Length to 60 mm. White [image 1], yellow  or orange. Ample mantle lacks tubercles; usually covers whole body except rhinophores, oral veil and small part of foot. Superficially resembles dorid nudibranch, but gill and rhinophores not on mantle surface. On translucent white specimens, reticulate pattern  on mantle resembles lobules of prey, Oscarella encrusting sponge, and large transparent central spot revealing the dark interior of body resembles exhalent opening (osculum) of Oscarella. Markings often less clear on yellow and orange specimens  . Single corrugated gill  attached for two thirds of its length to the right hand side of body. Gill normally concealed by the mantle, but tip sometimes protrudes  . Occasionally mantle flexes up to aid exhalent respiratory flow and may reveal whole gill  . Genital opening to anterior of gill, and anus to its posterior.
Pair of enrolled rhinophores  project forwards from between anterior edge of mantle and oral veil.
Head usually concealed, apart from rhinophores and large oral veil extending forwards from under edge of mantle. Veil expanded  laterally into wide flat bilaminate oral tentacles slit open at the sides.
Sole  smooth, paler than dorsal surface of mantle. Anterior edge cleft  .
Only small portion exposed beyond mantle  .
Height (longest dimension) to about 30 mm; greater than 40% of body length. Permanently enclosed in body. Very fragile. Varies translucent or chalky white, or yellowish  . Shell containing viscera can sometimes be discerned  as dull orange-yellow shape inside live specimens with translucent white bodies. Large final whorl; rest of spire relatively very small ; large aperture extends entire shell height, except in very early growth stages  when rest of spire relatively larger.
- Mantle, lacking tubercles, covers whole body except protruding rhinophores, large oral veil and small portions of foot  .
- Anterior rim of mantle not curved up to form exhalent siphon.
- Shell length to 30mm, more than 40% of body length.
Pleurobranchus membranaceus (Sea Slug Forum)
- Body length to 120 mm.
- Mantle brown with large soft retractile conical tubercles.
- Mantle often curved up at posterior to form exhalent siphon.
- Shell to 50mm long, about 40% of body length.
- Body length to 30mm.
- Body yellow to orange, like some Berthella plumula, but not like the translucent white ones with reticulate pattern and central transparent dark area.
- Shell to 7mm long, under 30% length of body.
Low water spring tide and sublittorally to 12m. Under stones and in clean rock pools near its sponge prey, Oscarella spp. (Enc.Mar.Life) . Simultaneous hermaphrodite. Spirally coiled cream-coloured ribbon of spawn attached to rock in late spring. Veliger larvae with external shells live as part of plankton before settling on the sea floor and transforming into adults.
Norway to Mediterranean, but possible confusion in records with Berthellina citrina in southern areas. (GBIF) Common on hard substrate shores around Britain and Ireland. Most records littoral; some sublittoral to 12m. Few records on coasts of North Sea and English Channel east of Isle of White. (UK interactive map. NBN)
References and links
The map provided here shows the distribution of the species based on Conchological Society data.