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Habitat Focus: Oyster Reefs

Wednesday, February 24th, 2016

But I don’t eat oysters, so why should I care about oyster reefs?

Even if you do not eat oysters, you should appreciate our Gulf coast oyster reefs. Oyster reefs are a historically common and important coastal ecosystem, they are responsible for many critical ecosystem services and, in some places, your tax dollars are being spent to restore them. ACER scientists are investigating the impacts of the Deepwater Horizon event on this critical coastal ecosystem.

The term oyster reef refers to an area in which clumps of oysters occur close together and in higher densities than in surrounding areas. The Gulf’s oyster, Crassostrea virginica, reproduces by releasing eggs and sperm into the water. Once sperm meet egg, and the egg begins to develop, a planktonic larva is formed. This free-swimming larval stage is carried by water currents and, usually within weeks, settles to the bottom. Although the exact cue for settlement is not known, larvae (known as spat once they settle) prefer to settle on hard surfaces such as other oyster shells.

This pattern results in the formation of clumps of oysters: aggregations of clumps are known as reefs. In the Gulf of Mexico, these reefs can occur subtidally (always covered by water) or intertidally. In some areas, clumps occur on top of clumps and the reef is multiple feet thick (a high relief reef): in other areas, clumps of shells and living oysters are spread across the bottom and the reef is said to have low relief.

Oyster habitat
A common sight along the northern Gulf coast are these low relief, fringing oyster reefs adjacent to marsh habitat. Photo credit: M. Schrandt

Oyster clumps provide surfaces and nooks and crannies in which other organisms can live. Invertebrate animals, such as barnacles, tube worms, sponges or mussels settle on the shell surface. More mobile animals such as crabs, snails and gobies (small fish), find homes within the spaces between shells. As fishermen know, larger organisms such as blue crabs, drum and rays are found in close proximity to reefs, eating oysters or these associated species. Thus, the density of organisms (number of organisms per area) is higher on oyster reefs than in surrounding areas.

A spadefish caught hanging around a restored subtidal reef near Dauphin Island, AL. Photo credit: D. Byron

The oyster is a filter feeder. Small hair-like structures on an oyster’s gill beat to create a flow of water through an oyster’s body and trap small particles carried in this current. By removing these particles from the water, either ingesting them or rejecting them but depositing them on the bottom, oysters improve water clarity and quality. For an aggregation of oysters like a reef, this filtration capacity is truly amazing. An individual oyster can filter about 75-190 liters (20-50 gallons) per day: simple multiplication demonstrates that, for example, an area with 100 oysters per square meter can filter 7500-19,000 liters (2000-5000 gallons) per day. A reef that is 10 meters long and 5 meters wide would have about 5,000 oysters which would filter 375,000 to almost 1 million liters per day. Lastly, the physical structure of the reef affects wave energy hitting shorelines and may serve to reduce rates of shoreline erosion. These ecosystem services - creating habitat, providing food, improving water clarity and quality and protecting shorelines - are important ecosystem services provided by oyster reefs.

Globally, the area covered by oyster reefs has dramatically declined. In the Gulf of Mexico, scientists estimate that this decline is between 85-90% of their historical abundance. The decline is due to a variety of factors; historical overharvesting, degraded habitat quality, harvesting techniques, hurricane impacts, disease and even disrupted food web relationships. With this dramatic decline came the loss of associated ecosystem services. ACER scientists are clarifying and quantifying the impacts of the oil spill on oyster reef diversity and the associated ecosystem services. Results will be used to develop scientifically relevant restoration plans and to guide future oyster reef restoration activities.

Sampling for oysters
Captain Joe Dear and intern George Jarvis sampling an intertidal oyster reef on Grand Isle, Louisiana. Photo credit: M. Schrandt