The base and lower part of the column are expanded and form two lobes which envelop the hermit crab and its gastropod shell. The column of the anemone lies beneath the crab and the lobes meet on its dorsal side. The upper part of the column is short and cylindrical. The span of the tentacles is up to 5 cm. The column, disc and tentacles are white with the lower part of the column being chestnut in colour. The column is spotted with lilac-pink or crimson with a line of the same colour running around the upper margin. Long acontia (defensive white threads) are emitted from the lower part of the column when the anemone is disturbed.This anemone is almost always found on the hermit crab Pagurus prideaux
. The association appears to be obligatory between the 2 species and they are not generally found apart in normal circumstances and both degenerate quickly if separated. The base of the sea anemone secretes a chitinous membrane which effectively increases the size of the gastropod shell so as the crab grows it does not need to change shells.