Up to 40mm long. Ample mantle covers whole body. Dull white with increasing amounts of brown as it grows, generally: all white at 1mm long ; brown stem on rhinophore and brown at base of gills at 2mm; dorsal panel, gills and basal half of rhinophore all brown at 5mm [image 3]; brown intenser, and periphery has some fawn-brown at 7mm ; dorsal panel and rhinophore bases dark brown, rest of mantle freckled fawn-brown, and gills lose brown to become translucent dirty white at 40mm . On all sizes, dark brown dorsal panel usually stops at or behind rhinophores, leaving a pale area  in front of rhinophores. Some specimens deviate from foregoing, e.g. a few stay completely dirty white to adulthood. Round-headed, club-shape tubercles of various sizes cover mantle , including within the gill ellipse . Dark brown dorsal panel lacks larger tubercles. Brown pigment usually absent from tubercles, so show as whitish spots among the brown.
Apex truncated. Bulbous base recessed in pit . Up to 16 oblique lamellae  on adults. Translucent whitish with brown basal pigment  increasing with age until extends two-thirds of height.
On adults, about 25 large , translucent, whitish unipinnate  gills curving untidily outwards from transverse ellipse, and often about six smaller gills  within the ellipse. About 20 large tubercles and many smaller ones within gill ellipse. Gills on juveniles  often dark brown.
Wide, curved, oral veil, often with wavy edge , white with slight yellow tint.
Sole translucent white or yellowish white, showing yelow or purplish viscera . Anterior of foot gently rounded, not indented by mouth. Rounded posterior protrudes from mantle when moving.
Key identification features
- Dark brown dorsal panel .
- Translucent, whitish gills curving untidily outwards  from transverse ellipse (brown on juveniles ).
- Pale unpigmented club-shape tubercles on mantle .
- Many large tubercles within gill ellipse .
No other British or Irish species is likely to be confused with O. bilamellata.
Ecologyy and behaviour
On lower shore and sublittoral fringe to about 20m. Feeds on barnacles, such as Semibalanus balanoides and Elminius modestus (Flickr). Simultaneous hermaphrodite. Spawn masses, resembling everted, high, white collars  , often deposited on barnacle rich shores in large obvious amounts by hundreds of congregated adults in February and March. Adults die after spring spawning, so inconspuous until next generation grows (about 5mm long by late August). Date of breeding may be later when severe winter persists, and breeding can occur at other seasons. Planktonic veliger larvae before transforming into adult form. Unlike most other nudibranchs, breeding does not occur until near full size is attained.
Distribution and Status
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society.
Current taxonomy: World Register of Marine Species (WoRMS)
Irish distribution map and prey organisms:
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland (EML)