Oerstedia dorsalis is a small and rather stout species: up to 30 mm long, but most specimens are 10-15 mm in length and 1-2 mm wide. The bluntly rounded head, which bears four distinct eyes, is not demarcated from the body.
The species is extremely variable in colour. Individuals may be a more or less uniform brown or reddish-brown with a single pale yellow, cream or dirty white mid-dorsal stripe, which may extend the full body length, or be irregularly interrupted, but the marbled or banded varieties are more commonly encountered. In these the background colouration ranges from a pale yellowish-brown to cinnamon-brown or reddish-orange, speckled with brilliant white to yellowish flecks, and marked with transverse bands or irregular patches of dark brown, chestnut or brownish-yellow (O. dorsalis). There is sometimes a distinct brown lateral line on either side of the body. The ventral surface in all colour varieties is usually paler than the dorsal one. It breeds during the period September to November, but deeper-water forms have been found containing mature ova as early as in June.
An intertidal or shallow sublittoral species to the depth of 80 m or more. It is found on a wide variety of substrata below the low water mark (mud, gravel, sand, stones and shelly sediments) but generally occurs intertidally on small algae growing in rock pools (especially species of Ceramium , Chondrus , Cladophora , Corallina and Ulva ) or between the holdfast branches of Fucus and Laminaria species. Occasional specimens have been found beneath intertidal stones, on Zostera , amongst ascidians or on the submerged surfaces of boats and hulks.
The species is widely distributed in the northern hemisphere and is found in the west Baltic Sea, the North Sea, the Mediterranean, on eastern Atlantic coasts from northern Europe to Madeira, and on both Atlantic and Pacific coasts of North America as far south as Mexico.
Based on electrophoretic screening of six enzymes (Sundberg, 1988), one of the forms is considered to be a separate species: Oerstedia striata .