Oyster Habitat
The oysters farmed in Texas are eastern oysters (scientific name Crassostrea virginica), also known as the American oyster, Atlantic oyster, Gulf Coast oyster, common oyster, or Virginia oyster. They live in shallow saltwater bays, lagoons, and estuaries in the Gulf of Mexico and Atlantic coast between Mexico and Canada. They prefer to settle 8 to 25 feet deep (although they do quite well in cages and baskets near the surface where water is rich with the phytoplankton and algae they feed on) in water between 28 to 90°F. Eastern oysters are highly adaptable, an important quality for creatures living in bay waters where conditions change quickly depending on season and weather. They can tolerate a wide range of temperatures and salinities and are able to snap their shells firmly shut to survive in less ideal conditions, even short periods out of water.
In the wild, oysters cluster in large groups attached to solid objects like piers or rocks and often layer on top of each other over generations to create solid oyster reefs. These reefs are important habitats for sea life including sponges, crabs, and fish.
Oyster Life Cycle
When the ocean warms up (from late spring to early fall in Texas), oyster spawning season begins. Male and female oysters release clouds of sperm and eggs, which are fertilized in the open water. Females can release up to 100 million eggs, but only about 1% of fertilized eggs survive to the next stage. Change happens quickly: Within hours of fertilization, the eggs develop tiny shells and the ability to move independently, and they spend the next two to three weeks as rice-sized larvae swimming through the water column in search of a place to attach. Once they attach (or set) to a solid surface (or substrate) with a glue-like substance, they transform into juvenile oysters called spat (or seed). In hatcheries, larvae can be induced to set into spat either by providing finely ground oyster shell pieces that individuals can attach to or using chemicals to cause larvae to change into spat without attaching to a substrate (learn more about oyster hatchery techniques).
In ideal conditions, spat grow rapidly and may reach 1 inch (at which point they are called oysters) in as little as three months. In Texas waters, it generally takes 18 to 20 months to reach market size of around 3 inches. They reach sexual maturity at just 7 weeks after hatching and have been known to grow up to 10 inches long and live up to 20 years.
Oyster Anatomy
Oysters are a type of bivalve, a group of mollusks that gets its name for the two-part shell held together by a hinge and two strong adductor muscles. Oysters use these muscles like humans use biceps to bend their arms: Tightening the adductors closes the shell, whereas relaxing the muscles opens it. The shells are concave, producing the spoon-like cavity that oyster farmers refer to as the cup. Inside, the body is surrounded by a soft, fleshy tissue called the mantle — that’s the part you eat. The mantle protects the internal organs and secretes the calcium carbonate that makes up the shell.
Oysters are filter feeders and herbivores, capturing tiny particles of algae and phytoplankton as water passes over their gills located along the outer edges of the mantle where the shell opens. Tiny moving hairs called cilia covering the gills move food particles trapped in mucus toward the mouth (near the shell’s hinge) and into the stomach. Once the oyster extracts the nutrients from digestible materials, it pushes feces out through the rectum and anus. However, not everything the gills capture can be digested. Particles like grains of sand bypass the digestive system and are trapped in the mantle’s mucus until the oyster ejects them as pseudofeces or biodeposits, which sink back to the ocean floor.
Ploidy
One of the unique things about oysters is that they can be polyploid, or have more than the standard two sets of chromosomes, a fact that scientists and farmers use to produce bigger, meatier stock. Wild oysters are diploid, meaning that, like humans, they contain two sets of chromosomes (one from the mother, one from the father). However, it is possible to breed oysters to be polyploid. In commercial oyster farming, polyploid oysters are either triploid (three sets of chromosomes) or tetraploid (four sets of chromosomes). The main advantage of tetraploid oysters is that they grow larger and plumper than diploid and tetraploid oysters, even in the hot summer months when oysters typically don’t grow as well. Most oysters sold on the half shell are triploid. Triploid oysters are almost always sterile, so they cannot breed with commercial or wild stock. Tetraploid oysters are fertile and do not have the same size advantages as triploids. Their purpose is primarily for breeding: A diploid crossed with a tetraploid will produce triploid offspring.
Read our page on biosecurity to find out how farmers must consider ploidy when stocking oysters.
Pearls
Oysters aren’t just known as a culinary delicacy; they’re also a prized source of saltwater pearls. However, eastern oysters usually don’t produce pearls, so you don’t have to worry about cracking your tooth on one during an appetizer of oysters on the half shell. However, it’s worth knowing how pearls form.
Several mollusk species form pearls, including the gold-lipped pearl oyster (Pinctada maxima), abalones, conchs, pen shells, and whelks. As mollusks grow and repair their shell over time, cells in the mantle secrete calcium carbonate (in a crystal form called aragonite) and a binding agent called conchiolin. These substances form alternating layers that together are known as nacre, or mother-of-pearl. Nacre gives the inside of mollusk shells their smooth, iridescent appearance, which is not only beautiful but protective.
Pearls form when an irritant — usually a parasite and not, as commonly believed, a grain of sand — works its way inside the oyster’s shell. The mantle then goes into protection mode, encasing the irritant with layers upon layers of nacre, thus forming a pearl. The layers of nacre are extremely thin, so it can take two or more years to form a 3 to 5 mm pearl.
Natural pearls are extremely uncommon — found in about one in 10,000 wild oysters. Most pearls sold in jewelry stores are cultured, meaning humans insert an irritant into an oyster to stimulate it to make a pearl.
