Mollusks

Oyster Reef
Oyster reef along the Matanzas River at low tide

NPS Photo

When it comes to eating oysters it seems that people either love them (raw, baked, steamed, however) or they hate them (don’t let that glob of phlegm near my plate). But in terms of oysters’ place in the environment, informed people can agree- they are essential to a healthily functioning ecosystem. At Fort Matanzas oysters can be found growing along the Matanzas River shoreline and in the adjoining marshlands. Two species have been documented in the park- the eastern oyster (Crassotrea virginica) and the crested oyster (Ostreola equestris). Both of them are examples of bivalve mollusks, meaning they have two similarly-shaped shells hinged at one end. The hard shell is actually a protective covering that is produced by the living oyster using an organ of its body called the mantle. The mantle utilizes minerals present in the water in which the oyster lives to slowly construct the shell, beginning when the oyster is very young. Over a twenty year lifespan the shells take on variable shapes, becoming very corrugated, rough, and extremely sharp. (One oyster shell found in the park measured seven inches long!)

Larval oysters, minute and free-swimming forms called veligers and pediveligers, are attracted to adult oysters that have already formed shells. The larvae tend to settle on the older oysters (at which point the larvae are called “spat”) and in this way oyster reefs are created over long periods of time. It takes spat about eight months to reach maturity and become capable of spawning. When they spawn they release eggs or sperm into the water depending on the sex of the oyster. But the trick with oysters is that some are hermaphroditic, possessing both male and female sex organs. An individual oyster can spawn many times per year, and each time release a different set of gametes (sperm or egg cells) depending on whether it’s functioning as a male or female!

As oyster reefs grow over time they can become quite extensive and end up being significant structural and functional components of estuaries, places where fresh and saltwater mix. The reefs act as habitat for many smaller fish and invertebrate species, which are in turn eaten by larger fish and wading birds. The physical structure of the reef helps tamp down wave energy and thus protects adjacent shorelines from erosion and storm surge. The oysters themselves are a food source for many animals, including humans! But what oysters are really good at is filter-feeding. The water column is full of particulate matter which can make a body of water look extra murky. Oyster gills (by which it obtains oxygen from the water) are covered with cilia, minute hair-like projections that are able to trap organic particles that are present in the water flowing over the gills. Amazingly, even one adult oyster is capable of filtering up to fifty gallons of water per day through its body! Multiply this by the thousands of oysters that can be found in a large reef and it’s easy to see how these colonial organisms have such a positive impact on water clarity in an estuary.

But oysters can only fulfill their important ecological role if they themselves are healthy. Unfortunately, oyster habitat has decreased by 85% worldwide in historic times. Reasons for this alarming decline include destruction of habitat to make way for development, water pollution, overharvesting of reefs, sedimentation as soil from cleared land is washed into waterbodies, and acidification- excess carbon dioxide being absorbed by the ocean, making it harder for creatures like oysters to construct their calcium carbonate shells. National parks alone can’t solve all these problems. It’s going to take all of us!

Last updated: April 18, 2017

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