Ecology

Home to some two million plant and animal species, coral reefs are the most biologically diverse ecosystem in the ocean and the second most diverse in the world—only the tropical rainforest supports more plant and animal species.

There are almost a thousand different coral species, each with its own growth and reproduction pattern. Each also has its own unique style: Some resemble wrinkled brains and mushrooms, while others look like pillars, tabletops, moose antlers, wire strands, and cabbages.

Coral reefs provide a habitat for a wide range of sea creatures, including mollusks and urchins. Sea fans, anemones, and sponges fasten onto the coral. Small creatures find nourishment and protections amid the coral, and in turn attract large sea species like sea turtles, rays, and sharks.

The building block of the coral reef is the coral polyp, a tiny, soft creature that attaches itself to hard surfaces in shallow sea areas. Polyps can range from the size of a pinhead to that of a football. The polyps have slit-like “mouths” at the top surrounded by tentacles, which they use to sting and trap food—mainly plankton. Cells on the bottom of the polyps produce calcium carbonate, which builds islands and reefs. When a polyp dies, it hardens into “rock,” creating the reefs that we know.

Coral reefs grow slowly. While some reefs can grow as much as two feet per year, most grow only a few inches. The coral reefs that exist in the Virgin Islands have been growing for millions of years.

Small algae called zooxanthellae live symbiotically within coral polyps. The algae get shelter and food from the polyp, while the polyp gets food from the algae via photosynthesis. Photosynthesis requires sunlight, so coral reefs can only grow where the ocean is shallow and clear. Coral also requires ocean currents, which bring it plankton, the tiny organisms that sustain it, and warm ocean temperatures—between 75 and 85 degrees.

The polyps and algae are a food source for other sea creatures, and the reef’s caves and crevices are ideal locations for breeding and protection from large predators. More than one-quarter of all marinelife is found in coral reefs.

Life on the Reef

Coral reefs are cities under the sea. Even when the surface of the sea is glassy and there is not a sound except the wind, the underwater reef is teeming with life. It is a joy to float above and observe.

There are two main types of coral: hard coral and soft coral. Hard coral, the bricks and mortar of the coral reef, takes on a fantastic array of shapes and colors. Elkhorn coral looks like clusters of antlers reaching out sideways toward the surface of the ocean. Brain coral colonies are spheres imprinted with a pattern that makes them look remarkably like brains. Pillar corals grow like candelabras reaching to the sea surface, and staghorn coral looks like great colonies of orange starfish. Pay particular attention to fire coral, yellow stony coral that looks something like giant lichen. A brush against one of these can leave you with painful cuts.

Like hard coral, soft coral is made up of colonies of coral polyps. But unlike hard corals, soft corals have skeletons consisting of needles encased in softer, more flexible material. Sea fans are a common kind of soft coral. Others resemble whips and plumes.

A host of other sea creatures live among the coral. Sea anemones are soft, translucent tentacled creatures often seen anchored in cracks between the reef. The sharp, black barbs of sea urchins can easily be seen poking out from under and between rocks. There are many types of sea sponges—soft, multicelled animals that act as filters for the ocean. Crustaceans including lobsters and crabs are also found on the reef, often beneath a dark ledge. It takes a practiced eye to find these reclusive animals.

Fish, of course, are the star of the show on the coral reef. They dart around, nibbling, weaving in and out of the current, showing colors ranging from deep purple to bright yellow and every hue in between. One of the most common reef fish is the parrot fish, which comes in colors ranging from red to rainbow to black. Look closely and you will see its distinctive fused teeth, which it uses to scrape chunks of coral into its mouth. Using a mill at the back of its throat, the parrot fish grinds the coral into a powder, which is excreted as sand.

The yellowtail damselfish begins life a vivid shade of dark blue with bright blue spots before turning to a nondescript brown as an adult. Look for the feelers on the lower lip of the goatfish. One of the most common types of reef fish is the wrasse, a small fish often with bright stripes of color near its head. Triggerfish, one of many types of fish that can change colors based on its surroundings, are distinguished by their tails, which extend to two long points.

Some fish have very distinctive shapes. Trunkfish are easy to spot because of their triangular build. Long and thin, the trumpet fish eats by sucking unsuspecting fish into its vacuum-like mouth. You will often encounter them standing on their heads to mimic some part of the underwater landscape. Colorful angelfish are flat and can grow to be the size of a dinner plate. They swim elegantly around the reef and will surely catch your eye. Butterfly fish resemble angelfish but are distinguishable by a large black dot near their tails, meant to confuse predators.

Look out for squids, easily recognized by their large, white eyes, which look almost human. Most reef squid are small—about a foot long—and can be seen swimming around just like a fish. The octopus, on the other hand, often hides.

One of the greatest joys of snorkeling on the reef is seeing a school of fish. Near shore, you may find yourself engulfed in a mass of tiny fry. Pop your head above water and you will no doubt see pelicans diving for dinner. On the reef, schooling species include goatfish, grunts (named for a noise they make), and spadefish. You may also encounter schools of jacks and silversides.

You will also see larger sea creatures on the coral reef, attracted by the presence of so much potential food. Hawksbill turtles, one of the three kinds of sea turtle in the Virgin Islands, look for food and shelter around the reef. Green turtles prefer sea grass beds, and leatherbacks are most often seen in the open ocean.

© Susanna Henighan Potter from Moon Virgin Islands, 4th edition