Trinchesia caerulea (Montagu, 1804) | The Conchological Society of Great Britain and Ireland

A variable species, but all forms have intermediaries.
Body (excluding appendages)
Up to 26 mm long. Translucent white, usually with a streak of white opaque pigment running from front edge of head to tip of tail, bifurcating to go round the pericardium. On adults, streak is a dorsal line, but on juveniles a wide diffuse band covering much of dorsum [image1]. Sometimes diffuse white pigment on flanks of juveniles [2].
Cerata
Arranged in distinct rows on each side of the body [7]. About 3 per row on 3mm long juvenile, about 6 per row at 15mm, larger specimens up to 8 per row. Translucent, revealing green [7], blue, grey-brown [3] or almost black internal digestive gland. Forms with surface pigment most fully developed have two orange bands with intervening blue band which conceal most of digestive gland [2]. On some the blue is missing [4]. At the minimum, only the terminal orange band is present [6]. Apex translucent revealing orange cnidosac [8].
Rhinophores
Smooth translucent with diffuse pigment flecks and suffused pale lemon [1].
Head
Circular outline in ventral view [3].Translucent white. Rose pink interior of head visible on juveniles [2].
Oral tentacles translucent with pigment flecks, suffused pale lemon [1].
Foot
Rounded propodial swellings often prominent on adults, but no propodial tentacles [5]. Sole translucent white.

Key identification features

Rhinophores and oral tentacles suffused pale yellow [1].
Cerata have distal band of orange pigment and orange internal cnidosac [8], and, often, a basal orange band and intervening blue band [2].

Similar species

Cuthona viridis (EML) resembles form of C. caerulea [6] with green digestive gland and only a single orange band of ceratal pigment.

Cerata have green digestive gland and swollen white or yellow internal cnidosacs filling tips, (Don’t confuse with surface pigment).
White or pale yellow pigment marks on front of cerata, and, sometimes, in a thin indistinct subterminal ring.

Ecology and behaviour

Usually sublittoral to 270m, few records at ELWS. On or near its prey, Sertularia polyzonias (EML), and S. gayi (EML). Also Halecium halecinum (EML), Hydrallmania falcata (EML) and Eudendrium racemosum. Simultaneous hermaphrodite. Spawn an anticlockwise spiral with few turns, April – September.

Distribution and status

Western Mediterranean to southern Norway, (Brazil and Florida records may be related species) (GBIF map). Western coasts of Britain from Dorset to Shetland and Kincardineshire. Records mainly sublittoral (UK interactive map. NBN).

References and links

Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
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).