Final answer:
Coral reefs are characterized by their high biodiversity, dependence on a mutualistic relationship with symbiotic algae in shallow waters, and their support for a vast number of fish species. They are formed by the accumulation of calcium carbonate skeletons from saltwater polyps and are crucial for coastal protection and marine life sustainability.
Step-by-step explanation:
A characteristic of coral reefs is their high biodiversity. These ecosystems are built by colonies of saltwater polyps, members of phylum Cnidaria, which secrete a calcium carbonate skeleton. Over time, these skeletons accumulate to form the reef structures. Coral reefs thrive in warm, shallow waters within the photic zone, predominantly found around 30 degrees north and south of the equator, such as the Great Barrier Reef near Australia.
In shallow waters, corals exist symbiotically with photosynthetic unicellular algae, which provide them with most of their nutritional requirements through photosynthesis. This relationship is crucial in nutritionally poor waters, enabling the large growth of corals. Conversely, some corals in deeper and colder waters do not engage in this symbiosis; instead, they capture prey with their stinging cells. Coral reefs are not just important for their biodiversity; they also protect coastlines from erosion, offer food and shelter to a multitude of marine organisms, and support around 4,000 fish species. However, these vital ecosystems are in global decline due to their sensitive nature to temperature changes and human activities.