Underwater World

American Oyster

North American Atlantic coast

At a glance | Vital stats | Curiosities | Closeup | The impact of disease | Ecosystem engineers | Making the grade

At a glance

When it comes to oysters, people tend to be more interested in how to prepare them as appetizers than in their unique traits as a species. Yet the biology of the American oyster is truly fascinating. Oysters have an intricate set of organs that control their digestion, respiration and reproduction. Their bodily systems are highly efficient, and an oyster's life can be surprisingly long: up to 20 years. Almost all of that time is spent in one location-aside from a very brief stint as a free-moving larva, the oyster is immobile.

Vital stats

Scientific name:
Crassostrea virginica
Taxonomy:
Ostreidae
Region:
Gulf of St. Lawrence and Bras d'Or Lakes (there are small populations in West-Nova)

Curiosities

Oysters are protandric, meaning they are all born male. Half of the population converts to female after birth so the species can reproduce. But if an oyster colony (called a 'bed') ends up lacking males, some females will convert back to being male to balance out the sexes.

Closeup

About the American oyster

The American oyster has a thick, rough, asymmetrical shell, usually a mix of brown, gray, green and white in colour. The shell is hinged so that it can open and close. The muscle that does this opening and closing is called the adductor. When the oyster's shell is shut it forms an airtight, watertight seal that's very hard to pry open. This is why it can have a shelf life of over 3 months. (It takes 20 pounds of pull to open a healthy 7.5 to 10-centimetre American oyster.)

Inside the shell is the soft, headless body of the oyster. Under the mantle-the sheet of tissue that covers the body-are its organs, including a three-chambered heart that pumps colorless blood full of oxygen throughout the body, and kidneys that keep the blood free of waste.

The mantle has three folds or ridges, each with its own special purpose. The ridge closest to the edge of the shell has glands that build and repair the shell. The centre ridge is made up of two rows of fine, dark feelers that sense changes in water chemistry and light and tell the adductor muscle when to close the shell to reduce exposure to the elements. The inner ridge-the largest of the ridges-is muscular and controls the flow of water in and out of the oyster.

The American oyster has a set of four gills. These are used for respiration and to collect food. The gills are covered with tiny hairs that filter incoming water and catch food particles such as live and dead plants (phytoplankton or seston).

Lifecycle and reproduction

In the southern Gulf of St. Lawrence, oysters reproduce (spawn) from late June to early July when the water temperatures are around 20 degrees Celsius. During spawning, eggs and sperm are released in intervals over the course of four to six weeks. Female oysters release eggs by clapping their shells gently-as many as 70 million in a season.

Eggs are fertilized on the outside of the oyster's body, resulting in a low survival rate. When successful, a tiny larvae forms. Young oysters go through several developmental stages. Fertilized eggs hatch into oyster larvae called trocophores. Only one percent of larvae reach the next stage of development. Those that survive become 'veligers' and begin to swim and drift, feeding on tiny plants. Eventually, the veliger becomes a pedi-veliger, producing a foot-like formation and settling onto a hard, clean patch of the ocean floor (substrate). Once there, the juvenile oyster, called a 'spat', attaches itself to the floor with a cement secretion from a gland in the foot.

The impact of disease

Deadly infection-causing micro-organisms can invade and destroy an oyster population. Between 1900 and 1945, disease wiped out 95% of the oyster population in the Gulf of St. Lawrence. Those that survived were used to populate the area once again-but reaching just 10% of the level of the past.

The underwater world of the American oyster

The shell of an American oyster tells the story of how and where it has grown. A long, narrow, mangled-looking shell suggests life in a crowded oyster bed on a muddy bottom. A round, strong shell with deep cups inside suggests more comfortable conditions: lots of space on a hard, clean bottom.

Crabs and seastars are the oyster's main predators. Sudden changes in environmental conditions also pose threats, such as the sudden overflowing of streams, freezing temperatures, suffocating plant growth and pollution. In recent years, North America's Atlantic coastal ecosystem has an overabundance of nutrients due to human development in coastal regions. This has led to an unhealthy over-production of plants, reducing the amount of clean substrate for oysters to settle on.

Ecosystem engineers

American oysters are sometimes called 'ecosystem engineers' for the role they can play in restoring reefs. When a struggling reef (a rocky ridge that is often home to many different species) is 'seeded' with live oysters, the reef often reaches its ideal size within a year. This attracts other species usually found on such reefs-so that within two years, the overall ecosystem is just like that of a natural reef.

Fishing or harvesting the species

The device most commonly used to harvest American oysters combines two rakes attached by a hinge. By opening and closing the rakes, the fisher can scrape the sea floor. This process, called 'tonging', not only lifts away the oysters but can also clean the substrate on the ocean floor, helping new oyster spat to settle.

Making the grade

In Canada, American oysters harvested for sale (three million pounds per year) are classified as Fancy, Choice, Standard or Commercial. The grade of the oyster depends on the length and width of the shell, with larger oysters being considered of higher quality.

Oyster farming, or aquaculture, is a way of making sure there are enough oysters to meet the demands of the fishery and also to seed reefs along the Atlantic coast to restore those important ecosystems. Farmed oysters are grown in cages over a three-year period, by the end of which they're ready for market.

Fisheries and Oceans Canada manages the harvesting and aquaculture of American oysters in this country.

Revised: October 2009