Gaze into the rock pool at low tide and the gently waving tentacles of the beautifully colored sea anemones will make it immediately apparent why these animals were once called ‘plant-animals’. Anemones are not flowers, of course, and in fact they are greedy predators, catching and eating fishes, worms, crabs and any other creatures that touch their waving arms.
Like the other members of the coelenterates, the tentacles of anemones bear stinging cells and their body plan is similar to that of the freshwater hydra.
Anemones are more muscular, however, a have several rings of tentacles around the mouth. The stomach is divided by partitions to increase its surface area so that larger prey can be digested.
Anemones may look fairly static in the rock pool but if you could watch for long enough you would see that they are continually on the move, contracting and extending the body and slowly gliding about.
What is coral?
Coral, which people often collect as decoration, is the hard, dried skeleton composed mainly of calcium produced by colonies of small animals very similar to anemones. The polyps of living coral live in tiny cups in the skeleton and are continually adding to its mass.
The coral grows into a variety of shapes. Some species produce numerous branches and in others the polyps are arranged in twisting rows on a rounded mass.
Coral thrives in warm, shallow water and it forms extensive reefs along tropical shores, particularly in the Pacific and Indian Oceans.
The protection the reefs provide make them ideal habitats and they support crowded communities of fishes, many kinds of invertebrates and seaweeds.
How are coral reefs formed?
There are three types of coral reef: fringing reefs grow close to the shore in shallow water; barrier reefs grow parallel to the shore but are separated from it by a deep channel which may be several miles wide; and atolls which are circular islands of coral enclosing a lagoon, often hundreds of miles from any other land.
There are numerous theories to explain the formation of these types of reefs. The most popular is that one type developed from another in a gradual transition so that all three are different stages of the same process.
If a landmass with a fringing reef begins to sink into the sea, as long as the coral can grow at an equivalent rate to the subsidence, the reef will grow further and further from the coast.
Eventually a barrier reef is formed some way from the shore. If the process continues and the landmass disappears altogether, a ring of coral will be left forming an atoll. This is the simplest explanation for the formation of reefs; the actual processes involved are more complex.