Body (excluding appendages)
Up to 38mm long. Translucent white revealing opaque white blobs of the ovotestis [image 4]
and, behind the rhinophores, the pink oesophagus 
. Medial line of white pigment on the surface of the tail 
. Iridescence, which is most prominent on the cerata, sometimes extends on to parts of the body 
Usually conceal posterior half of body. Arranged in distinct groups on each side of the body with a clear gap between the first and second clumps 
. Semi-transparent whitish, revealing yellow, pink, red or brown internal digestive gland (ScotNud
) which extends over three quarters of the way up the ceras. Digestive gland often tapers towards its tip on its posterior face 
. On the almost transparent distal end there is usually a band of white surface pigment, broad at the front and narrowing behind. Irregular splashes of white surface pigment on front of cerata. Distinctive iridescence on front faces of cerata, varies; pale purplish pink 
, light blue 
, purple-blue 
, dark blue (EML
). Specimens from deep water usually have the most extensive and darkest iridescence. Some littoral specimens need close examination to detect any iridescence; sometimes absent. Juvenile 
specimens may lack iridescence, but usually have white pigment splash on front of ceras.
Column translucent white with series of lamellae alternating between complete and incomplete disks. Lamellae vary from very faint sandy 
tint to orange-brown 
. Cigar shape stripe of white surface pigment on distal half of front face of column, but may be indistinct on pale specimens.
Head & oral tentacles
Head translucent white, often tinted reddish in front of rhinophores 
. Oral tentacles, twice as long as rhinophores, translucent white with white surface pigment distally 
. The head and oral tentacles are the parts of the animal that most frequently share the iridescence of the cerata 
Short sickle shape propodial extensions. Narrow translucent white sole, revealing white blobs of ovotestis.
Key identification features
- Coral pink oesophagus visible behind the rhinophore .
- Usually iridescence on cerata and, sometimes, on head and body .
- Very faint sandy  to orange brown  lamellae on rhinophores.
- White pigment marks down front of cerata .
- Slender, foot no wider than body.
- 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.
Ecology and behaviour
Fairly frequent at LWS on rocky shores, but commonest below 15m. Feeds on Tubularia indivisa, (EML), Laomedea flexuosa and Obelia (EML). Simultaneous hermaphrodite. Spawn laid May to October as Grecian key line arranged in a spiral. Shelled veliger larvae drift as plankton before settling on the sea floor and transforming into adults.
Distribution and status
Western Mediterranean to Norway. (GBIF map, obvious misplacement of some records inland.) Common all round Britain and Ireland on hard substrate sea beds and on rocky shores where food organisms occur. (UK interactive distribution map, N.B.N.)
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Anderson, J. Scottish nudibranchs (ScotNud
) (Wide range of colour forms shown.)
Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society.
World Register of Marine Species (WoRMS
Irish distribution maps & prey organisms:
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland