Oysters reefs are lively, noisy places
NEW ORLEANS (AP) — Though oysters spend most of their lives cemented in one spot, oyster reefs are lively, harboring and attracting hundreds of species from clams and other bivalves to worms, shrimp, snails, crabs and fish.
"Oyster reefs are noisy places," said Bryan Legare, a natural resource specialist with Texas Parks and Wildlife. "You hear snapping shrimps and other shrimps moving around," the crunch of stone crabs and black drum crushing oyster shells to eat the meat. There's also the knocking, purring, and grunting sounds made by seatrout and drum vibrating a specialized sonic muscle against their swim bladders.
The noise made by red and black drum is "halfway between a drumbeat and a grunt. Sometimes it's a grating, like down a washboard almost," Legare said.
Gulf Coast oyster harvests have dropped by as much as two-thirds in the four years since an oil well owned by BP PLC blew out, creating the nation's worst offshore oil disaster. BP blames other factors for oysters' decline in the region, and scientists agree the effects of the oil on the bustling underwater communities of bivalves won't be clear for some time.
Not all oysters create reefs. Those that do include the native Eastern oyster harvested from Canada to Venezuela, including the Gulf of Mexico. Their larvae need hard surfaces to affix to, and older or dead oysters are prime candidates. Although a century of oystering has kept beds in Galveston Bay scraped down almost to the bottom, those left unharvested in Sabine Pass at the Texas-Louisiana line are up to 7 feet high, Legare said.
Some cover a square yard or two, others stretch out for thousands of acres.
The live oysters are on the top and sides of the reef, which could be seen as condo complexes for tiny critters living in relatively shallow water. Some reefs are above water at low tide, and though Eastern oysters can be found up to 30 feet below the surface it would be exceptional to find them deeper than 20 feet in the Gulf of Mexico.
"Typically when we get out into 20 feet or more it's a pretty soft muddy bottom" rather than the hard surfaces oysters prefer, said William Walton, a biologist and aquaculture specialist at Auburn University. Additionally, oyster diseases are less prevalent in the less salty water — as are predators.
The oysters are ecosystem engineers. Sponges, sea anemonies and other molluscs latch onto their shells. Tiny fish and crustaceans find food and shelter in the spaces, often less than a quarter-inch across, between shells. Oyster farms offer "tremendous habitat for juvenile blue crabs," Walton said.
Larger predators gather around to feed on the smaller animals. Drum aren't the only oyster-eaters; cownose rays eat the spat when their shells are still small and soft. Blennies, gobies, spadefish and snappers also gather around reefs.
As filter feeders, the oysters clean the water around themselves, making aquatic plants more likely to grow nearby because more light comes in through the clearer water. Those in turn serve as habitat for fish and other marine life.
The amount of water filtered by oysters depends on many factors, including the oyster's size and the temperature and salinity of water. "It's a substantial number," Walton said. Fifty gallons a day is a frequently cited figure, but Walton said that would be for an Arnold Schwarzenegger-size oyster living in prime conditions. The Nature Conservancy's estimate is conservative at 20 gallons a day, said Anne Birch, its marine conservation director.
Legare said the oysters eat only a couple of kinds of algae. Silt and everything else they filter bypasses their digestive system and is deposited nearby.
"It feeds everything around them," Legare said. "The crabs, the shrimp, other bivalves, like clams and mussels, and other filter feeders, like tunicates."