Sand Diving Secrets

If you enjoy diving away from the crowd and need a change of scenery, take a detour away from the reef to the nearest sand flats. Chances are good that your fellow divers will think you've lost your way. You can surprise them when you bring back a detailed list and a roll of film of all the things you saw on the sand that they didn't see. Most scuba diving takes place over reefs, which account for a very small fraction of the actual diveable area. There is a lot more sand than coral. Divers who get dropped over sand flats usually keep their heads up and their fins moving until they find "the dive site", thereby missing some of the most interesting animals and one of the most fascinating habitats underwater. That's the Sand Diver's Secret.

To appreciate the sand flats properly you have to get right down on the deck with your mask a few inches from the sand. Notice that the sediment differs in coarseness from one sand flat to another, and within a sand flat from spot to spot. The smallest particles collect where current velocities are lowest, and vice versa. Water movement can easily carry off fine particles, whereas coarse, gravelly sand requires faster currents to move it. The texture of the sand provides an index of the average current velocity. Big chunks indicate high speeds. Fine, soft sand means low current velocity. The pattern marks in the sand can also tell you something about water movement and direction. High-speed currents heap coarse particles into big sand waves, while slower currents produce only ripple marks in finer sand. Ripples and sand waves are oriented 90 degrees to the average direction of water movement, just like sea fans, and can be used to help you navigate underwater.

Many burrowing animals live in sandy environments, including clams, shrimp and other crustaceans and worms. Most of them are large enough to see, but remain buried below the surface where they are invisible to divers. Deeper burrowers have tubes leading to the surface so that they can get food and oxygen and discharge wastes. They may filter plankton from the water or feed on microscopic animals that live in the sand. Other animals live on or near the surface of the sand, sometimes feeding on other animals or small plants. Brine shrimp, heart urchins, hermit crabs and conchs can be found here by following their trails across the sandy bottom. Sea stars, long-spined urchins, and big snails are also found here. Quite a few species are nocturnal, spending the daytime buried in the sand or beneath the edges of coral heads. A variety of predatory fish feed on these animals. Stingrays, flounders, guitarfish, and skates treat the sand flats as a cafeteria with an excellent selection. Stingrays dig obvious pits in the sand by flapping their "wings" and excavating worms and clams. These pits are sometimes six feet across and several feet deep. Other fish such as bar jacks and trunkfish hover above feeding rays, picking out any stray morsels that are stirred up. You can attract both jacks and trunkfish by simulating a ray and stirring up the sand. Many fish will be attracted to the sand plume you create hoping to find their next meal.

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