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

A sea slug (Onchidoris bilamellata)

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



Onchidoris bilamellata can grow to 4 cm in length and are usually white with a brown pattern on the mantle. However immature specimens and the occasional adult may be completely white in colour. It bears many (usually white) club shaped projections (tubercles) on its mantle and has two ringed sensory tentacles on its head. The numerous (up to 29) retractile gills are speckled brown around the edges and arranged in a horseshoe shape. The head extends to form an oral veil which lacks tentacles. The spawn is in the shape of a flattered coil attached to the substratum by one edge and contains many thousand eggs.

Recorded distribution in Britain and Ireland

Recorded all around Britain and Ireland.

Global distribution

Its range in the eastern Pacific is from Alaska to Baja California, Mexico. Occurs in the eastern and western Atlantic basins. Widely distributed in north west Europe reaching its southern limit on the French Atlantic coast.


Onchidoris bilamellata can be found in the intertidal and shallow sublittoral, to a depth of 20 m.

Depth range


Identifying features

  • Grows up to 4 cm in length.
  • Dirty white colour with a brown pattern.
  • Horseshoe shaped gills.
  • Club shaped projections (usually white) on the mantle.

Additional information

This species produces acid secretions from the mantle when disturbed. It is gregarious and feeds exclusively on barnacles, especially Semibalanus balanoides.

Listed by

- none -

Further information sources

Search on:


  1. Hayward, P.J. & Ryland, J.S. (ed.) 1995a. The marine fauna of the British Isles and north-west Europe. Volume 2. Molluscs to Chordates. Oxford Science Publications. Oxford: Clarendon Press.

  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. JNCC (Joint Nature Conservation Committee), 1999. Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line]

  4. National Biodiversity Network (NBN) Atlas website. Available from:  Accessed 01 April 2017

  5. Picton, B. E. & Morrow, C.C., 1994. A Field Guide to the Nudibranchs of the British Isles. London: Immel Publishing Ltd.

  6. Picton, B.E., 2000. Nudibranchs of the British Isles., 2003-09-11

  7. Thompson, T. E. & Brown, G. H., 1976. British Opisthobranch Molluscs. London: Academic Press. [Synopses of the British Fauna, no. 8.]


This review can be cited as:

Edwards, R.V. 2004. Onchidoris bilamellata A sea slug. 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 21-09-2018]. Available from:

Last Updated: 08/04/2004