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

A sea slug (Okenia leachii)

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



Okenia leachii can grow up to 4 cm in length. It has a translucent pink/brown body with white colouring on the tips of its processes, gills, rhinophores and around the edge of the mantle. The long, tapering processes arise from the mantle rim and form three rows down the back. Approximately 11 gills are arranged in a circle towards the rear. Eggs are spawned in a long sausage-shaped mass attached by a filament at one end.

Recorded distribution in Britain and Ireland

Only recorded on the west coast of Scotland from Shiant Island in the Outer Hebrides, Loch Fyne and off Talisker bay on the Isle of Skye.

Global distribution



Usually found in deep water below 25 m, on muddy sand seabeds.

Depth range


Identifying features

  • Up to 4 cm in length.
  • Three rows of long tapering processes arise from the mantle.
  • White pigment on tips of all processes, gills and edge of foot.

Additional information

It probably feeds on burrowing ascidians such as Molgula oculata (Picton & Morrow, 1994).

Listed by

Further information sources

Search on:


  1. Allen, J.A. 1962. The fauna of the Clyde Sea area. Mollusca. Millport: Scottish Marine Biological Association.

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

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


  1. NBN (National Biodiversity Network) Atlas. Available from:

  2. OBIS (Ocean Biogeographic Information System),  2021. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2021-12-03


This review can be cited as:

Place, A. 2007. Okenia leachii A sea slug. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 03-12-2021]. Available from:

Last Updated: 26/06/2007