Body
Usually up to 4mm long, occasionally 8mm. Smooth body with no tubercles or appendages, apart from curved rhinophoral crest [image1] above and in front of each eye. Crests absent on juveniles, and may be weakly [4] or well [5] developed on adults. Body black [1] or dark brown [4] apart from translucent white, spotted with white pigment, on dorsal rim of foot, dorsal surface of metapodium (tail), eye areas and rhinophoral crests [1], and, often, the central area of the back [3]. Usually, light spots on dark areas, and sometimes small greenish patches with a few reflective flecks of green. Anus a short distance behind mid point of body, set to right of dorsum. (Often difficult to see, except when defecating.)
Head
Anterior edge truncated, extends beyond foot [2]. No digitiform rhinophores or oral tentacles.
Foot
Sole translucent white spotted with white pigment [2]. Reveals viscera, often two shades of green. No propodial tentacles or extensions, but anterior of foot slightly swollen.
- Curved rhinophoral crest above and in front of each eye [1], no ridge below eye, and no digitiform rhinophores. At some angles of view, crests can be mistaken for digitiform rhinophores [6].
- Dorsal surface of metapodium (tail) [3], eye areas and rhinophoral crests all whitish [1].
- Anus a short distance behind mid point of body.
- Not in brackish water. (But may be swept into salting pools in full salinity sites.)
- No tentacles or rhinophoral crests but some have ribs/sharp edges to recessed eye-area, above, in front of and below eye) [1Ld] .
- Pale metapodium (tail) absent or negligible.
- Dorsal anus close to posterior.
- In brackish or fully marine salinity (though slow to adapt to change in salinity).
- Pair of tentacles on head when 2mm long or larger, with or without anterior crest.
- Pale metapodium.
- Anus a short distance behind mid point of body.
- Full salinity, also brackish Fleet lagoon, Dorset and variable salinity pools at MHWS on exposed coasts.
Arctic to Mediterranean ( GBIF map). Common and widespread around Britain and Ireland. ( UK interactive distribution map, N.B.N.)
Eliot, C.N.E. 1910. A monograph of the British nudibranchiate mollusca. London, Ray Society. Supplementary Volume.
Thompson, T.E. 1976. Biology of opisthobranch molluscs 1. London, Ray Society.
Current taxonomy; World Register of Marine Species (WoRMS)