These fossil reefs are prime locations to look for fossils of many different types, besides the corals themselves.Solitary corals: Corals are not restricted to just reefs, many solitary corals may be found in rocks where reefs are not present (such as Cyclocyathus which occurs in the Cretaceous period Gault clay formation of England).Reefs from both the Silurian and Carboniferous periods have been recorded as far north as Siberia, and as far south as Australia. However, these ancient reefs are not composed entirely of corals.Algae and sponges, as well as the fossilized remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites that lived on the reefs help to build them.Great structures in the midst of sedimentary rocks: At certain times in the geological past corals were very abundant, just as modern corals are in the warm clear tropical waters of certain parts of the world today.And like modern corals their fossil ancestors built reefs beneath the ancient seas.Usually growing close to the surface: Although sea anemones can catch fish and other prey items and corals can catch plankton, they obtain much of their nutrient requirement from symbiotic unicellular dinoflagellates (type of algae) called zooxanthellae.Consequently, they are dependent upon growing in sunlight and for that reason usually found not far beneath the surface, although in clear waters corals can grow at depths of 60 m (200 ft).
This has increased the importance of coral biology as a subject of study.
In an early symptom of environmental stress, corals expel their zooxanthellae; without their symbiotic unicellular algae, coral tissues then become colorless as they reveal the white of their calcium carbonate skeletons, an event known as coral bleaching.
Coral reef in danger: Scientists are predicting that over 50% of the coral reefs in the world may be destroyed by the year 2030. Many governments now prohibit removal of coral from reefs to prevent damage by divers taking pieces of coral.
The colony of polyps function essentially as a single organism by sharing nutrients via a well developed gastrovascular network, and the polyps are clones, each having the same genetic structure.
Each polyp generation grows on the skeletal remains of previous generations, forming a structure that has a shape characteristic of the species, but subject to environmental influences.
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In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a kind of incremental dating, which combined with geochemical analysis of each band, can provide high-resolution records of paleoclimatic and paleoenvironmental change.