Facts about Sea Anemones

Sea anemones are colorful polyps named after an equally colorful terrestrial flower, anemone. They are water-dwelling, filter feeding animals belonging to the phylum Cnidaria, class Anthozoa and order Actinaria. Sea anemones are found in sizes ranging from less than 1¼ cm (½ in) to nearly 2 m (6 ft) in diameter. They lack locomotive ability, hence spend most of their life attached to some rock. The only way they can change their habitat is by hitching a ride on a passing crab or they may just detach from the rock and drag themselves in the water.

Facts about Sea Anemones

Anatomy
A sea anemone shows typical anatomy of polyps with a small sac attached to the bottom by an adhesive foot. It has a column-shaped body which ends in an oral disc. There is a cavity at the center which acts as mouth as well as anus and is surrounded by numerous tentacles. These tentacles are equipped with many cnidocytes, the cells that function as protection mechanism as well as for capturing prey. The stinging elements in cnidocytes are called nematocysts. In each nematocyst there is a small vesicle filled with toxic compund - actinoporins, an inner filament and an external sensory hair. When some organism touches the hair cell explosion occurs, firing a harpoon-like structure which attaches to the organism and injects a small amount of poison. This gives the anemone its characteristic stingy feeling. The poison contains neurotoxins which help in capturing the prey by paralyzing it. It is then moved by the tentacles into the mouth for digestion, inside the gastrovascular cavity. The toxins also serve as a defense mechanism against potential predators when mixed with water.

The internal anatomy of anemones is comparatively simple with a gastrovascular cavity and a single opening on outside, which functions as mouth as well as anus. There is a non- central, primitive nervous system which coordinates the processes involved in maintaining homeostasis. It also co-ordinates the biochemical and physical responses to various stimuli. The muscles and nerves of anemones are simple as compared to other animals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis) have microfilaments which together form contractile fibers. These cannot be considered as true muscles as they are not freely suspended in the body cavity. Since skeleton is absent in anemone, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton. The stability for this hydrostatic skeleton is provided by the anemone shutting its mouth closed, which keeps the gastrovascular cavity at a fixed volume making it more firm.

Reproduction
There are separate sexes in sea anemones. Some species are also hermaphrodites. Reproduction can take place sexually or asexually. In sexual reproduction, male releases a sperm stimulating the female to release eggs; and then fertilization occurs. The eggs or sperms are released through the mouth. Asexual reproduction occurs by budding, binary fission (division in halves) or pedal laceration. Pedal laceration is a process in which pieces of the pedal disc break off to regenerate into small anemones.

Lifecycle
In other cnedarians larvae go through a free, swimming stage called medusa. This stage is absent in anemones. The fertilized egg directly develops into a planula which settles down somewhere and grows into a single anemone. Sea anemones establish symbiotic relationships with green algae, clownfish and hermit crabs. Anemone provides the algae a safe harbor and exposure to sunlight and in return receives oxygen and sugar, the byproducts of the algae's photosynthesis. They also form symbiotic relationships with clownfish and hermit crab by protecting them from predators and getting scraps of food in return.

More than 1,000 sea anemone species are found in the oceans and coastal reefs across the world. The largest and the most varied species are found in coastal tropical waters. Due to lack of skeleton most Actinaria do not form hard parts that can be distinguished as fossils, but a few fossils do exist. The oldest known fossil of sea anemone is Mackenzia, from the Middle Cambrian Burgess Shale of Canada.