Oysters are bivalve mollusks found in many oceans of the world. Humans view them as an economically valuable animal because they can provide a source of food and pearls, distinctive lustrous objects used in jewelry and ornaments in many cultures. In addition, they appear to be effective scrubbers of polluted water, as indicated in several experiments in the United States in 2006. Many other animals are also fond of oysters, thanks to their flavorful, protein rich flesh.
A mollusk is a soft bodied animal that protects itself with a shell. Bivalves have two shells which are connected with a small hinge. The animals can open their shells to intake food and expel waste, and they can close their shells if threatened by predators. Like other mollusks, oysters have relatively simple biological systems, and they can be found in brackish water as well as salt water. Some may also range further inland to fresh water, although these species are more rare.
All oysters are in the family Ostreidae. The animals are filter feeders, opening their shells to allow water to pass through their gills, supplying them with food and necessary oxygen. Because of this, they can be used to clean impure water, although this may hurt the oysters themselves as they accumulate toxins. They tend to root into place on a rock, allowing the tides to fulfill their needs.
Humans are among the major predators of oysters, although the animals are also eaten by marine mammals and organisms like starfish. The relationship between humans and oysters is quite old; many early humans greatly enjoyed them since they are relatively easy to harvest and high in nutrition. Some humans regard the mollusks as delicacy, and they are often eaten raw for their supposedly aphrodisiac quality. They may also be cooked in seafood stews and chowders, although they can get rubbery with excessive cooking.
One mechanism of oyster defense is of particular interest to humans. When an irritant such as a rock or grain of sand enters the shell, the animal will cover it in layers of nacre, a secretion that will harden into a smooth, glossy ovoid shape better known as a pearl. Other mollusks such as abalone will also form pearls, but oyster pearls are among the most widely harvested around the world, and in some nations people actually farm oysters to cultivate pearls for commercial sale.