Up to 38mm long. Chestnut brown mantle [image0] covers less than half the body, has conspicuous medial dorsal ridge with offshoots, and a prominent erect rim  . Juvenile paler . Rest of dorsal body surface translucent whitish tinted chestnut brown. Dull brown or bright orange may replace chestnut brown  especially in Oosterscheld estuary, Netherlands, where large population feeds on alien invader Botrylloides violaceus . Whole dorsal surface bears tubercles and flakes of white pigment. Two dorsal ridges to rear of gills . Penis in brown sheath to rear of right rhinophore .
Smooth digitiform apex offset to anterior . Truncate apex projects beyond lamellae [4a]. Incomplete lamellae leave anterior face smooth . Adult brown. Juvenile translucent white with a few brown flecks.
Up to 9 brown gills  in a complete circle around the anus on adult .
Transparent colourless on juveniles.
Oral veil  with white pigment marks on surface, formed into wide flat oral tentacles with S-shape anterior edge. Ridge runs along each tentacle .
Sole translucent white with white pigment marks, revealing pink viscera centrally . Anterior edge notched adjacent to mouth, and rounded with no propodial extensions. Projects behind mantle.
Key identification features
- Chestnut brown mantle .
- Mantle with erect edge, does not cover sides of body .
- Ridge on oral tentacle .
- Anterior edge of oral veil S-shaped .
- White “lacy” appearance with some yellowish pigment.
- No red-brown colouring.
- Oral tentacles not as wide as on G. castanea, and lack S-shaped edge.
Ecology and behaviour
Usually sublittoral to 25m, rarely ELWS on rocky shores. Feeds on ascidians, including Botryllus schlosseri , B. leachi (EML), Dendrodoa sp.(colour match) (EML), Ascidia mentula (EML) and the alien invader Botrylloides violaceus (Seapeach). May be recessed in pit eaten into prey. Simultaneous hermaphrodite. Spawn laid on or under stones in a spiral ribbon , recorded February – October. Veliger larvae drift as plankton before settling on the sea floor and transforming into adults.
Distribution and status
South Norway to Mediterranean (GBIF map), frequent sublittorally in Oosterschelde estuary, Netherlands. Widespread round Britain and Ireland on hard substrate. Rarely, if ever, common intertidally. (UK interactive distribution map. NBN.).
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Ermak, T. SeaPeach Bio Marine Research, USA. (Seapeach).
Groeneveld, R. & Reijs, S. Dutch Sea slugs (Diverosa)
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)