Coral reefs often are described as “underwater gardens” or “rainforests”. Coral reefs are complex communities of plants and animals. Colonies of corals that grow next to and on top of each other form the basis for a fascinating community, providing food, shelter, and living spaces for many kinds of plants and animals. Coral polyps have the unique ability to grow in nutrient-poor water, thereby supporting fish and marine life in waters that otherwise might be barren.
Coral reefs can be made of hundreds of different species of coral. Coral consists of two types: “hard” with an outer skeleton of calcium carbonate and “soft” corals that embed bits of calcium carbonate inside their bodies. All coral, irrespective of shapes and sizes, consist of tiny polyps (a tiny animal that resembles a jellyfish upside down). Reefs are formed when hard coral colonies grow next to and on top of each other.
The hard or reef-building corals that generate coral reefs in Hawaii consist of several types of coral including branching, finger-like and table-like forms. Each of these forms is adapted to live in a different type of habitat, such as shallow or deeper waters to avoid wave action. Non-reef building corals live in very deep water, including Black Coral, Hawaii’s official state gem, below where the sun penetrates.
Corals are plankton-eating invertebrate animals (polyps) that often are mistaken for minerals because of their skeletal, rock-like structure and slow-growing, immobile nature. In hard corals, polyps sit in little cups that they build out of calcium carbonate. During the day polyps stay in their cups. At night, they emerge to catch plankton with their stinging tentacles. But plankton is only part of their diet. Each polyp has a special algae in their body (zooxanthellae). These one-cell plants use sunlight and carbon dioxide to produce oxygen (photosynthesis).
In addition, coral polyps reproduce, using sperm and eggs. Sperm from male polyps fertilize eggs within female polyps. Baby corals leave the polyps and float to surfaces to which they can attach and grow into new coral that are exact copies of each other. New corals may grow over dead ones into many generations that result in huge reefs.
Thus coral plays a unique dual role in the underwater ecosystem as both producers of food for herbivores and supporters of carnivores. Coral “producers” feed the herbivores that live around a reef and, in the process, supply herbivores to carnivores such as moray eels, monk seals, and sharks. Darting among the corals, however, also are many beautifully colored “omnivores,” fish that feed on both plants and animals. These omnivores include moorish idols, reef triggerfish and the raccoon butterflyfish.