Oysters are invertebrate (lacking a backbone) mollusks which live in shallow, brackish or salty water and tend to grow in piles or clumps known as beds. Mollusk means soft body in Latin and octopus, squid, and snails are included in this classification. Oysters need hard bottom habitat—shell or rock—and cannot survive if buried in mud. They are sessile, spending most of their entire life attached to something and cannot move from that location. Oysters prefer oyster shells as substrate, although they can live on any hard surface. This surface material or substrate is called cultch. Cultch that is clean and free of mud and debris is preferred, since some pollutants, including oil can inhibit the setting of oyster larva. That is why it is important to return old shell to the oyster beds to provide cultch for future oysters.

Oysters spawn or reproduce during the summer by releasing eggs and sperm into the water (external fertilization): usually almost all the oysters on a bed will spawn at once when the temperature reaches about 75°F. The first oyster to spawn will release pheromones (chemical signals) which stimulate the oysters to spawn. The egg and sperm fuse in the water and become a fertilized egg or zygote and begins cell division. After about four hours, the zygote has developed tissues and organs and is called a trochophore, then continues to develop and is called a veliger. Up until now, the larva has been free floating, moving with the tides and winds. After about two weeks, the larva is ready to settle down and find a permanent home. It falls to the bottom and with its temporary foot, searches out clean, hard cultch. When it finds a place, it secretes byssus cement from its byssus gland to glue itself down to the cultch. There it will stay for the remainder of its life. This is called settling or spatting as the larva once settled is referred to a spat.

Oysters are especially fecund fertile; the female laying over 5,000,000 eggs and the male 2,690,000,000 sperm. Unfortunately for the oyster, most of these will not make it to adulthood. Only about one in a million fertilized eggs will survive to settle and grow. The rest will persist due to water conditions (temperature and salinity), perdation, inability to find appropriate cultch and other factors. But the planktonic oyster larvae still have a very important function in the ecosystem as a food source for other critters and part of the food chain.

To look at an oyster, even an open one, it would be difficult to determine its gender. Oysters, like many invertebrates are hermaphroditic. Most specifically they are sequential hermaphrodites. This means they have the ability to be both sexes although not at the same time. Oysters are protantrous hermaphrodites, meaning that htey start off life as male, then most will change to female. Oysters reach puberty and have the ability to reproduce after about one year. After that point, almost all of them are male, but they begin to change to female at this time. By the third year, about 90% have become female. Although other species of oysters can change back and forth between male and female, Crassostrea virginica cannot. Therefore, one can make an educated guess as to the gender of an oyster depending on size, but it is important to realize that you may not be correct in your guess. Larger oysters are probably females while the small ones are males. Why? Evolutionary advantage? Maybe it has something to do with more energy is needed to produce the larger eggs than sperm.

After three months, the spat is about the size of a dime. After one year it is sexually mature and can spawn. Market size (3 inches) is reached in two-three years in the lower beds and five-six years in the upper beds. Remember, growth is salinity and temperature dependent. Oysters can live up to 10-12 years, although it is uncommon as they are usually harvested, caught or eaten by predators before they reach that age.

The shell is manufactured by the mantle and this is how the oyster grows, by adding material from the mantle to the shell. The shell is predominantly (98%) calcium carbonate. Next is the adductor muscle, which is the muscle you must cut to open the oyster. This muscle holds the valves together from the middle of the oyster and is surprisingly strong, making it difficult to pry open the oyster. The mantle is the double fold of the body wall and encompasses the other organs. Oysters do not eyes, but do have pigmented spots that are light sensitive. If it senses a shadow, the oyster will close its shell for protection. Because it isn't going anywhere, it is unnecessary to have eyes to see. At the edge of the oyster, you will notice hairlike projections. These are called cilia and are used to draw in and expel water, allowing the oyster to feed on plankton in the waterflow. Continuing toward the center of the animal you will see layer—like gill for respiration. Above the adductor muscle, you see the stomach, which can be cut into to show the contents of the stomach (the students loved that!) and the heart, which if you are extremely careful when opening the oyster, you may be able to see beating. Oysters are blue-blooded, having copper rather than iron (red-blooded) to carry oxygen and carbon dioxide. Oysters do not have a true brain, but do have a nerve center called the ganglia located at the top toward the hinge. The gonalds (sex organs) are not usually visible, except during spawning.

One of the most interesting things about the oyster is that it can actually clean the water it lives in! The oyster feeds by pumping water through its body and filtering out its food (mostly algae and detritus—decaying plant material). This is called filter feeding. A healthy market-size oyster can filter approximately 50 gallons of water a day. At the height of the industry in the Chesapeake Bay, the oysters could filter the entire Bay in three-four days! Also, a natural oyster bed provides habitat—shelter and food—for a community that includes many other organisms: plants, crabs, worms, fish, etc. Among the creatures you may see are such predators as humans, oyster drills, moon snails, or whelks (snails that feed on oysters by drilling holes through the shells).

Who else eats oysters? The spat or larvae are very vulnerable and are eaten by a wide variety of fish and invertebrates. Larger oysters may be eaten by crabs, fish (oyster toadfish, rays, skates, drum), starfish, worms, or birds (oystercatchers). Boring sponges are parasites—they don't actually eat the oysters, but they can kill them. You can use the oyster to demonstrate the fifteen sysbiotic relationships in anture; parasitism, mutalism, and commensalism. In parasitism, one organism benefits and one is hurt (boring sponge), commensalism is when one animal benefits and the other is not harmed by the interaction (slipper snail) and mutualism is when both organisms benefit (clownfish and anemone). Anenome provides protection and clownfish provide food. Another organism is the oyster crab which can sometimes be found in the oyster. The crab is filtered in as plankton and then grows inside the oyster. It eventually will harm the oyster as it eats the cilia and thus inhibits the oysters ability to feed.

©1998 robert d. owens