"He was a bold man that first eat an oyster," wrote Jonathan Swift. And indeed with its hard rock-like exterior and gray, slimy insides, it's amazing that anyone opted to eat them. But since the days of the ancient Greeks, oysters have been a delicacy, and they were an important food source for early Americans. Today, Americans consume more than 50,000,000 pounds of oysters each year.
The eastern, or American, oyster (Crassostrea virginica) can be found from the Caribbean Sea to Canada's Gulf of St. Lawrence and flourishes in estuaries like the Chesapeake Bay. It can survive in a wide range of temperatures (28°F to 90°F) and salinities (10 ppt to 30 ppt). It can even tolerate daily exposure to air as the tide recedes, exposing portions of its home, or reef. Considering these hearty life history characteristics, one would expect the oyster to survive most anything, but human impact and natural diseases threaten the oyster's viability.
The oyster's life begins in late spring/early summer when water temperatures approach 70°F. Male and female oysters release sperm and eggs into the water column where fertilization occurs. Hours after an egg is fertilized, it develops into a trocophore larval stage, and a day or two later a veliger which spends the next three weeks swimming and feeding in the water column. After three weeks, the veliger (only 0.01 inches long) settles on a hard substrate, often another oyster, and metamorphoses into a juvenile oyster, or "spat." Now settled, the tiny juvenile oyster spends its days feeding by pulling water over its gills and filtering out particles from the water column. A single adult oyster can filter 60 gallons of water per day, making it nature's own water filter! It grows to maturity in about a year and reaches market size (~3 inches) in one to three years.
The eastern U.S. coastline was once replete with oysters, but as early as the late 1800s over-harvesting caused the oyster populations to suffer significant decline. This decline has been exacerbated by habitat loss due to water quality, sedimentation problems, and oyster diseases.
MSX and dermo are the two best-known diseases impacting oysters. Neither of these diseases can affect humans, but they can be fatal to oysters. MSX first appeared in the late 1950s in the Delaware Bay and now spans much of the east coast. MSX causes poor meat quality and death. Dermo, which surfaced in the 1940s, is associated with warm water temperatures (though it can be found as far north as Maine) and causes poor growth and death. In 1988, a new disease, Juvenile Oyster Disease (JOD), made its appearance in New England in hatchery-produced oysters. JOD is believed to be caused by a bacterium and results in stunted growth, uneven shell lengths, and mortality rates exceeding 90% of juvenile oysters. It is currently restricted to certain areas in the Northeast. Scientists at the University of Maine are conducting a number of studies on JOD to answer questions and find potential solutions to the disease.
Data Activity
In this data activity, students will use research data from the University of Maine to determine if oyster size, time of planting, or water temperature is significant to the onset of juvenile oyster disease.
Have students work in pairs or threes. Print out the data tables and assign each group a cohort (age class) of oysters. Have students graph cumulative mortality (death) over time and shell height over time for their cohort. Using their graphs, have students answer the following questions:
- What percentage of oysters in your cohort died by the end of the study?
- Was there a sharp increase in the mortality rate during the study? If so, when did it occur? What was the average shell height of the oyster at this point? What was the water temperature?
- Was there a point when the mortality rate slowed down? If so, when did it occur? What was the average shell height? What was the water temperature?
Once each group has completed their graphs and questions, have them share their results with the rest of the class.
- Overall, which cohorts had the most survivors? When were they deployed?
- Which had the worst survival?
- Do you see any connection between mortality rate and the size of the oyster, time of planting, or the water temperature?
- What impacts might these results have for oyster growers?
The results of this study indicate that both oyster size, time of planting, and water temperature are important factors in how well an oyster fairs against JOD. Scientists have concluded that, in general, the disease does not affect oysters over 25 mm in shell height. They also found that increased water temperature increases JOD-induced mortality. The oysters deployed early in the spring that reached 25 mm in length before mid-summer and the oysters deployed in the fall after the heat of the summer had much lower mortality rates.
These findings have had a large impact on oyster growers. Previously, seed oysters were produced and planted throughout the spring and summer. Now, oyster growers realize the need to plant all their oysters very early or very late in the season in order to avoid JOD.
Special thanks to Dr. Ryan Carnegie and at the Virginia Institute of Marine Science and Dr. Bruce Barber at the University of Maine for providing scientific information for this Data Tip.
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