Authors: Ian Smith & Malcolm Storey. May 2012
Body
Up to 45mm long. Smooth with no tubercles. Dark brown with green [image 8] and/or red [1] cast, varying with diet. Often darker when on Codium (EML) than when on other algae. Rarely pale orange [16]. Usually speckled glistening blue or turquoise and, sometimes, green [6]. Some have extensive white or pink marks [1]. Two large parapodial lobes can conceal dorsum, open wide to expose it, or be held ajar [17]. Most have a white mark on rim of each lobe [17]. Leaf-like when upper surface of lobes bright green with dendritic [2] digestive gland.
Head
Distinct groove [6] runs next to anterior edge of parapodial lobes and across sole, separating head from rest of body. Anterior rim thick [7] divided by vertical cleft into two curved halves. Large smooth enrolled rhinophores [7] start to grow on juveniles [15] when body 3mm long. No oral tentacles. Eyes clearly visible, in pale eye patches [6] behind rhinophores.
Foot
Sole translucent pale, tinted by viscera [18]. Sometimes with many white or pinkish flakes [5] and fine brown pigment specks [7] . Anterior of foot swollen [5] but rarely extended into distinct curved propodial tentacles as drawn by Meyer & Möbius, 1865 (in Thompson, 1976). Foot expansion may give an almost pentagonal [3] outline to head when viewed from above. Strong transverse groove crosses sole behind head [7].
- Large parapodial lobes [8] (start to form at 3mm body length).
- Speckled with glistening blue or turquoise.
No similar British or Irish species.
Lower shore and shallow water. Reported foodplants include Codium [13], Cladophora [11], Bryopsis [9], Chondrus, Cytoseira, Zostera and Enteromorpha. Food preference may vary, e.g. Cladophora rupestris in Isle Of Man, but in Orkney common on Codium [14]. Chloroplasts [12] are sequestered from Codium, and probably other plants, and live symbiotically in the body, presumably providing benefit for the Elysia (Taylor, 1968). Mobile parapodial lobes not used in active swimming, but may assist when drifts on currents. Large surface may assist respiration, or photosynthesis of chloroplasts. Spawn, May - October. Veliger larvae in plankton before metamorphosis.
Shetland and Norway to Mediterranean (GBIF map). Widespread around Britain and Ireland, locally common, but few records from North Sea (UK interactive distribution map, N.B.N.), where Codium scarce or absent (Codium distribution map, N.B.N.)
Groeneveld, R. & Reijs, S. Dutch Sea slugs (Diverosa)
Taylor, D.L. 1968. Chloroplasts as symbiotc organelles in the digestive gland of Elysia viridis (Gastropoda, Opisthobranchia). J. mar. biol. Ass. U.K. 48 : 1 – 15.
Thompson, T.E. 1976. Biology of opisthobranch molluscs 1. London, Ray Society.
Current taxonomy; World Register of Marine Species (WoRMS)
Irish distribution maps
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland (EML)