Sunlight: The Start of the Food Web for Coral Reefs

The shallow sunlit waters provide the perfect conditions for plant growth. The algae provide a rich and solid foundation to the food web on the reef, supporting a wide range of herbivores. But one of the most important groups of organisms on the reef are the specialized algae known as zooxanthellae that live in a symbiotic partnership with the corals and help to maintain the health of the reef itself.

How does the partnership between algae and coral work?

In the simplest terms, the corals provide a home for the zooxanthellae within their tissues. Furthermore, the waste products of the coral polyps also feed the tiny algae. In this secure and well-supplied environment, the zooxanthellae prosper and huge numbers of these microscopic plants are packed into coral tissue, helping to determine the color of the overall coral. In return for their accommodation, the zooxanthellae supply the coral with oxygen and sugars as they photosynthesize.

The relationship between these organisms is a long-standing one but not unique — similar relationships exist between various symbiotic algae and animals such as jellyfish, giant clams, and sponges. During periods of the year when sunlight is especially bright, the corals even protect their algal guests with screening pigments. However, if conditions become especially harsh for long periods of time, the stressed corals expel the algae. This phenomenon is known as coral bleaching and very often corals do not recover from it.

How else do corals feed?

The contribution made by the zooxanthellae to the coral's overall nutritional requirements varies between coral species, but in some cases, the algae may provide up to 75% of the coral's needs. However, the coral polyps are capable of feeding themselves. Each polyp in a colony can function like a miniature anemone, trapping drifting zooplankton in the water with poisonous stinging cells resembling miniature harpoons or, in some cases, with sticky mucus.Then they retract their arms and pass the prey into their mouths. Most of this feeding is done at night, when the zooplankton are most abundant and the coral predators — diurnal fish such as butterflyfish — are gone from the reef. With this, and the daytime photosynthesis of the algae, the coral polyps have a round-the-clock feeding system.

Are other kinds of algae found on the reef?

Algae forms the foundation of the coral reef food web. These tiny plants are present as free-floating phytoplankton in the water column and as the encrusting algae that grows over the reef itself. Phytoplankton are one of the major foods of the zooplankton and, ultimately, the larvae of reef fishes and invertebrates. The encrusting algae form a thin covering over the sunlit structures of the reef, where they are extensively grazed by a host of different herbivores, from urchins to tangs. The algal covering, sometimes called an algal turf, may be composed of one or a combination of different types of algae, including red. green, brown, and blue-green — and is itself home to communities of tiny invertebrates and bacteria.