Parasitic Anemone

Calliactis parasitica is up to 100 millimetres (3.9 in) tall, and 80 mm (3.1 in) wide, with the base of the column being slightly wider. The surface of the column is rough and leathery with a grainy appearance, but has no tubercles and is not divided into sections. It is variable in colouring, but is usually cream or buff in colour, with blotches and streaks of reddish or greyish brown, which tend to form vertical stripes.

Calliactis parasitica is found in the north-eastern Atlantic Ocean and the Mediterranean Sea. Its Atlantic range extends from south-western Europe as far north as the west coasts of Wales and Ireland, and the English Channel. Although this species has been recorded from the southern North Sea, those records are considered dubious. The depth distribution of C. parasitica ranges from a depth of 60 metres (200 ft) to the sublittoral zone; it is rarely found in the littoral zone.

Although Calliactis parasitica will occasionally attach to stones or empty shells, it is typically found on a gastropod shell inhabited by a hermit crab, and several individuals may live on the same shell. In the British Isles, the hermit crab is usually Pagurus bernhardus, but other species may be associated with C. parasitica in other parts of its range. C. parasitica is thought to use a chemical signal to detect its favoured shell, that of the whelk Buccinum undatum, because it has been observed in aquaria to mount the shell of a living B. undatum, although the whelk ensures that the sea anemone does not remain there.