This anemone is a common and well-known species occurring in a bewildering variety of forms which differ considerably in their general appearance. Two distinct varieties are recognized in British waters although these are probably only the extremes of a long series connected by numerous intermediate forms (M. senile var. pallidus). In spite of this wide variation the basic characters of M. senile are very constant and easily determined.
Base wider than the column, moderately or firmly adherent, its outline often ragged due to basal laceration. Column divided into a smooth scapus and a relatively long capitulum, with a parapet and fosse; in full expansion the parapet often forms a salient, collar-like ring. Inconspicuous cinclides present on the scapus; acontia emitted reluctantly. Disc fairly wide, with prominent protruding lips around the mouth.
Tentacles: Very variable in length, relatively long in small specimens, and often very numerous - several thousand may be present in a large individual. Catch-tentacles are rare in British specimens.
Colouration: Plain, with no pattern on the disc but often one or two opaque white bars are present on the basal part of each tentacle; disc and tentacles otherwise translucent. The commonest colours are white or orange, but brown, grey or occasionally red or yellow varieties occur. Bi-coloured specimens, e.g. brown column, white disc and tentacles, are not uncommon.
Reproduction: Basal laceration is habitual.
Nematocysts of acontia: b-mastigophores measuring about 30-70 x 3.0-5.0 öm invariably occur and often a range of smaller, more slender b-mastigophores is present; p-mastigophores measuring about 30-70 x 3.0-6.5 öm are common in small specimens but are rare or absent in large individuals.
Var. dianthus: This is the well-known large Plumose form. Scapus smooth, pillar-like in extension, terminating above in a broad collar-like parapet; capitulum long and flaring out toward the disc. Disc in older, not necessarily large specimens more or less deeply waved or folded, often tending to form lobes. Tentacles relatively short and slender, those of the inner cycles larger than the rest; very numerous - up to several thousand - and imparting a densely fluffy appearance to the expanded anemone. Colouration as above, sometimes lacking the white bars on the tentacles. Large specimens often attain 300 mm in height, with a basal diameter and tentacle span of 150 mm or more.
Var. pallidus: A small form not exceeding about 25 mm across the base. Column not usually very tall, its regions clearly defined. Disc flattish or slightly undulating, never deeply waved or lobed. Tentacles long and slender, not usually more than about 200. Colouration as above; usually very translucent, with prominent white bars on the tentacles; occasionally with greyish radial lines on the disc, amongst the tentacle bases. This variety, which varies in detail in different localities, appears to be a genuine dwarf race which becomes sexually mature at a small size (Rawlinson, 1934).
The two varieties described above are linked by a series of forms of intermediate size and structure which may exhibit characters of either variety. A common form frequent on the shore has a tall column and a slightly waved disc, occasionally forming lobes, and up to about 500 tentacles of moderate length. Such specimens are probably stunted forms of dianthus, their growth being inhibited by the littoral environment, but young anemones developing from basal lacerations often bear a close resemblance to pallidus.
Attached to any suitable hard substratum from the lower shore to moderate depths, at least 100 m. Smaller shore-dwelling forms tend to occur in shaded places - beneath overhangs, in caves, under stones, etc. When the tide is out they are often found hanging in a limp, uncontracted condition from the roofs of caves and overhangs. Large specimens of dianthus are almost exclusively sublittoral, occurring in situations with strong water movement - frequently on piers or harbour walls. They are often found in abundance, just below the water surface at ELWST, on projecting structures such as pier piles or sewer pipes. The var. pallidus and other small forms may occur in conditions of reduced or variable salinity - estuaries or brackish creeks.
Common and widespread on all coasts of the British Isles and north-west Europe, from the Biscay coast of France northward to Scandinavia and Iceland.