The Coral Reef Corner

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Coral reefs are some of the most complex, yet poorly understood, ecosystems on Earth. Their existence appears to stem from the ocean’s desire to remind us of its formidable capacity for biological innovation. Built by colonial cnidarians and teeming with fish and invertebrates, these underwater paradises exist in a state of constant flux.

This article appeared in the Spring 2014 issue of Current Exchange Magazine.

So specialized are reef organisms to their particular ecological niche that merely a decade ago, it was nearly impossible for a person to recapitulate the delicate and awesome beauty of a reef in their own home. Today, things are different. A dedicated community of hobbyists has generated a vast amount of information about the care and maintenance of captive reef ecosystems, thanks in no small part to the advent of the Internet. I have been a reef enthusiast since I was 13 years old, and I grew up in online message boards such as Reef Central ( and in monthly reef meetings scattered across Long Island. The truth is, everyone who keeps a coral reef aquarium becomes a little bit of a fanatic so today, and in future issues of Current Exchange, I will attempt to explain why. Who knows—maybe you’ll find that you’re a bit of a fanatic yourself.

What are corals?

The first and perhaps most astonishing fact about corals is that they are metazoans, or animals. More specifically they are cnidarians, a phylum that also includes sea anemones and jellyfish. An easy way to think of corals is to imagine them as upside-down jellyfish. A mouth, set in a central disc, is surrounded by a ring of 6 or 8 tentacles. This simple organism is a coral polyp, and the corals that most people are familiar with are actually colonies composed of many hundreds or even thousands of polyps.

Soft corals can exist as free-living single polyps, or in colonies. This is a diverse and hardy group of corals, united in their distaste for CaCO3 secretion. Pictured: A few tiny green star polyps (Briarium) will rapidly grow into a “lawn” of colonial polyps, whereas the red mushroom polyp (Discosoma) will divide slowly to produce large individual polyps capable of relocation. A hermit crab (unidentified) examines the scene. Bonus fact: Red Discosoma corals were the source of dsRed (a form of RFP).

Many coral colonies also secrete a calcium carbonate “skeleton,” which eventually contributes to the overall structure and substrate of the reef. Many corals have a symbiotic relationship with photosynthetic dinoflagellates called Zooxanthellae, which are the source of the many astonishing colors of corals. Zooxanthellae live inside the clear bodies of coral polyps, and produce sugars to nourish the corals in exchange for a safe place to live. Coral bleaching, which occurs when a coral is severely stressed, occurs when Zooxanthellae “jump ship” and abandon the dying coral. Today, mass coral bleaching is occurring at an ever-accelerating pace due to human disruption of the ocean environment. It is sobering to imagine that, one day, corals may only live on in the aquaria of reef enthusiasts.

Types of Corals

It is important to limit the number of fish in a reef aquarium, as they produce a lot of waste and may snack on corals or other invertebrates. Pictured: Clownfish (Amphiprion ocellaris) are among the safest fish for a reef aquarium. These juvenile clowns are presently male, but in time the dominant fish will grow larger and adopt different coloration as it morphs into a female.

Hobbyists typically describe corals as belonging to one of three groups, based on polyp morphology and the presence of a calcium carbonate skeleton. Small polyp stony corals (or SPS) have intricate and often branching skeletons, bright colors, and tiny polyps. They are both the most immediately recognizable corals and the most difficult to grow successfully, requiring tremendous amounts of light and current and pristine water conditions to truly thrive.

Small polyp stony corals (SPS) secrete calcium carbonate skeletons with intricate branching patterns, and require large amounts of light and current in order to thrive. Pictured: Montipora and Acropora species, with red Gracilaria macroalgae in background.

Large polyp stony corals (LPS) tend to be easier to maintain, and their large polyps with long “sweeper tentacles” are able to catch a meal to supplement their diet. Care must be exercised in positioning these corals, as their long tentacles can sting and kill nearby corals overnight.

Large polyp stony corals (LPS) also secrete a CaCO3 skeleton, but their polyps are larger and extra tissue is apparent. LPS corals can supplement their diet with food captured via their long “sweeper tentacles.”. Pictured: Red Chalice coral, Echinophyllia sp.

Soft corals are by far the easiest to maintain in captivity. As their name suggests, these corals do not secrete a skeleton, and they can be found in colonies or as free-living polyps. Most reef hobbyists start out keeping soft corals, and then move on to their more finicky stony brethren.

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