Flabellina verrucosa and Coryphella verrucosa rufibranchialis are both accepted names in WoRMS.
Two forms of Flabellina verrucosa with very distinct appearances occur in Scandinavia and northern Britain. One with short stout cerata closely resembles the original description and illustration by Sars (SSF). The other with longer, more slender cerata may be the form described in Alder & Hancock as Eolis rufibranchialis Johnston, 1832, which is the form described in this account. DNA sequencing by R.Eriksson et al. (2006) suggests that two Scandinavian forms are the same species genetically. Mating of the two forms has been photographed by Erling Svensen and intermediate forms also photographed, but opinion is still divided on whether British F. rufibranchialis are the same species. 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 has been photographed in Scotland by George Brown, and it seems it extends as far south as Strangford Lough in N. Ireland. 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.
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 .
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.
Small anterior expansions  of foot, can assume form of short stout tentacles. Sole translucent white.
- Coryphella lineata (Lovén, 1846)
- White lines run along dorsum and sides of body, and the full lengths of the rhinophores and oral tentacles.
- Coryphella gracilis (Alder and Hancock, 1844)
- Narrow subterminal opaque white ring on cerata.
- Maximum size 15 mm (ovotestis lobules when 8mm).
- Rhinophores white.
- Coryphella browni Picton, 1980
- Interrupted white medial line from the rear cerata to tip of the tail.
- Broad white ring on cerata distally, but no pigment on apex.
- Rhinophores white.
- Flabellina pellucida (Alder & Hancock, 1843)
- White pigment covers entire distal end of cerata, except tiny extreme tip of apex.
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.
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 (UK distribution map NBN).
The map provided here shows the distribution of the species based on Conchological Society data.