As we learned, healthy coral reefs are full of many different species of marine life. They’re like the “cities of the seas,” in part because corals create stony structures that offer shelter and attachment points for things like small fish and invertebrates.
Can you think of other, non-marine habitats that are also very biodiverse? What do these habitats have in common?
The Coral Reef Habitat
Forming the southeastern border of Florida Bay and Biscayne Bay, is a chain of islands we call the Florida Keys. The Keys have attracted thousands of people looking for their own slice of paradise. Today, houses and hotels sit on the ancient graveyard of what was once, a massive barrier reef.
During the Pleistocene epoch, when the world’s oceans were much higher, the Florida Keys were thriving submarine habitats. Once living corals were the architects of the rocky islands that were left behind when the waters receded.
Just past the barrier Key islands, is a younger, submerged, living coral reef system. This is the largest living barrier reef in the continental US, and the third largest in the world.
Corals are actually animals… marine invertebrates in the phylum Cnidaria, the same phylum that includes jellyfish and anemones. In some places gorgonians and soft corals decorate the seafloor like a colorful garden. But it’s the hard corals, like this brain coral and this branching elkhorn coral that are the true architects of the oceans.
A hard coral is a community of individual coral organisms called polyps. At night their tentacles reach out for food. As a polyp grows, it secretes a stony cup of calcium carbonate around itself for protection. Over time, a colony of coral polyps may keep reproducing to construct a massive structure that becomes a haven for other marine life.
The polyps of hard corals grow best in the warm, clear, shallow and unpolluted waters found in the Earth’s tropical belt. They need clear water, because the polyps act as hosts to a special kind of photosynthetic organism called zooxanthellae that lives within their tissues. Zooxanthellae creates food energy through photosynthesis, and much of that food energy is passed on to their host, the coral polyp. Corals need that extra nutrition, so this is a fantastic example of symbiotic mutualism. The zooxanthellae get a home, and in return, the polyp gets food energy.
Coral reefs cover about 100,000 square miles of Earth’s marine environment, or only about one tenth of one percent, but, along with tropical rainforests, reefs are among the most diverse and complex habitats in the world.
The hard coral architecture of the reef is one of the main reasons for its incredible biodiversity. Within its intricate shapes are nooks and crannies where crustaceans such as lobsters and crabs find safety in small caves. The goatfish probes the sandy areas along the edge of the reef. It has barbels under its mouth for flicking sand as it searches for tidbits. The shelter of the reef is close by, if necessary.
Sea turtles visit the reef and search for jellyfish – even at night. Night is also a good time to spot schools of big-eyed reef squid cruising for shrimp. The nocturnal squirrel fish also has large eyes for night feeding. The reef habitat never sleeps.
The corals also offer attachment places for sponges, soft corals, anemones and other invertebrates. Worms, like Christmas Tree worms, burrow right into the coral rock. More species and more total biomass is squeezed into one square foot of coral reef than any other marine habitat.
The reef is a dynamic place where many factors are in balance. Parrotfish browse off the algae that tend to grow over the living polyps, cleaning the rock where new coral colonies might become established. When the parrotfish graze, they may sometimes bite out chunks of living coral as well. Turned into coral sand within the fish, the sand streams out as the parrotfish swims along. Predators like the green moray, the nurse shark and the reef shark are on hand to make sure the grazers don’t get overpopulated.
Predators are actually incredibly important to the whole balance of the food web. When you see sharks on a reef, that’s a good sign. Unfortunately, sharks are disappearing quickly. The culprit, us. Humans have been slaughtering them specifically for their fins. Shark fin soup is a delicacy in China, and as many as 100 million sharks are killed each year to satisfy this demand.
But where sharks are protected, where they are thriving, we often find thriving coral reefs. Nowhere is this more obvious than Cuba’s Jardines de la Reina, The Gardens of the Queen. This remote archipelago is home to some of the last, truly healthy coral reefs in the Caribbean, and the sharks are everywhere. The entire ecosystem is a colorful explosion of healthy corals, schooling fish, and lots and lots of Caribbean reef sharks.
The greatest dangers facing corals today come from human impacts. Corals are delicate creatures. They are easily damaged by reckless divers, dragging boat anchors, when the water is polluted by human communities or when water temperature rises. Global warming is a big killer. One of the early signs of coral in decline is a disease known as coral bleaching. When conditions are bad, coral polyps may expel the colorful zooxanthellae living within their tissues. When that happens, they often can’t get enough nutrition to survive. The increased transparency of the polyps makes the entire colony look like it’s been dipped in bleach. In South Florida, this is becoming an increasingly common event, especially during the hottest part of the summer. Scientists estimate that the Florida Keys have lost half their coral cover in the past 200 years. Bleaching is a warning sign that we are not doing enough to protect our amazing coral communities. We need to get to work to save our incredible coral reefs.