Fish Keeping 101

Aquarium Talk: Coral in the Marine Aquarium

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Many a marine aquarist desires to keep beautifully colored, exotic corals in his saltwater aquarium, but doing so requires an understanding of the animal's biology.

What is the most important discovery of the twentieth century about coral reefs? While some may argue the correct answer, Svein Fossa and Alf Jacob Nilsen, authors of The Modern Coral Reef Aquarium, clearly believe it is the discovery of the symbiotic relationship between zooxanthellae and coral. Understanding this symbiotic relationship, even if only at the fundamental level, is important for the marine aquarist who wishes to keep corals.

Corals Are Not Colorful; Algae Are!

While aquarists are often drawn to the colorful array of corals, in truth, it is not usually the coral itself that is colorful. The bold fluorescent colors, especially under actinic lights, actually are due to algae that live in the tisues of the coral. These algae-known as zooxanthellae-are endosymbionts. Endosymbionts, simply stated, are organisms that live within the cells of another organism (derived from the Greek "endo" or "inner," "sym" or "together," and "biosis" or "living"). Therefore, it is the zooaxanthellae living in the endoderm of many corals that gives them their appealing coloration.

Essential Algae

"But aren't algae in the aquarium bad," the aquarist might ask. That depends. There are many, many different kinds of algae. Some algal species are clearly a nuisance, but others, like zooaxanthellae, are essential to the biology of both wild and captive colonies of coral. Providing an atmosphere wherein these algae can thrive is critical to the coral's health.


Why are zooaxanthellae so essential? The answer to this question is quite complicated, but it has to do with calcium carbonate synthesis. Any marine aquarist who has run a calcium reactor knows that the reactor chamber is filled with calcium carbonate media. The calcium carbonate media dissolves in the reactor as a result of running CO2 into the reactor chamber thereby lowering the pH of the water. The effluent produced provides the system with calcium and carbonate, which is essential for calcification. In short, a calcium reactor promotes the growth of coral in the reef aquarium, and a very similar process happens naturally within the coral itself. The zooaxanthellae take CO2 from the calcium bicarbonate and create calcium carbonate that provides for the calcification of the host animal. In addition, the primary benefit afforded by the zooxanthellae is the energy the provide to the coral; they provide as much as 90-percent of the coralís nutritional requirements.

Bright Lights a Must for Many

This is why providing appropriate lighting for coral really can make the difference between life and death. If the light is not of sufficient intensity (and within the right spectrum) to permeate the host coral's tissue and reach the zooaxanthellae, then photosynthesis will not take place. If photosynthesis does not take place, than carbonate synthesis will not take place, and, eventually, the coral will die. Note that corals do periodically discard dead or dying zooaxanthellae, and this should not alarm the aquarist. This happens when the algae cells have divided and the coral expels the older cells. The aquarist will recognize this occurring when he or she sees a long stringy mass of brown zooaxanthellae coming from the coral's mouth.


Not all corals function in this manner, but the beginning aquarist should always be sure to research a given coral's light and temperature requirements before purchasing the animal. Nearly all stony corals have dinoflagellate symbionts. In many cases, corals will not thrive (or even survive) without quality aquarium lighting such as metal halides. If the aquarist plans to keep corals that host zooaxanthellae, he or she will need to make the sizeable investment in a lighting system that can support these remarkable animals. Doing so, however, will allow the aquarist a rare glimpse into one of the more amazing processes of a coral reef's biology.
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