Coral are found in every ocean in the world, although most people think of stunning tropical coral reefs full of marine life. Coral reefs, or colonies of coral polyps, create the largest biological structures on Earth. Although more than 6,000 species of coral are known, coral share some physical characteristics--tentacles that emerge from the top of a tube-shaped body. The life cycle of a coral reef is important to understand, as coral reefs are host to almost 25 percent of all marine life.
Coral reefs are made up of both living creatures and limestone skeletons. Adult coral, also known as polyps, whip their stinging tentacles through the water hoping to catch zooplankton. The tentacles pull the food toward the simple mouth atop the tube-shaped body. During the course of its life, the polyp produces calcium carbonate, which is secreted from its living tissue. The calcium carbonate builds up around the tube, creating an exoskeleton. When the coral dies, the exoskeleton remains, and new coral polyps attach to it, starting the process all over again. Many coral reefs are thousands of years old.
An adult polyp will develop eggs or sperm within its body. Depending on the lunar cycle and triggers of night and day, coral somehow know to release both at the same time, ensuring higher success for fertilization. Most coral species spawn in late spring, a few days after the full moon. Both eggs and sperm are released into the water, where fertilization takes place. When eggs and sperm come together, the fertilized eggs drift along until larvae emerge.
The larvae, known as planulae, swim toward the ocean bottom, using the current to drift as well. Planulae are a prime food source for many sea creatures, so of the millions that develop, many never grow to maturity. Planulae that survive attach themselves to something solid, whether an existing coral reef, a rock or sunken ship. Once attached, it only takes a few weeks for the planulae to develop into polyps.
Polyps naturally form colonies by a process known as splitting. This reproductive process simply requires a grown polyp to divide, creating an identical offspring. The new polyp usually stays attached to the parent, and will share the same characteristics. The new polyps then divide, creating third-generation polyps. Thousands of polyps can be created in a short amount of time. Thus, coral reefs grow by both sexual and asexual reproduction.
As the polyps die, the calcium carbonate casings remain, only to be built upon by new polyps. In a coral reef, just the topmost segment contains living, feeding coral polyps. Underneath is an empty calcium carbonate network. The coral reef structure is somewhat fragile, with everything from chemicals to fluctuating temperatures threatening both the polyps and the calcium carbonate structure.
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