A reef is a rock formation in the sea that approaches the surface of the water, and a coral is a tiny animal that spends its adult life anchored in place and often grows in colonies. The adult animal secretes calcium carbonate (limestone) to form its skeleton and anchor. This skeleton remains behind after the animal dies and is also called coral.
Not all corals form colonies, and those that do are found only in warmer waters, where the water temperature remains at or slightly above 70 degrees F (21 degrees C) or higher all year. In addition, they grow predominantly in shallow waters, where sunlight can easily penetrate.
> Types of Coral Reefs <
There are three types of coral reef, categorized by location and growth characteristics.
* Barrier Reef
The barrier reef runs parallel to the coastline, separated from it by a shallow lagoon.
* Fringing Reef
A fringing reef also follows the coastline, but extends up onto the beach in some spots.
* Atoll
The third kind of coral reef, the atoll, is not associated with coastlines. An atoll is doughnut-shaped, with a calm lagoon at the center, and often forms around extinct underwater volcanoes. Atolls probably started as fringing reefs around volcanic islands. After the volcano went dormant, the island slowly started sinking, eventually disappearing completely and leaving the reef behind as the only evidence of its existence.
> Coral Reproduction and Reef Formation <
A coral may look like a plant, but it is actually an animal, whose adult stage is completely non-motile. The adult coral animal is called a polyp, and it can reproduce either sexually or asexually.
*Sexual reproduction
Sexual reproduction of corals typically involves the release of eggs and sperm into the water, where the sperm fertilize the eggs. A few species fertilize the eggs within the polyp, but this is rare.
Fertilized eggs become motile larvae called planulae, which eventually settle onto a hard, stable surface. They may settle back onto the coral reef from which they came, or they may float away to settle onto a rock or the ocean bottom and start a new reef.
Once the planulae has found a solid surface, it begins its transformation into a polyp. The coral secretes calcium carbonate to build its skeleton and anchor itself in place. Finally, the polyp develops a stomach and a mouth surrounded by tentacles that draw food to it. The tentacles contain specialized stinging cells called nematocysts that paralyze the coral’s prey.
*Asexual reproduction
Coral asexual reproduction is called budding. In this process, polyps grow buds that eventually turn into new polyps and grow their own buds. These buds usually remain attached to the original polyp, but may also break off and become a new animal. The new animal may either settle onto its original reef or may be washed away to start a new colony. This continuous budding and skeleton building creates huge colonies that are all interconnected by their skeletons.
Both types of reproduction occur in coral reefs and serve to enlarge the coral colony and, by extension, the reef. A reef contains many different species of corals, each with their own growth rates. Overall, however, coral reefs grow at most 10 cm per year, and it takes tens of thousands of years for a reef to fully form.
In addition to the corals, which are the biggest contributors to the formation of coral reefs, some mollusks and algae also secrete calcium carbonate. A particular type of algae, the coralline algae, acts as a glue to bind together the various individual colonies that make up a coral reef.
The vast biodiversity of a coral reef may not contribute directly to reef formation, but the balance between all the inhabitants contributes to the health of the entire ecosystem, allowing for the continued growth and beauty of these magnificent formations.
