Actinothoe sphyrodeta

(Gosse, 1853)

Very soft and delicate in texture, which expands its tentacles very readily. Diameter of base up to 20 mm, span of tentacles to 30 mm; specimens occurring on the shore are usually about half this size.
Base a little wider than column, lightly adherent. Column usually short but capable of becoming taller than wide in extension. Cinclides visible as dark dots on upper part of column, acontia are emitted through them very readily.
Tentacles: Moderate in length, characteristically stout at the base and tapering to fine points; they are usually irregularly arranged, up to about 120.
Colouration: Column rather uneven in colour, translucent dirty white or greyish, with longitudinal patches of pure white tending to form stripes. Tentacles white, translucent. Disc may be white, with the pigment patchily distributed, bright yellow or pale orange, usually with vague dark markings surrounding the tentacle bases.
Nematocysts of acontia: p-mastigophores 43-86 x 6.0-9.0 µm, b-mastigophores 22-32 x 1.6-2.5 µm.

Attached to rocks or other hard substrata, usually on flat surfaces rather than in holes or crevices like most other sagartiids; also occurs on various algae, e.g. Laminaria, or Himanthalia buttons. Occasional on the shore in shaded situations, usually on vertical or overhanging surfaces. More common and widespread sublittorally, down to about 40 m. Frequently occurs in the company of Caryophyllia and Corynactis.

Locally abundant on west coasts of British Isles, as far north as Shetland but more common in the south. Southern limits of its range are uncertain; it occurs at least to Biscay but is apparently absent from the Mediterranean.