(de Quatrefages, 1844)
This a shell-less species, up to 6 mm in overall length, dark brown, with paler areas on the head. The animal is smooth and graceful and it has some resemblance to a sacoglossan sea-slug such as Limapontia , from which it can be distinguished, however, by the presence of three simple gills under the rear lip of the mantle in Runcina .
This species is found in clean coralline rock pools. Its exact diet is unproven, but it seems that it feeds either upon the coralline algae of the pools, or upon the diatoms and other microscopic forms that live epiphytically on these dominant plants.
Distributed from the Atlantic and Mediterranean coasts of France and from Portugal, always in very shallow waters; rare in the North Sea (Distr. R. coronata).