Coral reefs are made up of polyps and algas. Packed into every available spot in polyp cells, algas give corals their brilliant colors. And they often exceed the polyps in weight, sometimes by as much as 3 to 1, making corals more plant than animal! The algas’ main function, however, is to photosynthesize organic compounds, 98 percent of which they give to their host as “rent.” Polyps need this nutrition not just to survive but also to build reef-forming limestone skeletons.
The algas benefit from the alliance in at least two ways. First, they get food in the form of the polyps’ waste products—carbon dioxide, nitrogen compounds, and phosphates. Second, they enjoy the protection of a tough skeleton. Algas also need sunlight; hence, coral reefs grow in clear, well illuminated water.
When coral is subjected to stress, such as a rise in water temperature, the polyps eject the algas and become bleached. Death by starvation may follow. In recent years scientists have observed an alarming increase in coral bleaching around the world.
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