Link : Anemones of the Eastern Cape
Sea anemones are a group of water dwelling, 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.
A sea anemone is a polyp, attached at the bottom to the surface beneath it by an adhesive foot, called a pedal disk, with a column shaped body ending in an oral disk. The mouth is in the middle of the oral disk, surrounded by tentacles armed with many cnidocytes, which are cells that function as a defense and as a means to capture prey.
Anemone 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. 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. Anemonefish, small banded fish in various colors, are not affected by their host anemone's sting and shelter from predators within its tentacles.
The internal anatomy of anemones is simple. There is a gastrovascular cavity (which functions as a stomach) with a single opening to the outside which functions as both a mouth and an anus: waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut. A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis as well as biochemical and physical responses to various stimuli.
Anemones range in size from less than 1 cm to nearly 2 m in diameter. They can have a range of ten tentacles to hundreds.
The muscles and nerves in anemones are much simpler than those of other animals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis) have microfilaments that group into contractile fibers. These fibers are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Since the anemone lacks a skeleton, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton.
The anemone stabilizes itself by shutting its mouth, which keeps the gastrovascular cavity at a constant volume, making it more rigid.