anemones - corals - sea fans - sea pens
There are no medusal stages
in this class which contains the corals, sea pens, sea
fans and the sea anemones. There are around 6 500 recorded species worldwide, and most can be found in soft sediments. The size range for individuals is 1 mm - 10 m, though many seem much larger because they live in communities.
Anemones (see right) range considerably in size, with the largest
being about ten centimetres in diameter and about twenty centimetres long. They
attach to the substratum by a pedal disc. They get their common name because of the flower-like appearance of the expanded oral disc. Although they are found worldwide they are more abundant in warmer waters. In the U. K. there are over 40 different species, usually found attached to rocks or weeds.
Some form mutualistic relationships
with other animals, e.g. the hermit crab, where they are attached to the shell
of the crab, but most live as solitary polyps.
Urticina felicina, the dahlia anemone, right, is found in waters below 22 oC, on the lower shore areas with strong wave action, although it can be found as deep as 200 m. Its tentacles are short and stubby, and usually arranged in multiples of 10. A fully mature species can have up to 160. With its tentacles it catches shrimps, small fish, crabs, muscles, in fact anything it can. It can be up to 15 cm in diameter at the base, and 20 cm across the tentacles. Its colour is variable. It can form dense carpets.
The sea anemone Stichodactyla helianthus, found in shallow Caribbean waters, has a compound in its toxin that is being trialled as a treatment for multiple sclerosis.