Oysters 101
Peter Frederick, Department of Wildlife Ecology and Conservation
University of Florida, Gainesville FL.
What other animal do you know of that strongly affects water flow, navigation, and ratings of gourmet restaurants? Furthermore, you can see them from space, so they MUST be important.
Oyster reefs are landforms made entirely of animals and their shells. Reefs provide the construction materials for many roads, buildings, and villages, food for civilizations, and have been the basis for wars, shifting state lines, and granting the first underwater property rights in history. The taste of a raw oyster has been described as “kissing the ocean on the lips”. People tend not to be ambivalent about oysters.
Figure 1. An intertidal oyster bar at low tide in South Carolina.
The American or Eastern Oyster (Crassostrea virginica) is found along the Atlantic and Gulf of Mexico coasts of North America, and its full range includes Nova Scotia to Brazil.
Oysters are bivalves, which means they have two shells, hinged at the umbo. The shell can be closed tightly for much of the day, depending on feeding opportunities and stresses like temperature, tide, and predation. Once settled, oysters do not move.
Filter feeding - Oysters feed by filtering water, at a rate of 2 – 4 gallons per hour per oyster. In so doing, oysters clean the water of sediment, flocculent material, and remove phytoplankton (diatoms, algae) as food from the water column. This process removes large amounts of nutrients from the water which is deposited in the substrate in the form of pseudofeces. Once deposited, the nutrients are in a form that does not go back into the water column easily. Fewer nutrients in the water means less chance of algal blooms, which may negatively affect dissolved oxygen, and light. Thus oyster reefs enhance conditions for many kinds of fish and shellfish, and help buffer conditions for submerged vegetation by keeping the water clear.
Reproduction and life cycle: Like most bivalves, oysters reproduce by broadcasting their sperm and eggs directly into the water column. Adult oysters begin reproduction when water temperatures become greater than about 68oF (~20oC). In much of the southeastern U.S. this occurs from May through October. A fertilized egg develops into a planktonic (free-swimming) trochophore larva in about 6 hours (Figure 2). A fully shelled veliger larva is formed within 12 to 24 hours. The larva remains planktonic (floating near the surface of the water) for about three weeks, and is only able to weakly move about in the water column, mostly being moved by tides and currents. Towards the end of this period it develops a “foot” and settles to the bottom of the water column where it seeks a hard substrate. Oyster larvae are attracted by the chemical signature of other oysters, and when a suitable surface (ideally adult oyster shell) is located, the larva cements itself and metamorphoses to the adult form.
This newly attached oyster is known as a "spat." Spat are highly vulnerable to predation, having very thin shells, and may be eaten by a variety of crabs and fish. “Spatfalls” often involve millions of spat cementing at the same time, and can literally cover the surface of oysters, rocks, boots, cement blocks, pilings, and clog the intakes of salt water pipes. Adult oysters change sex – they start off life as males when small, and become females as they grow larger.
Fig 2. Life cycle of the Eastern Oyster
Reef building: Oysters cluster together in reefs or bars for several reasons. Typically, bars are in good areas for feeding, which means they have high tidal flow and a good supply of food in the water. Oysters also need firm substrate to attach to, and therefore tend to attach to other oysters, a runaway process that results in cohesive reefs of continuous oysters. Once a reef gets started, it tends to grow at 90 degrees to the flow of water – which obviously alters the flow of water (see Figure 3). Reefs come in a spectacular array of shapes and sizes, largely depending on water flow, depth and shape of underlying substrate.
Fig 3. Oyster bars seen from the air near Cedar Key, Florida.
Salinity effects: Although most oyster species around the world live in ocean-strength salinities (36 parts per thousand salt, or ppt), the Eastern Oyster is a specialist on intermediate salinities. Estuaries are places where fresh and salt water mix, often where a river meets the ocean, and Eastern Oysters occur in their largest reefs and densities in estuaries. Oysters are typically found in sounds, bays, and tidal creeks from brackish water (5 ppt) to full strength seawater (35 ppt). You can easily use your tongue to assess salinity into high, medium and low categories.
Salinity and disease: Oysters can withstand wide fluctuations in salinity. However, at very low salinities, Eastern Oysters are physiologically stressed, do not filter water, and growth rates are extremely slow. At very high salinities, Eastern Oysters are vulnerable to a variety of marine predators, including predatory snails, fishes, crabs, turtles and rays. At high salinities, oysters also are vulnerable to parasitic diseases, including boring sponges, and several protozoal organisms. Perhaps the most common and devastating disease in the southeast is called Perkinsus marinus, or “Dermo”, which is vastly more common and virulent at high salinities – it saps the oysters of energy, greatly reduces growth and survival, and may curtail reproduction and result in death. With all these predators and diseases, oysters in the southeast typically do not live beyond 2 – 3 years of age. Eastern Oysters have fastest growth rates and highest survival and reproduction at intermediate salinities – 8 – 25 parts per thousand (Figure 4). In this range, the marine predators and parasitic diseases are reduced, but salinities are not low enough to cause physiological stress.
