This shell is extremely similar to that of 0. semicostata . It differs in that the costae occur only on 1-2 adapical whorls, if at all, and that the protoconch is larger (330-575 µm in diameter).
The brown bands found on the last whorl of semicostata are usually absent in this species. Up to 3 mm high, 1.25 mm broad; the last whorl occupies about two thirds of the shell height, but is variable in this respect; the aperture about 40%.
0. aculeus lives in situations very similar to those in which O. semicostata is found, but it avoids the siltier habitats which the latter species prefers and is commoner on algal rather than stony substrata. It is also more frequent in brackish water. It is a panarctic species and has so far been recorded locally only from the east and west coasts of Scotland and in Galway Bay: it may well prove to be much more widespread. There are recent records from southern England.
The animals breed in Denmark (Rasmussen, 1951, 1973; Waren, 1974), the only locality for which information is available, from November to March. Capsules, usually with only one egg, are attached to weed, sand grains, or the shell of other animals, Each capsule has the usual hemispherical rissoid shape and has a diameter of 650-700 µm. A free larval stage is absent and young snails emerge from the capsules after a rather long developmental period.