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 muricata)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.
Only coastal and marine records shown



Onchidoris muricata has a maximum length of 1.4 cm and is usually white in colour but pale yellow individuals have been recorded. Also some rare individuals have been recorded with brown speckling on the mantle. Club-shaped stalked projections (tubercles) are visible on its mantle. Up to 11 gills can also be found at the rear of the mantle. The head is extended to form an oral veil, which lacks tentacles. The eggs are spawned in a ribbon, coiled several times.

Recorded distribution in Britain and Ireland

More common on the west coast of the British Isles particularly in the north. Few recordings on the east coast of the British Isles.

Global distribution

This widespread species has been recorded on both sides of the north Atlantic, and in Alaska, British Columbia and Washington in the north Pacific.


Onchidoris muricata is found on the lower shore or in sublittoral areas to depths of 15 m.

Depth range


Identifying features

  • Grows up to 1.4 cm.
  • White or pale yellow in colour.
  • Rounded tubercles with flattened tops.

Additional information

Likely to be confused with Adalaria proxima, however Onchidoris muricata has club-shaped tubercles whereas Adalaria proxima tentacles are pointed. Also Adalaria proxima is a large species, it can grow to 1.7 cm in length.

Listed by

- none -

Further information sources

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  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. & Morrow, C.C., 2000. Encyclopaedia of Marine Life of Britain and Ireland., 2003-09-18

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

  8. 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 muricata 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 20-08-2018]. Available from:

Last Updated: 08/04/2004