While often eaten these shelled animals are some of the most important aquatic animals in the Atlantic. Not only do eastern oysters clean the ocean they also provide home for countless animals, plants, and algae. However, humans have devastated their numbers although conservation efforts have been implemented in vital areas to help them make a comeback.
Taxonomy and Evolution
The eastern oyster goes by many names including the Atlantic oyster, the American oyster, the Virginia oyster, the Chesapeake Bay oyster, and the Blue Point oyster to name a few common names. All these names refer to the same species, Crassostrea virginica, making it one of the ‘true oysters’ which are members of the family Ostreidae. It is hard to tell true oysters apart from other animals that we often refer to as oysters and this difference is based more on genetic and family classifications than how they look. In fact, the true oysters often belong to a different order, Ostreida, to the other oysters. Ostreida first appear in the Triassic period and some of their fossils are amazing – one species is known to have attached themselves onto the shells of ammonites! Oysters are bivalves – the class of molluscs (other molluscs include cephalopods and snails) that live in shells that also include clams, mussels, scallops, and cockels.
Biology and Behaviour
When you see an eastern oyster you are likely seeing an adult or when they are in the ‘spat’ stage where they have developed a shell. Oysters will spend their entire life in the shell once it has developed with the shell opening to feed and reproduce. These shells can vary in length ranging from 100 to 250 millimetres in length, or 10 to 25 cm although the larger lengths are incredibly rare. Eastern oysters do not reach sexual maturity at a specific age but more a specific size, 75 mm, which can be as quick as four months if the temperatures and nutrition levels are right. Oysters generally live for 6 years but if not threatened they can live for an incredibly long time – 30 years have been recorded for some long lived oysters. Eastern oyster shells are a grey colour, in the shape of a pear, and are covered in bumps, but the inside is a glistening white.
During the spring eastern oysters spawn. In huge clouds the oysters release sperm and eggs into the water where they will hopefully create fertilised eggs. Any fertilised eggs will be plankton floating in the sea for two days and then they develop the ability to feed. They then develop an ‘eye’, really light sensitive spots, and a foot that their future shell will develop around. This takes from 12 to 20 days. The oysters will then hopefully land on something hard as this will be where their shell cements itself for the rest of their lives. Now ceasing to be plankton they have entered the ‘spat’ stage. Eastern oysters are almost entirely male before sexual maturity and upon reaching maturity some will become female. Some oysters then will transition again becoming male. Spawning takes a lot out of oysters with them losing half of their weight during this – understandable considering over 75 million eggs are released during spawning! However, only a tiny fraction of those eggs will get to be fertilised, develop into a spat, find a suitable place to deposit themselves, and then become an adult.
There is a lot going on inside that shell. Inside the oyster is muscle with the very few organs it has working to keep it alive. Brains, for example, use a lot of energy so oysters have evolved to not have one, where several sensors negate the need for a brain. Some of these sensors detect light so if threatened they can clamp their shell shut. Eastern oysters try to keep their shells open as this allows them to suck water into their shells which passes over cilia, and from this cilia they get food and oxygen. They are incredibly efficient at this being able to suck in 5 litres of water an hour! Water actually becomes cleaner thanks to this as they are even able to remove toxins from water before expelling it. Some of the waste is not fully expelled with it eventually coming together forming a pearl, however this is incredibly rare and the pearls from eastern oysters are not valued.
Distribution and Habitat
The common name for these oysters is a reference to them being found on the eastern seaboard of the United States. They are not limited to the United States with them being found as far north as the Gulf of St. Lawrence and as far south as Brazil. Being found over such a wide area it is no surprise that eastern oysters can handle a wide variety of temperatures and salinities. The populations found in the Gulf of Mexico and West Indies do mature faster than more northern populations, largely as the warmth helps them respire faster. Oyster survivability is reliant on where they land so they are more commonly found near coasts and bays where they can safely develop. Sometimes ships have been known to have oysters growing on their hulls! Often, oysters themselves are the habitat. Oysters can grow on top of one another like corals do which can form huge reefs creating safe homes for countless species of algae, animals, and plants. Sadly most of these giant oyster beds have been wiped out but only a couple centuries ago ships could be endangered by these beds. Their hard shells and the quantity of them could easily do damage to a ship. Eastern oysters have been introduced to the west coast, Hawai’i, and parts of Europe and have become invasive as a result.
Diet and Predators
Oysters are filter-feeders taking microscopic organisms from the water, principally algae. This does a huge service to the ocean as they remove bits of dead organisms and the algae that feeds on that from the water. As a result, the water becomes cleaner and richer in oxygen allowing for life to flourish. Oysters are also capable of taking small amounts of toxins from the water making them essential in reversing the destruction humans have done to the oceans. Oysters are also an essential part of the food chain. As larvae filter feeders will eat them, as spats crabs can eat them, and the adults are eaten by a wide range of animals. These range from sharks to seabirds (including the American oystercatcher) and if close to the shore raccoons can eat them.
Conservation and Threats
Things are not looking pleasant for eastern oysters. The giant oyster beds that people wrote about even a century ago have been depleted, most famously in New York. For as long as humans have lived in New York they have used the oyster bed as a source of food (and their shells as jewellery). Thomas Downing became the ‘New York Oyster King’ based on his knowledge of oysters. Born to parents who had been enslaved in Virginia he moved to New York in the 1820s and used oystering to make a living before opening his first oyster restaurant. His story is remarkable and I cannot cover it in detail here – he would use the money rich whites paid him for the oysters to fund abolitionist movements – but by his death in 1866 the New York oyster bed was able to feed a city. However, sewage pumped into the New York-New Jersey bay put too many toxins into the water harming the oysters’ ability to spawn, and increased demand among the New York elite led to the beds being stripped bare. By 1927 the bed had vanished. Similar stories can be found everywhere with the Chesapeake Bay beds being reduced to 1% of its pre-industrial level. The infamous Deepwater Oil Spill in the Gulf of Mexico caused hundreds of oysters to be unable to spawn or directly killed them. While not evaluated by the IUCN the US conservation organisation NatureServe has classified eastern oysters as ‘G3 – Vulnerable to Extinction’.
There have been movements to help eastern oysters. The most noted examples of this are the Million Oyster Project in New York and the Massachusetts Oyster Project. These projects aim to rebuild the depleted oyster beds, albeit very slowly. The return of the oyster beds will help clear the coastal waters of toxins which will rejuvenate the local seas. However, it is a long task that will be disrupted by erratic weather caused by climate change. Furthermore, continued demand for oysters means they are still being harvested and oyster farms could be better used to help restoring the lost oyster beds. Time will tell if we can see the oyster beds return to their former glory, or if the damage is permanent.
Bibliography:
- Katherine McFarland and Matthew Hare, ‘Restoring oysters to urban estuaries: Redefining habitat quality for eastern oyster performance near New York City’, PLoS ONE, 13:11, (2018)
- Richard Wallace, ‘Cultivating the Eastern Oyster, Crassostrea virginica‘, Southern Regional Aquaculture Center, 432, (2001)
- ‘Eastern Oyster’, NOAA Fisheries, (07/06/2023), [Accessed 23/03/2024]
- ‘Eastern Oyster’, NatureServe, (15/01/2014), [Accessed 25/03/2024]
- Paula Osborne, ‘Crassostrea virginica‘, Animal Diversity Web, (1999), [Accessed 23/03/2024]
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