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Common name(s): Sea anemones
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria. They are named for the anemone, a terrestrial flower.
Anthozoa often have large polyps that allow for digestion of larger prey and also lack a medusa stage.
As cnidarians, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra.
A sea anemone is a polyp attached at the bottom to the surface beneath it by an adhesive foot, called a basal disc, with a column-shaped body ending in an oral disc. Most are from 1.8 to 3 centimeters (0.71 to 1.2 in) in diameter, but anemones as small as 4 millimeters (0.16 in) or as large as nearly 2 meters (6.6 ft) are known. They can have anywhere from a few tens to a few hundred tentacles.
A few species are pelagic and are not attached to the bottom; instead they have a gas chamber within the pedal disc, allowing them to float upside down in the water.
The mouth, also the anus of the sea anemone, is in the middle of the oral disc surrounded by tentacles armed with many cnidocytes, which are cells that function as a defense and as a means to capture prey. Cnidocytes contain nematocyst, capsule-like organelles capable of everting, giving phylum Cnidaria its name. The cnidae that sting are called nematocysts. Each nematocyst contains a small vesicle filled with toxins (actinoporins), an inner filament, and an external sensory hair. A touch to the hair mechanically triggers a cell explosionâ€”which launches a harpoon-like structure that attaches to organisms that trigger it, and injects a dose of venom in the flesh of the aggressor or prey. This gives the anemone its characteristic sticky feeling. The sea anemone eats small fish and shrimp.
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. Actinoporins have been reported as highly toxic to fish and crustaceans, which are the natural prey of sea anemones. Anemonefish (clownfish), small banded fish in various colors, are not affected by their host anemone's sting and shelter themselves from predators among its tentacles. Most sea anemones do not present a serious risk to humans, but a few highly toxic species have caused severe injuries and are potentially lethal.
The internal anatomy of anemones is quite complex.
Unlike other cnidarians, anemones (and other anthozoans) entirely lack the free-swimming medusa stage of the life cycle; the polyp produces eggs and sperm, and the fertilized egg develops into a planula that develops directly into another polyp.
Anemones tend to stay in the same spot until conditions become unsuitable (prolonged dryness, for example), or a predator attacks them. In that case anemones can release themselves from the substrate and use flexing motions to swim to a new location. Most sea anemones attach temporarily to submerged objects; a few thrust themselves into the sand or live in burrows; a few are parasitic on other marine organisms and some have symbiotic relationships with hermit crabs.
The sexes in sea anemones are separate in some species, while other species, like the brooding anemone (Epiactis prolifera), are protandric hermaphrodites. The gonads are strips of tissue within the mesenteries. Both sexual and asexual reproduction can occur. In sexual reproduction males release sperm to stimulate females to release eggs, and fertilization occurs. Anemones eject eggs and sperm through the mouth. The fertilized egg develops into a planula, which settles and grows into a single polyp.
Anemones can also reproduce asexually, by budding, binary fission (the polyp separates into two halves), and pedal laceration, in which small pieces of the pedal disc break off and regenerate into small anemones
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