Flabellina verrucosa is the accepted name in WoRMS.
Two forms of Flabellina verrucosa with very distinct appearances occur in Scandinavia. One with short stout cerata closely resembles the original description and illustration by Sars (SSF). The other with longer more slender cerata is the form described in Alder & Hancock as Eolis rufibranchialis Johnston, 1832, and is the form described in this account. DNA sequencing by R.Eriksson et al. (2006) has confirmed that they are the same species genetically, but interbreeding of the two forms has yet to be observed. The forms might be the result of different diets, so recording of food organisms is important. Meantime, it is suggested that records of the Johnston form are kept as Flabellina verrucosa rufibranchialis. The form with short cerata is absent or extremely rare outside of Scandinavia, but two images of it from Norway are included to help recognition if it is found elsewhere  & . Also see (SSF).
Body (excluding appendages)
Usually up to 25 mm long, occasionally 35mm. Translucent white, revealing white ovotestis when mature (can mature at 5mm). Opaque white continuous dorsal line, with occasional clear spots, on tail , continues as a broken dorsal line to pericardium, but often obscured by overhanging cerata. Line on back may be only slightly developed on juveniles. Cerata Often obscure dorsum of body. Arranged in 5 – 7 diagonal rows on each side of the body, rising from a notal ridge  which does not continue between ceratal groups. Translucent white revealing brown, maroon or crimson internal digestive gland. Narrow, fragmentary, opaque white subterminal pigment band  circles the translucent white tip containing cnidosacs. Small round cerata frequently at base of ceratal groups .
Rhinophores Wrinkled , translucent yellowish , sometimes tinted pinkish. Opaque white pigment line along distal third . Head Translucent white oral tentacles with white pigment line along distal third. Prominent mouthparts, ventrally cleft, extend forward of foot. Internal pink organ below and to rear of rhinophores; a forward pointing V  in dorsal view and a diagonal band  in lateral view Foot Small anterior expansions  of foot, can assume form of short stout tentacles. Sole translucent white.
Key identification features
Flabellina verrucosa rufibranchialis
Continuous white dorsal line on tail, frequently broken dorsal line on body .
Narrow, often fragmentary, subterminal white ring on cerata .
Wrinkled yellowish rhinophores, sometimes tinted pink .
White pigment covers entire distal end of cerata, except tiny extreme tip of apex.
Ecology and behaviour
Sublittorally to 450 metres, and at LWS, where strong currents favour principle prey; Tubularia indivisa (EML). Many other hydroids eaten, especially when juvenile. Simultaneous hermaphrodite. Spawn a thin line arranged as a smooth spiral on flat rock, or looped around hydroids. Breeds April – June in Britain. Veliger larvae drift for about ten days before metamorphosing.
Distribution and status
Greenland and Spitzbergen to Britain and New England. Also Kamchatka to Japan, and western Canada. (GBIF map) Fairly frequent sublittorally in Britain and Ireland, and sometimes at LWS, as far south as Wales. Scarcer further south. (Interactive UK distribution map NBN)
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Eriksson, R., Nuygren, A. & Sundberg, P. 2006. Genetic evidence of phenotypic polymorphism in the aeolid nudibranch Flabellina verrucosa (M. Sars, 1829) (Opisthobranchia: Nudibranchia). Organisms, Diversity and Evolution6:1, 71-76
Odhner, N.H. 1922. Norwegian opisthobranchiate mollusca in the collection of the Zoologial Museum of Kristiana. Nyt. Mag.Naturv. 60: 1-47
Rudman, W. Sea Slug Forum (SSF)
Sars, M. 1829. Bidrag til söedyrenes naturhistorie. Pt 1. Bergen.
Thompson, T.E. & Brown, G.H. 1976. British opisthobranch molluscs. London.
Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society. Current taxonomy; See notes at top of this account.
Irish distribution maps
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland. (EML)
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