Diameter of base up to about 50 mm, span of tentacles to 70 mm.
The base is broad, wider than column, moderately or firmly adherent.
Scapus broad and cylindrical, rarely becoming taller than wide except in small individuals, smooth, terminating above in a wide, sometimes shelf-like parapet which encloses a deep fosse. Rounded, sometimes compound acrorhagi are present, mounted in the fosse; these may be hidden if the parapet is constricted, or by the overhanging tentacles.
Disc wide, flat or convex, the mouth sometimes on an elevated hypostome.
Tentacles: Tentacles moderate in length and hexamerously arranged, those of cycles 1 and 2 virtually equal and not easily distinguished, in six cycles, up to 192.
Colouration: Usually fairly uniform, with no pattern on the disc: various shades of red, from blackish maroon to pale scarlet, various shades of green, orange, or brown. Some individuals, particularly green ones, or young red ones, may have irregular spots or streaks of yellow, greenish, or pale blue on the scapus. The acrorhagi are almost invariably bright blue, occasionally white or pink; a narrow line of blue usually encircles the limbus and a blue spot may be present on each angle of the mouth. Bi-coloured specimens, e.g. green column, tawny orange tentacles, are frequent in some areas. Totally colourless specimens sometimes occur in permanently dark habitats - sea caves or beneath large boulders.
Typically a shore form occurring from high up, around HWN, to the lowest levels, and also in shallow water offshore, down to about 20 m. Occurs attached to any convenient hard substratum in exposed or sheltered situations, sometimes attached to rocks below a layer of sand, expanding above the sand. Also found in regions of variable salinity, such as estuaries or in the little rills of fresh water that often occur on beaches at low water.
Very common on all coasts of the British Isles and western Europe, including the Mediterranean, from north Russia (Kola peninsula) to the west coast of Africa, almost to the equator, and possibly further afield.