Two varieties of this species are recognized: the typical form, decorata, which is extremely variable in colouration, and the small ornata, which is relatively stable in colour and always bears a disc pattern.
Var. decorata: Diameter of base up to 50 mm, height in full extension to 120 mm or more, span of tentacles to about 50 mm. Base wide, usually equalling or exceeding the span of the tentacles; firmly adherent and tougher than in S. elegans or Cereus, more easily detached from the substratum without damage. In full extension the column is tall and pillar-like but this condition usually occurs only in darkness or when the anemone is buried; under normal conditions it remains relatively short. Suckers small and often inconspicuous; in freshly collected specimens they nearly always have gravel or other debris stuck to them but this may drop off after capture. Cinclides are present near the limbus and on the upper part of the column; acontia are emitted from them only with reluctance. Disc a little wider than the column but never markedly so, as in S. elegans or Cereus.
Tentacles: Moderate in length, neatly arranged in a regularly hexamerous manner, up to 192.
Colouration: This variety is probably more variable in colour than any other British anemone but unlike S. elegans does not occur in reasonably distinct, constant colour forms. Column usually drab: buff, greenish olive, dirty flesh-colour, yellowish grey, etc., almost invariably with pale endocoelic stripes running upward from the limbus to the middle of the column or higher. Suckers greyish, not always easily discerned, especially in pale forms or small individuals. Disc and tentacles are infinitely variable: almost any colour except red (e.g. scarlet, crimson, magenta) may occur, plain or variegated, with or without a pattern. The complete pattern is usually carried out in brown or blackish. This is frequently incomplete; in particular the discal elements are often missing or only vaguely indicated and the banding on the tentacles may vary in appearance or be lacking. The most constant elements are the B-shaped markings at the bases of the tentacles, these often being the only markings present; even in apparently unpatterned examples, traces of these B-marks may be discernible. When the full pattern is present a ring of 12 or 24 white spots on the older radii is often very distinctive. Plain specimens, or individuals with contrasting discs and tentacles are common, the variety of combinations being endless. White, brown, grey and orange are common but such unusual colours, for anemones, as lilac, violet and black are not infrequent. Often the rayed effect, described for Sagartia elegans is present; this may be regular, with the dark rays centred on the primary radii, or irregular; sometimes the dark patches extend onto the column.
Var. ornata: Generally the diameter of the base does not exceed about 15 mm. Form similar to decorata but rarely becoming very tall; the base may not be very wide and is often only lightly adherent. The suckers may be very inconspicuous and may not have adherent gravel; on the other hand the cinclides, though few in number, may be large and easily observed.
Tentacles: Usually the tentacles are long, with only four or five cycles present.
Colouration: Column and disc usually translucent, coloured in various shades of green or reddish brown. The typical pattern is invariably present, carried out in a darker shade of the ground colour, or blackish. Due to the smaller number of radii on the disc the overall effect of the pattern may seem different from that of decorata but a careful examination will reveal the relationship; the apparent differences are caused by the pattern elements being relatively larger in proportion to the disc, which alters their emphasis.
Typical examples of decorata and ornata are easily distinguished but many intermediate forms occur and in some cases it is difficult to decide where their affinities lie. Var. ornata may be regarded as the only stable, and thus recognizable, variety of an exceedingly variable species.
Reproduction: This species is habitually viviparous although the production of young is not often observed, probably because they are very small - usually less than 1 mm diameter - at birth. The var. ornata is capable of reproduction at a very small size, specimens having a basal diameter of only 3-4 mm having been observed to produce young.
Nematocysts of acontia: p-mastigophores 20-37 x 3.0-5.2 öm, b-mastigophores 12-30 x 1.5-3.5 öm.
A common and widespread species likely to be encountered in any habitat where anemones occur. The var. decorata is typically found buried in mud, sand or gravel, usually attached to a stone or shell fragment 100-150 mm below the surface. It may be very abundant on mud or sand flats exposed at low water in sheltered localities. Var. decorata may also be found on rocky shores, in pools, in holes or crevices, under stones, or in kelp holdfasts; specimens in such habitats tend to be smaller than burying ones. This is a typical shore fom occurring from about MTL down but is also found in the sublittoral to about 50 m depth.
Var. ornata usually occurs on rocky shores, in similar places to decorata and also amongst mussels, in empty barnacle shells, etc., often in exposed situations and at fairly high levels. This variety is rarely found buried in soft substrata but has been recorded on the roots and rhizomes of Zostera in tidal creeks. It is a common intertidal form and may well occur sublittorally, although this has not yet been reported.
Either variety may occur in brackish water localities - estuaries, tidal creeks, saline lagoons, etc.
Throughout western Europe from Iceland and Scandinavia to the Mediterranean. In Britain both varieties are common and widely distributed on all coasts but decorata appears to be rather uncommon in the English Channel.
True periderm is lacking in this species but commonly a loose investment of mucus and detritus covers the lower part of the column. In some specimens of ornata this may become glued to the surrounding substratum, successive layers building up into a firm wall around the anemone. Such specimens were described by Gosse (1858) as Phellia murocincta. This mucous covering is never adherent to the column and thus does not constitute true periderm.
Freshly dug-up specimens of decorata from soft substrata may be formed into a curious goblet-shape. This is not permanent and the anemone soon reverts to a more normal shape. A similar form may be assumed by buried specimens of Sagartiogeton undatus but, apart from differences in colour, the latter species lacks suckers and never has gravel stuck to its column.