Allison SmithOysters, scientifically known as ostreidae, are commonly used for food or pearls depending on the class of bivalve mollusk. They are found in oceans around the world, thriving with a wide range of oxygen levels, salinities, and temperatures; two of the most popular species are the eastern American oysters and the Pacific oysters. Typically found in shallow waters and near the shoreline, adult oysters tend to cluster in groups called colonies, beds, or reefs. Baby oysters, on the other hand, float through the water until they find a hard substrate to settle on, such as another oyster in a reef.
Oysters and Water Filtration
One benefit of oysters is that they filter the water around them. Up to 50 gallons of water can be filtered by a single oyster in just one day — equivalent to the water used in a ten minute shower. As filter feeders, oysters pull water into their shells through their gills before extracting food and expelling the remaining water. Cilia, microstructures in the gills that act as sieves, separate semi-solid and solid components from the water such as algae, phytoplankton, and nutrients that can be converted to food. Anything that the oyster can’t digest will be released as faeces or pseudofaeces trapped in mucus that will settle as biodeposits on the ocean floor for microorganisms and bacteria to convert to nitrogen gas. This is a part of the Nitrogen Cycle and ensures the ocean remains in balance.
Benefits for Global Ocean Health
Oysters are integral species for the marine ecosystem and provide many services that go beyond water filtration. As ecosystem engineers, their reef constructions provide habitats for other biodiverse species, food for predators, storm protection for underwater vegetation, and even nurseries for young sea life such as crabs and fish. Humans also harvest oysters as food as part of the fishing economy. Oyster generated $221 million in the United States in 2019 alone.
Moreover, oysters enhance water clarity by filtering out sediments and pollutants. This increases light penetration to support the growth of aquatic plants such as seagrass. Additionally, oysters filter out excess nutrients and prevent harmful algae blooms and dead zones. Scientists have found that bacteria in and on oysters regulate their ability to process excess nutrients such as phosphorus and nitrogen from agricultural runoff and sewage that can over-fertilize the ocean.
Additionally, oysters can help fight climate change in different ways. First, the use of oyster reefs instead of artificial breakwaters slows shoreline erosion and can save waterfront communities about $85,000 annually. Increased plant and animal life from cleaner water supported by oyster filtration also enhances the carbon sequestration capacity of the ocean. Oysters can even sequester carbon dioxide themselves, as they form their shells by extracting carbon from the water. Furthermore, since they clean their surrounding habitat and don’t require additional food or nutrients, oysters serve as a climate-friendly protein source as we grapple with the need for sustainable food systems.
Oyster Population Decline
About 85% of global oyster reefs have been lost in the last century, according to the Nature Conservancy. This is a sharp contrast to early descriptions of oyster reefs from explorers in the 17th century, when ships had to navigate around oyster reefs in the Chesapeake Bay and the Hudson River was the largest source of oysters in the world. Threats contributing to oyster population decline include overfishing, habitat degradation, invasive species, pollution, disease, human development, and climate change. Oysters are particularly vulnerable to environmental stressors caused by climate change since they are permanently attached to their reef habitats; these threats to their habitat include increasing temperatures, salinity fluctuations, rising sea levels, sediment accumulation, and ocean acidification. For example, scientists have established the correlation between climate variability and decreased historic oyster populations on the French Atlantic coast. Additionally, plastic pollution in the ocean poses a serious threat to oyster populations. Microplastics, or pieces of plastic 5 millimeters or shorter, are small enough to be uptaken and then expelled by oysters. Around 94.4% of oysters globally have been found to contain microplastics, making this mollusk a useful indicator of environmental pollution. Samples of oysters from the Mississippi Gulf Coast were found to contain microplastics in both external and digestive system tissues; lower concentrations found in digestive system tissues suggest that most microplastics are not ingested. However, scientists have documented that even the act of filtering and expelling microplastics, while seemingly having little to no effect on adult oysters, decreases oyster offspring. Furthermore, these polluted environments were shown to stunt subsequent offspring growth.
Oyster Conservation and Restoration Efforts
Conservation efforts thus far have included regulations to limit harvesting, legislation proposals, and artificial reef construction. State regulations have included catch limits, seasonal closures, and gear restrictions. Notably, scientists have called for the inclusion of Indigenous community members in developing restoration, harvest, and management strategies given the historical persistence of Indigenous oyster fisheries long before commercial fishing contributed to population declines. Additionally, legislation such as the Break Free from Plastic Pollution Act introduced to the Senate and the Save Our Seas Act passed in 2020 seek to combat plastic pollution contributing to the decline of marine populations like oysters. Artificial reef construction projects also aim to attract oyster larvae through building reefs from materials such as concrete, crushed limestone, and recycled shells. Researchers at North Carolina State University are also studying oysters’ ability to locate reefs through sound vibrations in efforts to understand how to control where oysters choose to settle.
The National Oceanic and Atmospheric Administration or NOAA has identified reef restoration as a conservation priority and has funded over 70 projects in 15 states. The largest restored population thus far has been in the Chesapeake Bay, where more than 1,200 acres of oyster reef have been restored since 2011. In a 15 year study tracking the health of these restored reefs, scientists have found that restored reefs can match their natural counterparts and sustain the population after six years. Restored reefs can also enhance commercial fishing through their ecosystem services; restoration projects in the Chesapeake Bay are projected to increase seafood sales by up to $23 million annually and support 300 additional jobs. Other ongoing restoration projects funded by both the government and nonprofits include areas such as the Delaware River, New York Harbor, and Belfast Lough in Northern Ireland. Scientists are working to uphold genetic diversity and monitor resource competition to address human intervention concerns for restoration projects.
Overall, oysters are mighty mollusks that can considerably serve the marine ecosystem, the climate, and the human population. Restoring oyster populations is thus an important part of improving water quality and fighting climate change. Nevertheless, tackling the problem at the source through strategies and regulations to reduce contaminants in our waterways and reduce carbon emissions remains the prime pathway to cleaner air and water.
Peer Editor: Priscila Santa Rosa