Runcina coronata is a shell-less species up to 6 mm long. The head is blunt at the front, with no tentacles. The body is dark brown, with paler areas on the head. Three small posterior gills protrude at the right hand end of the mantle which distinguish the genus from similar sea slugs e.g. Limapontia senetra.Runcina coronata and the sacoglossan Limapontia senestra are the only European opisthobranchs in which the shell makes no appearance at any stage in its embryonic development (Thompson, 1976). Spawning in Runcina coronata occurs in the spring months. The egg masses contain 20-30 eggs (Thompson, 1976).
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