Berthella plumula
(Montagu, 1803)

Family: Pleurobranchidae


Northeastern Atlantic from Norway to the Mediterranean Sea.


St. Kilda, Scotland, July, 1984, August 1985. Photos: Bernard Picton

The thin, transparent shell is internal. It is about half the body length, whicih may reach about 60mm. Stellate calcareous spicules are present in the skin. In colour it is pale lemon-yellow to orange, often with net-like markings in the middle of the smooth dorsal mantle. The skin can secrete defensive sulphuric acid if attacked. This is the type species of Berthella, described in 1803 by Montagu. It is a common species around the British Isles and extending from Norway to the Mediterranean. It is normally found beneath rocks on the lower shore but I have seen it frequently at St. Kilda on the west coast of Scotland down to depths of 30m. on steep rock faces. It seems to feed on the sponge Oscarella lobularis (actually the Oscarella in the British Isles is probably not named and distinct from Mediterranean O. lobularis). The appearance of Berthella seems to be providing camouflage on Oscarella, with a central transparent patch mimicking an oscule of the sponge and the reticulate pattern looking similar to lobules of the sponge. Thompson (1984, 1988) suggests that Berthella plumula may feed on tunicates but there have been no published observations to support this idea.

• Thompson, T.E. (1988) Molluscs: Benthic Opisthobranchs (Mollusca: Gastropoda). Synopses of the British Fauna (New Series). No. 8. 2nd Edition. E.J.Brill/W. Backhuys: Leiden. 1-356.

Authorship details
Picton, B.E., 2002 (January 3) Berthella plumula (Montagu, 1803). [In] Sea Slug Forum. Australian Museum, Sydney. Available from

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