page_title: Sea anemone
section_title: Mutualistic relationships
hierarchical_section_title: Sea anemone / Behaviour and ecology / Mutualistic relationships
caption_attribution_description: Español: Amphiprion ocellaris, en Heteractis crispa
context_page_description: Sea anemones are a group of marine, predatory animals of the order Actiniaria. They are named after the anemone, a terrestrial flowering plant, because of the colourful appearance of many. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Hexacorallia. As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra. Unlike jellyfish, sea anemones do not have a medusa stage in their life cycle.
  A typical sea anemone is a single polyp attached to a hard surface by its base, but some species live in soft sediment and a few float near the surface of the water. The polyp has a columnar trunk topped by an oral disc with a ring of tentacles and a central mouth. The tentacles can be retracted inside the body cavity or expanded to catch passing prey. They are armed with cnidocytes. In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, zooxanthellae or with green algae, zoochlorellae, that live within the cells. Some species of sea anemone live in association with hermit crabs, small fish or other animals to their mutual benefit.
context_section_description: Although not plants and therefore incapable of photosynthesis themselves, many sea anemones form an important facultative mutualistic relationship with certain single-celled algae species that reside in the animals' gastrodermal cells, especially in the tentacles and oral disc. These algae may be either zooxanthellae, zoochlorellae or both. The sea anemone benefits from the products of the algae's photosynthesis, namely oxygen and food in the form of glycerol, glucose and alanine; the algae in turn are assured a reliable exposure to sunlight and protection from micro-feeders, which the sea anemones actively maintain. The algae also benefit by being protected by the sea anemone's stinging cells, reducing the likelihood of being eaten by herbivores. In the aggregating anemone (Anthopleura elegantissima), the colour of the anemone is largely dependent on the proportions and identities of the zooxanthellae and zoochlorellae present. The hidden anemone (Lebrunia coralligens) has a whorl of seaweed-like pseudotentacles, rich in zooxanthellae, and an inner whorl of tentacles. A daily rhythm sees the pseudotentacles spread widely in the daytime for photosynthesis, but they are retracted at night, at which time the tentacles expand to search for prey.
  Several species of fish and invertebrates live in symbiotic or mutualistic relationships with sea anemones, most famously the clownfish.  The symbiont receives the protection from predators provided by the anemone's stinging cells, and the anemone utilises the nutrients present in its faeces. Other animals that associate with sea anemones include cardinalfish (such as Banggai cardinalfish), juvenile threespot dascyllus, incognito (or anemone) goby, juvenile painted greenling, various crabs (such as Inachus phalangium, Mithraculus cinctimanus and Neopetrolisthes), shrimp (such as certain Alpheus, Lebbeus, Periclimenes and Thor), opossum shrimp (such as Heteromysis and Leptomysis), and various marine snails.
  Two of the more unusual relationships are those between certain anemones (such as Adamsia, Calliactis and Neoaiptasia) and hermit crabs or snails, and Bundeopsis or Triactis anemones and Lybia boxing crabs. In the former, the anemones live on the shell of the hermit crab or snail. In the latter, the small anemones are carried in the claws of the boxing crab.
caption: Ocellaris clownfish among the tentacles of a sebae anemone
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