny of the protochordates: chordate evolution. Can J Zool 83 (1): 24–33


Usage of data from the Ascidiacea World Database in scientific publications should be acknowledged by citing as follows:

  • Shenkar, N.; Gittenberger, A.; Lambert, G.; Rius, M.; Moreira da Rocha, R.; Swalla, B.J.; Turon, X. (2020). Ascidiacea World Database. Accessed at on 2020-05-29. doi:10.14284/353
If the data from the Ascidiacea World Database constitute a substantial proportion of the records used in analyses, the chief editor(s) of the database should be contacted. There may be additional data which may prove valuable to such analyses.

Individual pages are individually authored and dated. These can be cited separately: the proper citation is provided at the bottom of each page.

Image credits

(left to right)
· Botryllus eilatensis (Noa Shenkar)
· Didemnum rodriguesi (Rosana Moreira da Rocha)
· Herdmania momus (Noa Shenkar)
· Pyura stolonifera (Marc Rius)
Website and databases developed and hosted by VLIZ · Page generated 2020-05-28 GMT · contact: Noa Shenkar