Vertical structure: In many northern parts of their range, the Eastern Oyster occurs only below the tide line, or in subtidal reefs. This is partly because if they are exposed by low tide in the winter, they may freeze repeatedly. But outside of the freezing zone, oysters and oyster reefs can grow vertically, well into the intertidal zone (between high and low tide marks). There are tradeoffs between daily feeding time (longest in subtidal zone) and risk of predation or parasitism (greatest in subtidal). In the more southern parts of their range, subtidal oysters may grow rapidly, but rarely form reefs because they are exposed to predators – they live fast and die young. In fact, the only place where oysters form subtidal reefs in the southeast is in estuaries where salinities are reliably intermediate. At the top of the intertidal zone, oysters may not be covered by water for a large portion of the tide cycle – and though they grow slowly as a result, they may live a long time. Obviously the best place to be is in the middle tidal zone. These limitations give oyster bars their characteristic mounded shape, with discrete edges.
Figure 4. Oysters can grow very quickly in good conditions. This block of concrete grew oysters 5” long in less than two months.
Ecosystem engineers: Because oysters can change landforms, alter flows and water quality, and create huge amounts of hiding places for other organisms, they are considered “ecosystem engineers”. During colonial times, oyster reefs in the Chesapeake Bay were so prevalent that they could filter the entire volume of the bay in two days. Oyster reefs have characteristic flora and fauna associated with them, and offer a huge surface area and numbers of nooks and crannies in which juvenile forms of fish, crabs and invertebrates are safe from larger predators. This makes oyster reefs very important as nursery grounds for larger fish and shellfish.
Oysters reefs also provide a number of other important ecosystem services, including serving as breakwaters during storms, reducing erosion on coastal areas and human structures, sequestering nutrients from the water column, and storing carbon in substrate. The value of these services has recently been estimated at $2,200 – $40,000 per acre. Thus oyster reefs are probably far more valuable for their ecosystem services than for the oysters we harvest from them. Because oyster reefs can grow vertically faster than sea level rise, they can be a remarkably sustainable “living shoreline”.
Endangered habitat: Oyster reefs are the most endangered marine habitat in the world, more so than coral reefs or seagrass beds. Over 85% of oyster reefs in the world are now considered dead or functionally irrelevant, having succumbed to some combination of the many threats – overharvest, pollution, sedimentation, disease, increased wave action, reduction of freshwater flows and other anthropogenic causes. Some of the last large, functioning populations are in the Gulf of Mexico.
In the Cedar Keys area, oyster reefs have declined in area by 66% overall, and offshore reefs by 88% in 30 years (Figure 5). Since these reefs are thought to be over 3,000 years old, this is an unprecedented rate of loss. In this otherwise pristine coast, the main cause is probably decreased freshwater flows, which results in many years with unnaturally high salinities. During these events, oysters are rapidly killed by exposure to predators and disease, leaving few or no live oysters on reefs. Wave action quickly erodes the surface of the reef, leaving little habitat for young oysters to grow on. When this happens, the reef is lost and cannot come back on its own.
Restoration: Oyster reefs can be restored using a wide variety of techniques, including putting dead shell out for spat to attach to, building mats of shell, making breakwaters of cement that oysters attach to, reducing harvest, changing water quality, spawning oysters in hatcheries and sowing the spat in likely places, and many other methods. In many places, oyster reefs are being considered an engineering tool to reduce erosion and increase water quality. In the case of the Cedar Key oysters, the ultimate solution for restoration is allowing more freshwater to make it to the coast. This is a large scale effort engaging the human population living in the entire northern quarter of Florida, with water users as far away as Jacksonville, and may take several political lifetimes to accomplish. In the interim, providing durable substrate in the form of shell in bags, and limerock is one method to allow young oysters to recolonize reefs following a dieoff event.
Figure 5. Lone Cabbage Reef at the mouth of the Suwannee River in 1982 (left) and 2010 (right), showing the rapid reduction in extent and size of the reef in only 30 years. The reef chain is hypothesized to hold fresh water against the coast, keeping conditions estuarine in the land-side of the reef.
Climate change, oyster reefs and coastal habitat: Climate change presents many kinds of risks for coastal habitats, and oyster reefs are not excluded. Changing weather patterns can substantially alter the amounts of freshwater that reaches estuaries, and in hotter, drier climates human use of surface water will increase, reinforcing the potential negative effects on estuaries. Climate change also means more variation in weather extremes, including more severe storms, in some cases heavier, more pulsed rainfall, and wider swings in weather behavior. Changing temperatures can also affect the distribution of many of the organisms, native and exotic, that make up coastal habitats. In Florida for example, mangroves may move northward, altering the makeup of salt marshes substantially. While many coastal habitats may be drowned by sea level rise, oyster reefs are somewhat robust to this process because they can grow vertically faster than current rates of sea level rise. For this reason, oyster reefs may be an important tool in managing the effects of sea level rise and concomitant erosion in the future. However, one of the effects of increasing storage of carbon dioxide in the oceans is to make the pH more acidic. This means calcium becomes less available for bivalves and many planktonic organisms to form shells, leading to an eventual collapse of marine food webs. Floridian coastlines may be somewhat insulated from this effect because the entire peninsula sits on limestone, which may buffer the effect of acidic ocean water. Perhaps the greatest question is the extent to which coastal habitats are able to move inland as sea level rises – both the speed of change and the degree to which humans limit or foment this process are critical variables.