information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

A colonial sea squirt (Botrylloides violaceus)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.



Botrylloides violaceus is a colonial sea squirt forming lobed sheets usually 2-3 mm in thickness. Individual colonies are always one colour. The colonies can be different colours, e.g. dark brown, brick red, orange, purple and yellow. The zooids are arranged in a variety of ways, roughly oval groups or meandering, occasionally branching, double rows or chains.

Recorded distribution in Britain and Ireland

Recorded from the southern coast of the UK at the following locations in 2004: Gosport, Southampton, Hamble, Poole, Exmouth & Queen Anne's Battery in Plymouth. Also recorded from Milford Haven.

Global distribution



Found on artificial surfaces in shallow water, especially in harbours and marinas. Also found attached to macroalgae and other unitary sea squirts, for example Styela clava.

Depth range


Identifying features

  • Thick sheets or lobes adhering to the surface; may grow back-to-back.
  • Uniform in colour, can be dark brown, brick red, orange, purple and yellow.
  • Zooids arranged in roughly oval groups or meandering, occasionally branching, double-rows or chains.
  • The form and structure of the larva is a conclusive feature to aid with identification.

Additional information

A non-native species from Japan recorded in the UK for the first time in 2004. The one-toned colouration of Botrylloides violaceus distinguishes it from Botrylloides leachi and Botryllus schlosseri. Furthermore, Botryllus schlosseri has star-like zooid arrangements.

Depending on the season (July to September) it is possible to see the larvae within the colony due to their large size and spherical shape. The larvae are brooded separately from the zooids and are usually a dark pink or purple regardless of the colony colour so stand out and are large enough to see without a hand lens. (Larval information; G. Lambert pers. comm.). The released tadpole larvae have a ring of 25-30 vascular ampullae around the trunk (see image).

Listed by

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Further information sources

Search on:


  1. Arenas, F., Bishop, J.D.D., Carlton, J.T., Dyrynda, P.J., Farnham, W.F., Gonzalez, D.J., Jacobs, M.W., Lambert, C., Lambert, G., Nielsen, S.E., Pederson, J.A., Porter, J.S., Ward, S. & Wood, C.A., 2006. Alien species and other notable records from a rapid assessment survey of marinas on the south coast of England. Journal of the Marine Biological Association of the United Kingdom, 86, 1329-1337.

  2. Howson, C.M. & Picton, B.E., 1997. The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

  3. Saito, Y., Mukai, H. & Watanabe, H., 1981. Studies on Japanese compound styelid ascidians II. A new species of the genus Botrylloides and redescription of B. violaceus Oka. Publications of the Seto Marine Biological Laboratory, XXVI , 357-368.


This review can be cited as:

Snowden, E. 2008. Botrylloides violaceus A colonial sea squirt. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 18-07-2018]. Available from:

Last Updated: 29/04/2008