Each polyp is between 0.5 mm to 10 mm in size. Thousands of polyps make up a colony. The body structure of a polyp consists of a mouth and a saclike cavity called the coelenteron. The coral polyp's skeleton is made of calcium carbonate. Calcium carbonate is actually what the polyp secretes. The skeleton serves as a home for the coral polyp and protects it from harm. When the coral polyp is threatened, it simply retracts into the skeleton. The polyps are carnivorous and feed on zooplankton by extending their tentacles, equipped with stinging cells called nematocysts, to capture the floating prey.
Coral polyps reproduce sexually and asexually. The reproduction of corals happens at very precise times during the year. Sexual reproduction occurs when the polyps produce planktonic larvae, which disperses and settles, establishing a new colony. Polyps can also reproduce asexually through budding or fragmentation. The coral polyp in budding simply replicates itself and divides. Fragmentation occurs when a broken piece of coral is able to regenerate if the coral lands on a suitable substrate.
Coral polyps supply housing and protection for zooxanthellae, a single-celled plant, which lives in the polyp's tissue. Coral also provides the chemical compounds needed by this algae for photosynthesis. The polyps, in return receive the "food" produced by the zooxanthellae.
This unusual symbiotic partnership gives corals a competitive edge in the nutrient poor waters that they inhabit. By being essentially able to live off of captured prey and sunlight they can survive when one food source is limited. They also, provide oxygen to the polyps and remove any excess waste the polyps produce. Through the exchange of nutrients, the zooxanthellae give the polyps their color, thus giving color to the reef.
Corals are generally classified as soft non-reef-building or hard, based on differences in polyp and skeleton structure. Soft corals, such as sea fans, appear to be colorful underwater plants, bending and swaying with the ocean waves. Soft corals consist of polyps that are 'housed' individually. The outer crust contains spinicules of calcium carbonate, while the inner core consists of gorgonin; a flexible, fibrous wood-like protein. This gives the soft coral colony the ability to flex with the ocean waves and currents. A soft coral colony has the growth potential of 2 to 4 cm per year.
Unlike soft corals, the polyps of hard corals are interconnected. All digestive-circulatory and gastrovascular cavities are linked to each of the colony members. This system of 'sharing' the food intake and digestion duties with the colony provides some resilience to starvation for individual colony members. Hard coral growth rates vary depending on the different species from less than 1 cm to 10 cm a year. Growth rates tend to increase in shallower waters, but growth has been found to actually slow down as the size and age of a colony increases.
Corals are very fragile and are susceptible to various diseases. Two of the most common diseases affecting coral colonies are Black and White Band Disease.
Black band and white band disease kill the polyp's tissue leaving a 'band' or ring of the skeleton behind. The ring makes room for invasions of algae, invertebrates, and other various organisms. Scientists believe this is not beneficial to the reef because the 'replacements' resilience towards storms is lower than that of a healthy colony.
White band disease ...
The bacterial pathogens that cause black band disease are believed to spread colony to colony by currents. Unlike white band, black band disease has been linked to human activities. Higher temperatures also increase the risk of black band disease.
Heavy traffic on the reef as a result of tourism and commerce has placed the health of North America's only living reef in jeopardy. South Florida's coral reefs have had a hard time competing with boat groundings, uneducated divers, hurricanes, pollutants, global warming, and algal blooms. Boaters create severe damage, when groundings occur. Not only does the boat scraping the bottom destroy the reef, but the leakage and dispersal of gasoline and oil is very hazardous to the reef. A boat's anchor, a more common occurrence, breaks the coral and destroys entire colonies. Due to the increase in diving activity, uneducated divers can cause just as much damage. Divers need to be aware that walking or standing on coral kills the polyps. Even touching the coral kills the polyps. The oils in our hands coats the polyps, thus suffocating them. Fishermen and commercial collectors use various harmful methods in an attempt to increase their yields.
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