Up to 6mm long. Fused mantle/cephalic shield; oblong, four times long as wide when fully extended [image1]. Anterior of shield folded down over front of foot , distinctly rectangular when viewed from above . Posterior of mantle more rounded. Smooth with no tubercles. Mantle/shield appears mostly black or pitchy brown with a whitish collar and a few light marks when seen on the shore . In strong lighting, such as photographic flash, mantle is translucent showing a clear collar of whitish spots anteriorally, many yellow spots on the mantle and, posteriorally, a U of yellow dots on the mantle and foot . Cephalic shield dark brown medially with few light spots; broad peripheral border of buff or yellowish brown . Anus under rear edge of mantle; white faeces  often visible on adjacent foot when grazing calcareous Lithothamnium.
Tuft of three, brown, simple gills may protrude  from below posterior of mantle on right side, or be concealed .
Concealed by cephalic shield . Under magnification, eyes  may be vaguely visible through pale border of cephalic shield.
Distinct  from fused mantle/cephalic-shield, which folds down over its anterior margin. Posterior of foot, usually extending well behind mantle, has broad medial black-brown band flanked by a few white spots and wide yellowish brown periphery. Yellowish brown margin of foot with yellow spots may protrude laterally  beyond mantle, or be hidden.
None. Only European opisthobranch, except Limapontia senestra, with no trace of shell at any stage of development (Thompson, 1976).
Key identification features
- Head concealed under rectangular cephalic shield .
- Fused mantle/cephalic-shield
- No tentacles, ribs or crests on anterior.
- Extensive metapodium with broad dark dorsal band extends at posterior .
- Tuft of brown gills  often protudes on right from under posterior edge of mantle.
- Anus under posterior edge of mantle .
Several spp. of Runcina In Mediterranean. (Sea Slug Forum).
- Head crests or tentacles, and distinct eyes in pale patches.
- No tuft of gills.
- Cephalic shield and mantle not fused.
- Foot extends into lateral parapodia.
- No exposed tuft of gills.
May be abundant in rock-pools  lined with encrusting pink or white calcareous Lithothamnion algae, or Corallina. Occurs plentifully in such pools at and above mean high water spring tide level at sites exposed to very strong wave action such as the tip of Portland Bill and north coast of Anglesey. Also on sea-weeds in shallow water at LWS of very sheltered land-locked bays and lagoons such as Finstown, Orkney and the Fleet, Dorset, and in pools near ELWS among Chondrus crispus and Rhodochorton algae. When grazing on suface of Lithothamnion, white faeces  suggest it ingests some Lithothamnion, obtaining nutrition from it, or from diatoms and other microscopic organisms living on its surface. High visibility of dark specimens against pale Lithothamnion suggests distasteful substances in body. Hermaphrodite. Spawn in spring. Egg mass contains up to 30 large ova (diameter 0.3mm). No veliger larval stage; miniature adults hatch from eggs. Occurs in fully marine salinity and brackish conditions, and able to withstand fluctuations in salinity as occurs in rock pools which are not refreshed with sea water at every high tide, and which experience salinity changing rainfall and evaporation.