Calliactis polypus
Encyclopedia
Calliactis polypus is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...

 in the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Hormathiidae
Hormathiidae
Hormathiidae is a family of sea anemones in the class Anthozoa.-Genera:Genera in this family include:* Actinauge Verrill, 1883* Adamsia Forbes, 1840* Allantactis Danielssen, 1890* Amphianthus R...

. It is usually found living on the surface of a sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....

 shell
Gastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 in which a hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

 is living.

Description

C. polypus can grow up to 8 cm (3 in) long. Several anemones can grow on one shell. The base is wide with wavy edges and pink striations and flares out over the shell surface. The column is wider at the base than further up and has pale brown and white patches and longitudinal striations. The oral disc has several whorls of long brownish translucent tentacles with paler bases surrounding the mouth.

Distribution

C. polypus is found in the Indo-Pacific
Indo-Pacific
The Indo-Pacific is a biogeographic region of the Earth's seas, comprising the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the seas connecting the two in the general area of Indonesia...

 area and the Red Sea
Red Sea
The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...

. It lives in the neritic zone
Neritic zone
The neritic zone, also called coastal waters, the coastal ocean or the sublittoral zone, is the part of the ocean extending from the low tide mark to the edge of the continental shelf, with a relatively shallow depth extending to about 200 meters...

 at depths down to about 25 metres (80 ft).

Ecology

C. polypus lives as a commensal with several species of hermit crab including Dardanus gemmatus
Dardanus gemmatus
Dardanus gemmatus, the jeweled anemone hermit crab, is a species of hermit crab native to tropical reefs surrounding the Indo-Pacific, typically at depths of ....

. The anemone shows no inclination to attach itself to a gastropod shell
Gastropod shell
The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

with or without a hermit crab. However the crab shows great interest in the anemone and taps and massages the base of the column with its legs until the anemone relaxes and the pedal disc becomes detached. The crab then picks up the anemone and holds it against the shell in which it is living. Both the pedal disc and the tentacles are very sticky and either can quickly form a firm attachment to the shell.
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