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CREATURE FEATURE 2 – EASTERN OYSTER

If Apalachicola Bay had an animal mascot, it would have to be the eastern oyster (Crassostrea virginica). While there is any number of more glamorous options, no other resident of the estuary is so physically, and ecologically, attached to the bay. At first glance, this simple bivalve (two shells) appears to be a pretty boring creature. However, on closer examination, we find that the oyster is an amazing animal and an essential part of the estuary ecosystem.

The evolution of the eastern oyster stretches back hundreds of millions of years and has resulted in an animal that has developed several unusual traits. The oyster starts its lifecycle as adult oysters release eggs and sperm into the water. A single female can produce 100 million eggs, but only about one in a thousand will survive. Fertilized eggs quickly develop into free swimming larvae, and shell development begins. After a few weeks the larvae start to settle to the bay bottom where they grow a foot that helps them explore, looking for a hard substrate to attach to. Once a suitable hard surface has been located, usually an existing oyster reef, a cement like substance is excreted, permanently attaching the left valve. This is where the oyster will spend the rest of its life. At this stage oysters are called spat. Oysters generally start life as males, changing to females as they mature. Some will change sex several times over their lifetime. I told you they weren't boring. Oyster reefs form habitats for a large number of estuary species, providing both food and shelter. Oysters, and the reefs produced by their shells, are an essential part of the estuary food web and ecosystem.

Oysters are filter feeders pumping as much as 50 gallons of water a day through their bodies. Whatever is in the water is in the oysters. Because of this oysters are an excellent indicator of the health of an estuary bay. Estuaries are very complex systems located where fresh water empties into and mixes with salt water. They are known as the "nurseries of the oceans" because so many marine species come to them to spawn. One of the essential pieces of the estuary puzzle is a clean, nutrient rich source of fresh water. The nutrients carried into the bay by the fresh water source form the base of the food web in the estuary. It is these nutrients that the oysters are filtering out for their food. These nutrients, either directly or indirectly, are also the food for many of the marine babies growing up in the estuary. This is one reason why the health and flow of the Apalachicola River is such a concern. If the river is compromised, the bay will be adversely affected. Low river flows mean an increase in salinity and a decrease in nutrients in the bay. Many of the oysters' predators prefer a higher salinity. So, in addition to a decreased food source, low river flows also means the oysters have a greater chance of becoming food for something else.

I don't know about you, but I prefer my oysters to be kind of salty, too. Humans have been harvesting and consuming oysters here for over a thousand years. Apalachicola Bay is famous for producing 90% of the oysters harvested in Florida and 10% of the national oyster harvest. But we have to be careful. If the salinity in the bay rises too high, and stays that way for too long, we could see this interesting and delicious bivalve disappear from Apalachicola Bay forever.

 

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