Diameter of base up to 120 mm in shore-dwelling specimens, often larger in examples from deep water offshore.
The base is broad and very firmly adherent.
Column usually shorter than wide, with a well-marked parapet and a deep fosse. Verrucae always present, variable in size but generally well-developed, largest in the middle of the column and extending on to the parapet where they tend to form longitudinal rows. They are strongly adhesive and, in shore dwelling specimens at least, invariably have numerous particles of gravel and other debris stuck to them, a contracted anemone having the appearance of a rounded mound of gravel. Disc not normally wider than the parapet.
Tentacles: Stout, short, occasionally moderate in fullest extension, their decamerous arrangement easily determined, up to 160.
Colouration: Very variable, a common form being as follows: column red, irregularly blotched with green, the verrucae grey; disc pale blue-grey shading to red around the mouth, with a prominent pattern of broad red lines amongst the tentacle bases; tentacles greyish, banded with dull red and white. The range of variation is endless: white, yellow, orange, red, blue, grey, purple and brown are all common colours, occurring in various combinations or, less frequently, plain. The distinctive and typical pattern of red lines on the disc is usually present, but it differs in detail: often the lines enclosing the primary tentacles are bordered with white behind the tentacles, sometimes they coalesce to form a broad red area amongst the tentacle bases, sometimes the whole disc is plain red. The tentacles are usually banded but may be plain; another common form is red all over except for grey verrucae and plain white tentacles.
Typically a form of the lower shore, occurring in its greatest abundance in the Laminaria zone, particularly on shores exposed to strong wave action. Small specimens are frequently encountered as high as MTL and large ones are occasionally found in deep, permanent pools above the usual levels. It attaches to rocks and boulders, often crowding into crevices at the bottoms of pools and gullies, forming a dense carpet; also in caves, amongst Laminaria holdfasts, partly buried in gravel, etc. Offshore it occurs down to at least 100 m depth; specimens in the sublittoral may not bear the coating of gravel characteristic of shore dwellers.
A widespread species, common, often very abundant locally, on all coasts of the British Isles. It occurs throughout Europe from north Russia to Biscay but is absent from the Mediterranean. Apparently a boreal-arctic, circumpolar species but knowledge of its full distribution is incomplete.