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

A sea slug (Jorunna tomentosa)

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



Jorunna tomentosa is usually sandy brown in colour with a loosely paired series of dark brown blotches down the sides of the body. It grows to about 6 cm in length.The mantle of Jorunna tomentosa is velvety, caused by the dense covering of small, uniform tubercles with an array of protruding spicules (caryophyllidia). Each tubercle has a characteristic retractile central projecting finger. There are up to 17 tripinnate gills, located within a gill pocket. The oral tentacles are short, slender and finger-like. The foot projects posteriorly when crawling. Jorunna tomentosa feeds on encrusting siliceous sponges, especially Halichondria panicea and Haliclona spp. Its colour and texture ensure this sea slug is well camouflaged amongst the sponges.

Recorded distribution in Britain and Ireland

Recorded from most of the coast of Britain and Ireland.

Global distribution

Recorded north to the Faroes and western Norway and south to Morocco and throughout the Mediterranean.


Common on the lower shore and shallow sublittoral on rocky coasts. Feeds on encrusting siliceous sponges especially Halichondria (Halichondria) panicea and Haliclona spp.

Depth range


Identifying features

  • Colour usually sandy brown or sandy.
  • Pair of short oral tentacles slender and finger-like.
  • Loosely paired series of dark brown blotches down sides of body.
  • Narrowly oval body.
  • Up to 17 trilobed gills in a tight circle.

Additional information

Spawning is recorded February to August (Thompson & Brown, 1984). The spawn is a broad ribbon laid in neat, tight coil. Each spiral spawn mass contains up to 145,000 eggs (Thompson & Brown, 1984).

Listed by

- none -

Further information sources

Search on:


  1. Hayward, P., Nelson-Smith, T. & Shields, C. 1996. Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

  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. MarLIN (Marine Life Information Network), 2005. SEArchable BEnthic Data (SEABED) Map [on-line]. Data Access Sub-programme, Marine Life Information Network for Britian and Ireland,

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

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

  7. Rudman, W.B., 1999a. Jorunna tomentosa [on-line].,

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

  9. Thompson, T.E. & Brown, G.H., 1984. Biology of Opisthobranch Molluscs, vol. II. London: Ray Society.


This review can be cited as:

Oakley, J.A. 2007. Jorunna tomentosa 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 16-08-2018]. Available from:

Last Updated: 23/08/2007