In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, with zooxanthellae, or with green algae, zoochlorellae, that live within the cells.

Some species of sea anemone live in association with clownfish, hermit crabs, small fish, or other animals to their mutual benefit.

When the animal contracts, the oral disc, tentacles and capitulum fold inside the pharynx and are held in place by a strong sphincter muscle part way up the column.

The venom is a mix of toxins, including neurotoxins, that paralyzes the prey so the anemone can move it to the mouth for digestion inside the gastrovascular cavity.

However, Amphiprioninae (clownfish), small banded fish in various colours, are not affected by their host anemone's sting and shelter themselves from predators among its tentacles.

Most sea anemones are harmless to humans, but a few highly toxic species (notably Actinodendron arboreum, Phyllodiscus semoni and Stichodactyla spp.)

[5] Sea anemones have what can be described as an incomplete gut: the gastrovascular cavity functions as a stomach and possesses a single opening to the outside, which operates as both a mouth and anus.

They have stomach lining on both sides, separated by a thin layer of mesoglea, and include filaments of tissue specialised for secreting digestive enzymes.

Circular fibers are found in the body wall and, in some species, around the oral disc, allowing the animal to retract its tentacles into a protective sphincter.

The anemone stabilizes itself by flattening its pharynx, which acts as a valve, keeping the gastrovascular cavity at a constant volume and making it rigid.

When the longitudinal muscles relax, the pharynx opens and the cilia lining the siphonoglyphs beat, wafting water inwards and refilling the gastrovascular cavity.

The fertilized egg develops into a planula larva, which drifts for a while before sinking to the seabed and undergoing metamorphosis into a juvenile sea anemone.

[8] Sea anemones have great powers of regeneration and can reproduce asexually, by budding, fragmentation, or longitudinal or transverse binary fission.

[11] Transverse fission is less common, but occurs in Anthopleura stellula and Gonactinia prolifera, with a rudimentary band of tentacles appearing halfway up the column before it splits horizontally.

When eggs and sperm (gametes) are formed, they can produce zygotes derived from "selfing" (within the founding clone) or out-crossing, which then develop into swimming planula larvae.

[17] Although some species of sea anemone burrow in soft sediment, the majority are mainly sessile, attaching to a hard surface with their pedal disc, and tend to stay in the same spot for weeks or months at a time.

[20] Stomphia coccinea can swim by flexing its column, and the sea onion anemone inflates and casts itself loose, adopting a spherical shape and allowing itself to be rolled about by the waves and currents.

[21] The sea onion Paranthus rapiformis lives on subtidal mud flats and burrows into the sediment, holding itself in place by expanding its basal disc to form an anchor.

[23] Sea anemones are typically predators, ensnaring prey of suitable size that comes within reach of their tentacles and immobilizing it with the aid of their nematocysts.

The lips can stretch to aid in prey capture and can accommodate larger items such as crabs, dislodged molluscs and even small fish.

[24] One of these is Peachia quinquecapitata, the larvae of which develop inside the medusae of jellyfish, feeding on their gonads and other tissues, before being liberated into the sea as free-living juvenile anemones.

[25] 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.

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.

[27] Other animals that associate with sea anemones include cardinalfish (such as Banggai cardinalfish), juvenile threespot dascyllus,[28] incognito (or anemone) goby,[29] juvenile painted greenling,[30] various crabs (such as Inachus phalangium, Mithraculus cinctimanus and Neopetrolisthes), shrimp (such as certain Alpheus, Lebbeus, Periclimenes and Thor),[31] opossum shrimp (such as Heteromysis and Leptomysis),[32] and various marine snails.

The majority of species cling on to rocks, shells or submerged timber, often hiding in cracks or under seaweed, but some burrow into sand and mud, and a few are pelagic.

Those living in shallow-water coastal locations are affected directly by pollution and siltation, and indirectly by the effect these have on their photosynthetic symbionts and the prey on which they feed.

[41] Anemones are also a source of food for fisherman communities in the east coast of Sabah, Borneo,[42] as well as the Thousand Islands (as rambu-rambu)[43] in Southeast Asia, Taizhou, Zhejiang (as Shasuan).

[48] †= extinct Actiniaria (Sea anemones) Antipatharia Corallimorpharia Rugosa † Scleractinia Zoantharia Alcyonacea Helioporacea Pennatulacea Penicillaria Spirularia The relationships of higher-level taxa in Carlgren's[49] classification are re-interpreted as follows:[47]