The oyster is found in North America from the Gulf of the St. Lawrence to the Gulf of Mexico in many bay areas and estuaries. They have even been introduced to Hawaii, west coast America and other spots around the world. Estuaries are areas where rivers meet the sea. Estuaries harbor unique plant and animal communities because their waters are brackish: a mixture of fresh water draining from the land and that combines with salty seawater.
Who Am I?
Oysters are considered shellfish because they create beautiful hard shells that covers their bodies. You may find them on the beach or on rocks. The soft-bodied animal inside is called a mollusk. There are two groups of mollusks, gastropods and bivalves. Gastropods have a shell made of only one valve and it is usually coiled. Bivalve mollusks like oysters have two valves or parts, and the two shells are connected by a hinge, which looks like a small row of teeth.
Oysters cling to rocks or are wedged into the sand to wait for food to come by. Often in that environment, their shells are slightly open when they are resting or eating. If frightened, they use strong adductor muscles to force themselves shut tight. They constantly feed on very tiny organisms, which are filtered out of the water by their gills. They affect plankton and algae, but also take particulate nitrogen out of the water drawing it down into the sediment, so the oysters make water safer not just for plants but also people.
Feeding and Fighting
Oysters are hearty creatures, often living in salty lagoons. They can handle variations in temperature as well as areas with sediment in which the water is suspended with debris, with a high salt or salinity level and dissolved oxygen. The amount of oxygen in the water is integral to the survival of organisms. A dissolved oxygen level that is too high or too low can harm aquatic life and affect water quality.
Oysters are referred to as filter-feeders. They suck or draw water in through one of two siphons situated over its gills, by beating hair-like cilia (pronounced SIHL-ee-uh). Thousands of tiny creatures, some too small for the naked eye, and others such as plankton, are trapped in the mucus of the oyster gills and taken to the mouth. There they are eaten and digested; and the expelled particles swirl through the water and resemble a smoke ring.
Also, parts of the digested product are feces or pseudofeces, which are suspended solids that are bundle, and then used by other organisms on the oyster reef for food. The filtered water then flows out of the oyster through the other hole or siphon. An adult oyster can filter up to 5 liters or 1.3 gallons on water an hour. That’s equal to 60 two-liter soda bottles a day, just for one small oyster.
Oysters also draw minerals from the water mostly in the form of calcium carbonate, a substance found in many rocks. The minerals flow through the mollusk’s bloodstream to its mantle (a thin membrane or layer of tissue) that covers the animal inside. This sets off special glands that create a liquid substance that turn the minerals into shell. The shell forms from its hinges outward, ring after ring in layers, and then hardens. Scientists can tell how old an oyster is by counting these rings.
Mature oysters produce millions of eggs, but only a few larvae survive. Since oysters spawn in warmer water, they release eggs and sperm into a water column at about 68 degrees (20 degrees C). The fertilized eggs develop into swimming larvae after a couple of weeks, then the surviving groups sink to the bottom and attach themselves to a solid surface. It could be a rock but mainly it attaches to another oyster in a big oyster bed. The new arrival is called a “spat."
Overfishing has created a disaster, as more than 85 percent of the oyster reefs have been lost, according to a study led by Michael Beck of the University of California, Santa Cruz. Dr. Beck led a team of marine biologists, who examined 144 former strongholds of the creatures in 40 regions around the world. The overall condition of the various species is poor.
Dredging, which is fishing where the ocean floor is dug up, coupled with global travel and disease have damaged many areas. You can help by eating farmed oysters and other sustainable seafood. Also, participate in a shell recycling program or get involved with a regional conservation organization. In addition, learn more about the species and talk about your findings with other conservationists.
Just So You Know: Pearls
Pearls are formed when a foreign particle such as sand, dirt or a piece of shell enter the mollusk’s body. Since it signals new damage, repairs begin immediately. The oyster uses its mantle to produce nacre, called mother of pearl, which is a substance that surrounds the foreign substance and hardens. Eventually, this grain or whatever gets layered up and becomes a smooth pearl. Not all pearls are round. Some form against a shell’s surface and one side will be flat!
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