Sea anemones are a group of water dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. As cnidarians, sea anemones are closely related to corals, jellyfish, tube-dwelling anemones and Hydra.
Anatomy
"A sea anemone is a small sac, 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. The mouth 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 cnidae, capsule-like organelles capable of everting, giving phylum Cnidaria its name [1]. 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. When the hair is touched, it mechanically triggers the cell explosion, a harpoon-like structure which attaches to organisms that trigger it, and injects a dose of poison in the flesh of the aggressor or prey. This gives the anemone its characteristic sticky feeling.
The poison is a mix of toxins, including neurotoxins, which paralyze the prey, which is then moved by the tentacles to the mouth/anus for digestion inside the gastrovascular cavity. Actinoporins have been reported as highly toxic to fish and crustaceans, which may be the natural prey of sea anemones. In addition to their role in predation, it has been suggested that actinoporins could act, when released in water, as repellents against potential predators. Certain clownfish are not affected by their host anemone's sting.[2]
Exploitation
The global trade in marine ornamentals is an expanding industry. In the early 1980s the estimated value of imported marine fish and invertebrates was US $24-40 million annually.[3] Current estimates place that value at US $200-330 million,[4] with the USA accounting for 80% of the industry imports.
The sea anemone has a foot which in most species attaches itself to rocks or anchors in the sand. Others also burrow into a stronger object. Some species attach to kelp and others are free-swimming.
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.
A few anemones are parasitic to marine organisms. Anemones tend to stay in the same spot until conditions become unsuitable (prolonged dryness, for example), or a predator is attacking them. In the case of an attack, anemones can release themselves from the substrate and swim away to a new location using flexing motions.."
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