A coral reef is an ecosystem, not a single plant or animal, and it has as its members both plants and animals. Unlike most ecosystems the plants are almost invisible, being mostly single celled and symbioticalgae, living inside the coral polyps. Coral polyps look like flowers but they are in fact animals belonging to the invertebrate phylum Cnidaria.
Coral reefs occur in shallow tropical waters which are poor in nutrients. Too much nutrient runoff from land can actually kill a coral reef. The reefs are formed by the little polyps of the hard corals that can build the calcium carbonate structures that are the basis of the reef ecosystem. These small animals begin life in the plankton and are related to jellyfish and sea anenomes. Eventually they sink to the bottom and make the little limestone houses that will eventually become an entire apartment blocks containing thousands of individual polyps in a variety of shapes: brain corals, staghorn corals and plate corals among others. Around these structures live all the fish and other animals that form the coral reef ecosystem.
So what do these coral animals live on if there is little food in the water? Like jellyfish and anenomes, coral polyps can filter feed but there are not enough particles in the water to sustain them. Instead they depend on their relationship with single celled algae called Zooxanthellae. These algae gain protection from herbivorese by living safe within the polyps’ calcium-encased bodies and in return they make sugars by photosynthesis, which nourish their hosts as well as themselves. This is why coral reefs are located in shallow waters where the sun can penetrate and provide the energy that the algae need to fulfill their role.
In this way, a coral reef is as dependent on its plants as any grassland or forest. It is just that they are almost invisible. The only hint of their microscopic presence is their wonderful variety of colours which turn the reef into a living rainbow. Other filter feeders such as clams also owe their survival and their brilliant colours to the presence of these microscopic algae.
The corals are preyed upon by many species of fish. Some of the most beautiful are the aptly named parrot fish, which come in bright colours and have a sharp beak for tearing apart the coral apartments to get at the tasty, sugar-filled polyps inside. Parrot fish are an important part of the reef, turning coral into sand that eventually can create a coral island on which sea birds and sea turtles can breed and where land plants and other organisms can take hold to create a new ecosystem.
Coral reef ecosystems are as rich and diverse as tropical rainforests. They abound in thousands of species of corals, anenomes, jellyfish, sea cucumbers, starfish, fish, sharks, sea turtles, sea snakes, whales, dolphins and other creatures. A typical reef can be home to 500 to 1500 species of fish alone. So it would appear that the reef is basically animal in nature. But underlying this incredible animal diversity are the humble zooxanthellae, the plants that make it all possible.
