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Marine life on the reefs
WBN09 - Sports - Dive
Marine life on the reefs
Night dives reveal hidden beauty
The fabled clear seas of The Bahamas are as much an illusion as they are a legend. Each gallon of water, apparently pure and lifeless, actually contains up to 2,000,000 invisible plants and animals—a rich nutrient soup on which all offshore life depends.
Coral reefs survive by filtering out this abundance. Anyone who has ever dived at night will know the transformation that comes over a reef after dark, when what looked like an ornate formation of rock in the daylight suddenly becomes a throbbing surface of tiny, voraciously feeding mouths.
“You see octopus at night and lots of crustaceans, crawfish and crabs, walking around,” says Stuart Cove, founder of Stuart Cove’s Dive Bahamas. “But the most spectacular change is in the coral. Because you take your own light down, you see the true colours. And all the surfaces become furry, as the polyps come out to feed.”
These polyps are tiny cylindrical creatures just like sea anemones and jellyfish, except that they secrete an outer skeleton of calcium carbonate. They thrive in water that only rarely drops below 70˚F so, strictly speaking, most of The Bahamas should be too far north to support them. However, the Gulf Stream provides a flowing, warm-water sanctuary, creating conditions so favourable that the 100-mile barrier reef east of Andros is the third-largest biological construction on earth (behind only the barrier reefs of Australia and Belize).
Reef destroyers
Different corals grow at different speeds. Star and brain corals average just an inch a year, while the large branching corals grow much faster. Each fragile tip of the staghorn coral extends itself four inches annually before branching in two. Like all the other corals, though, the staghorns are eaten back almost as quickly as they grow.
Sharp-beaked parrotfish are the most picturesque of the reef’s destroyers. Ranging in colour from delicate cream and rose to vivid midnight blue, they crunch away at the hard surface to eat the algae that live inside, symbiotically with the polyp. Algae are plants that produce energy and food by photosynthesis and in return get protection and the nutrition of the polyp’s waste. They also give the coral its colour—the growing edges of corals are often colourless because algae has not yet spread there.
The symbiotic nature of the coral is another limiting factor on how far it can spread, restricting it to shallow, sunlit waters. Sponges are even more primitive animals, but have a far greater range, from the surface of the reef to deep abyssal waters miles below the surface. They range in size from a few inches to ten feet across and in shape from indeterminate patches clinging to rocks all the way up to beautiful, lacey fan sponges waving in the ocean currents. Some have amazing inner lives. The huge loggerhead sponge has a labyrinth of internal cavities and passageways. As it grows it traps up to hundreds of pistol shrimps—named for the explosive sound of their snapping claws—inside these spaces, so that a loggerhead sponge often sounds as if there is a gunfight going on inside.
Strange behaviours
Most reef animals are territorial to the point of foolhardiness. The yellow, black-striped sergeant major and numerous small, bright species of damselfish will attack divers and even, on occasion, the Caribbean reef shark which is at the top of the reef’s food chain. Another highly territorial fish is the grouper, a type of sea bass that is a staple of Bahamian cuisine.
The most aggressive grouper are called supermales, which are female at birth but later change their gender when they reach a certain size—usually about two feet. This allows them to defend a territory. Another species that goes through this change is the bluehead wrasse. In fact the only wrasses that attain the copper-blue head are supermales.
Curiously, the greater an animal’s reputation for being dangerous, the more timid it seems to be. Ferocious-looking moray eels are shy creatures, unless you get too close. The three stinging species of ray found in The Bahamas—the delicate yellow, the hulking southern and the even larger spotted eagle ray, with an eight-foot wingspan—strike only if pestered or stepped on.
Fish in the open sea
Although the greatest abundance of life forms in Bahamian seas is clustered about the reefs, there are other habitats teeming with life. To most visitors, sea grasses and sea weeds are a nuisance when the waves carry them onto the beaches. But the grasses are vital ecosystems, helping to keep unwanted sand off the reefs, and providing food and shelter for many sea creatures. The most prolific and important sea grass is turtle grass, so named because it is the basic diet of the green turtle.
Because they could be kept alive aboard ship for months at a time, green turtles were a valuable resource for sailors during the colonization of the New World, when they were quickly hunted to less than one per cent of their pre-Columbian level.
They take up to 40 years to reach sexual maturity, and they may swim thousands of miles from their feeding grounds in The Bahamas to the mid-Atlantic islands where they breed and lay their eggs. The Bahamas is also home to loggerhead, hawksbill, leatherback and olive ridley turtles, each with its own complex, easily endangered pattern of migration and reproduction.
The open oceans, where turtles spend at least part of their life, are often described as the marine equivalent of empty deserts. The Tongue of the Ocean, off Andros, is a mile-deep trench, forming a part of the Great Bahama Canyon. But even here, partly because of the Gulf Stream, abundant life can be found in the sunlit upper few hundred feet of water. This is where the sport fish swim.
Bluefin tuna, which can weigh up to 1,500 pounds, spend their winters in The Bahamas, migrating north for their summer feeding frenzies. The prized blue marlin grows to a similar weight and has a sharp bill that it can use to spear free-swimming fish and squid.
Other species have evolved specifically to evade these fearsome predators, such as the flying fish, which can stiffen its fins into wings that allow it to glide above the water for more than 10 seconds at a time.
From the shallows to the depths, the cycle of life, with all of the fascinating adaptations and struggles that drive it, is clear to see throughout the waters of the Bahamian archipelago.
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