Body (excluding appendages)
Up to 55 mm long. Translucent white showing oesophagus behind rhinophores. Medial line of white surface pigment on the tail [2] may be discontinuous . Very rarely any iridescence.
Cerata
Often held to expose dorsum. Linear, often exceeding half body length [3]. Rather untidily arranged groups on each side of the body with the gap between the first and second clumps overhung by cerata of first group. Semi-transparent whitish, revealing light reddish brown to dark chocolate brown internal digestive gland; see Scottish Nudibranchs for colour range. White cnidosacs clearly visible inside the near transparent distal end. Small patch of white surface pigment extends a short way over digestive glands from cnidosacs [1]. No white pigment splashes on the rest of the ceratal surface; rarely a minute white spot or two (ScotNud). Usually no iridescence; occasionally a little on head, all over cerata very rarely.
Rhinophores
Translucent rosy fawn [3], yellowish or whitish (juveniles [2]) with up to 30 lamellae (all complete disks), distally whitish with small amount of white pigment.
Head
Translucent white, often with some white pigment between rhinophores [2], occasionally slight iridescence. Oral tentacles half as long as body, translucent white with white surface pigment on distal third. Small internal eye visible at posterior of rhinophore base [1].
Foot
Anterior edge convex with centre of curve indented. Slender sickle shape propodial tentacles. Translucent white sole, much wider than body [3].
- Cerata long, linear, rather untidy and lack iridescence [3]. (Occasionally, a small amount of iridescence on the head or oral tentacles; very rarely all over cerata).
- No white pigment splashes down cerata, apart from small patch near top [2]. (Sometimes a minute white spot or two on front of cerata.)
- Rhinophore lamellae all complete disks.
- Slender sickle-shape propodial tentacles [2].
- Usually iridescence on cerata and, sometimes, on head and body, occasionally not.
- Cerata usually neatly swept back, each group only slightly overlaps the next behind.
- Rhinophore lamellae alternate complete/incomplete disk.
- Propodial extensions shorter and stouter than on F. bostoniensis.
- White pigment splashes down front of cerata.
Facelina dubia (EML) Very similar to F. bostoniensis
- Smooth rhinophores lack lamellae.
- White pigment flecks on head and oral tentacles.
- In British & Irish waters, so far only recorded in Lough Hyne in SW Ireland.
New England, USA, and Southern Norway to Spain and western Mediterranean. (GBIF map) Widespread and common around Britain and Ireland. (UK interactive distribution map N.B.N.)
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society. Anderson, J. Scottish Nudibranchs (ScotNud).
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 maps and prey organisms:
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland (EML).