Eastern Oyster

Eastern oysters, otherwise known as the American oyster, Virginia oyster, or Crassostrea virginica, are mollusks located across the Western Atlantic coast extending from Canada to the Gulf of Mexico and down to Argentina.

Eastern oysters are critical to their ecosystems, as they provide shelter to other important marine species. 

Characteristics & Appearance

Weight & Length

Eastern oysters range between three and five inches in length and weigh between 30 grams and 150 grams. If an easter oyster resists threats, predators, and has a healthy supply of plankton to feast on, it can grow to be as large as eight inches in length.

Physical Characteristics & Color

An Eastern oyster has a soft body protected by two rough, ridged shells. They range in color from grey to white or tan. The bottom shell is cupped with a dark muscle scar on the inner side of the shell, while the top shell is more soft and flat. 

Because the same oyster species can appear so different, it is unknown exactly how many different oyster species exist. It is thought that there are between 30 and 100 different oyster species worldwide. 

Lifespan & Reproduction

As with any animal, marine or otherwise, an Eastern oyster’s lifespan is highly dependent on its environment, freshwater inflow, and potential threats or predators. Eastern oysters typically will live to be a couple of years old, but there have been instances of them living to age 20 when kept in captivity. 

To reproduce, adult Eastern oysters release sperm and eggs into the water during the spawning season in early summer. Impressively, female Eastern oysters produce approximately 20 million eggs annually.

Oysters are protandrous hermaphrodites, meaning that they first spawn as males before changing sex after two to three years to spawn as a female. As a result, younger oysters are male, while older oysters are female. 

Optimum spawning occurs when the water temperature reaches 24 degrees Celsius, but Eastern oysters can spawn freely once the water temperature reaches 20 degrees Celsius. 

Habitat

Where does the Eastern Oyster live?

Found in both subtidal and intertidal zones, Eastern oysters are highly tolerable to varying temperatures, salinities, and oxygen levels. Spanning across Canada to the Gulf of Mexico coast and south to Argentina, Eastern oysters are resilient and plentiful. 

When exposed to low tides, Eastern oysters tend to develop clusters of shells that are irregular in shape and elongated. Alternatively, subtidal oysters tend not to cluster and appear to have more rounded shells. 

Beyond an oyster’s shells providing a home for their soft bodies, oysters also provide habitats for other marine life, like the Blue Crab and Striped Bass. Starting as a free-floating larva, oysters attach to a surface in the water where they will live out the rest of their lives, accumulating and growing together to create a reef. 

This reef then makes a protective and effective shelter for other sea animals, providing a safe environment for other marine life like anchovies, flounder, herring, shrimp, Spanish mackerel, silver perch, and more. 

Food & Diet

What does the Eastern Oyster eat?

Eastern oysters are filter feeders who feast on phytoplankton by pumping water through their gills while their hard shell is open.

Because of their filter-feeding nature, oysters consume and filter nitrogen-containing compounds and sediments, improving water clarity. In addition, this process enhances sea grass’s ability to photosynthesize because sunlight can reach it more directly. 

Threats & Predators

Human Threats

Eastern oysters fall prey to humans, fish, and other invertebrates. Critically overharvested, oyster populations are in danger as they have reduced by half since 1999.

A study conducted by the Department of Natural Resources and the University of Maryland Center for Environmental Science found that Maryland’s oyster population drastically dropped from 600 million in 1999 to 200 million in 2002. 

While overfishing is arguably the biggest threat to Eastern oysters, they also face threats from habitat degradation, climate change, and invasive species. 

Climate Change & Global Warming

Eastern oysters are faced with climate change and global warming obstacles as well, as increased temperatures and heightened salinity foster disease outbreaks and may extend a predator’s hunting range and period. 

Predators

In addition to human threats, climate change, and global warming, Eastern oysters fall prey to several other marine animals. For example, the boring sponge bores into oyster shells, secreting chemicals that dissolve calcium carbonate.

Soon after, the oyster dies, and the boring sponge inhabits its shell. Another predator, oyster drills, penetrates a hole into oyster shells, consuming them. Similarly, whelks also drill into oyster shells and consume them. 

Crabs feast on oysters and mussels, in addition to fish, fungi, worms, snails, and bacteria. Oysters grown on the seafloor are most in danger from crabs. Using their claws, lobsters crush oysters to eat the meat inside. The lobsters locate the oysters by using their antennae to sense chemicals in the water. 

Oyster flatworms target oyster spat, consuming the meat of juvenile oysters as they pump their gills to feed. The worms then use the empty oyster shells to lay their eggs inside, providing them protection.

Starfish search for their prey using sensory tubes on their arms, penetrating the oyster’s shell and entering their stomach inside of the Eastern oyster to digest it. The starfish’s digestive chemicals disintegrate the oyster’s internal structure, leaving only an empty shell. 

Oyster toadfish and cownose rays are also mass oyster predators. Bottom dwellers, oyster toadfish feast on crustaceans, shellfish, squid, and small fish.

These predators use camouflage to stay hidden as they await their prey. Cownose rays use dental plates to crush their prey, including clams, oysters, and other invertebrates. 

Other threats

Finally, oysters can become infected by parasites carrying aquatic diseases Dermo and MSX and are also threatened by increased water salinity. Dermo attacks the blood cells of its victims.

The oyster catches Dermo through a single-cell parasite ingested by the oyster as it feeds. This parasite thrives in warm weather and puts the oyster population at significant risk as global warming progresses. 

MSX is a pathogen that greatly endangers Eastern oysters, which are not resistant to it, unlike their Pacific oyster counterparts. The disease works by reducing an Eastern oyster’s feeding rates, effectively forcing it into starvation from lack of carbohydrates. 

Water salinity is also a potential threat to Eastern oysters. Saltwater is measured by parts per thousand (ppt), in which Eastern oysters thrive between 5 and 35 parts per thousand.

Brine water, saline water, and brackish water measure in between 50+ ppt, between 30-50ppt, and .5-30ppt, respectively. Oysters typically thrive in waters between 5-35ppt. 

Conservation Status

Oyster harvesting has restrictions through catch limits, seasonal closures, and gear restrictions. Several regions have implemented artificial reefs using concrete, crushed rock, or oyster shells to provide a place for oyster larvae to attach to. 

Furthermore, the NOAA Restoration Center has provided funding for restoration techniques, including reef construction, creating hatcheries, dispersing shells to provide bases for oysters to attach to, and bagging shells to use as cultch for spat. 

Fun Facts

• Oysters have been around for over 200 million years.
• Oysters are a part of the phylum Mollusca, to which octopi, squid, snails, and other invertebrates are also a part.
• Oysters have high phenotypic plasticity, meaning that the same species can look vastly different depending on its surrounding environmental factors.
• Eastern oysters are native to the Chesapeake Bay. 
• Oysters filter 1.3 gallons of water per hour, balancing the marine ecosystem and feasting on phytoplankton. 
• An oyster’s flavor is derived from its environment, as they develop a flavor profile from the water they filter. These influences include the nutrients present, as well as different levels of salt. The flavor profiles come from characteristics like buttery, briny, metallic, mild, and sweet.