Shells Reveal Change in Oysters

400-year-old remnants show that creatures grew faster than they do today

By PAULA NEELY

A study of 400-year-old oyster shells discovered in a Jamestown well used by colonists shows that eastern oysters grew significantly faster then than oysters today, a clue that may help shed light on the plight of the modern oyster.

The Virginia Institute of Marine Science study provides the first documented evidence that oysters function differently than they did in the early 1600s.

They were larger than modern oysters the same age, which probably would have made them "exponentially more capable of reproducing, filtering water and making shell," said Juliana Harding, senior marine scientist at VIMS.

Roger Mann, professor of marine science at VIMS, said the slower growth rate of modern oysters may be the result of changes in water quality or sedimentation -- which can bury oyster habitat -- diseases, or a combination of these factors.

Researchers studied shells unearthed in a 1609-1616 well discovered inside the James Fort site at Historic Jamestowne in 2006. Archaeologists contacted VIMS and other organizations to find out what shells and other organic artifacts preserved in the watery environment could reveal about the Chesapeake before the impact of European colonization.



As they produce their shells, oysters, clams and other mollusks record biological information about their age and growth just like trees store information in growth rings. They also incorporate minerals from the water and lay down a record about their environment that can potentially provide information about water temperature and salinity levels.

"They're like a million environmental barometers spread all over the place that have recorded everything that's happened at that spot throughout their entire life history," Mann said.

To select shells suitable for comparison, Harding sorted through about 3,000 oyster shells deposited in the well after colonists began using it as a trash pit.

To determine changes in growth rate, researchers measured the shell lengths and compared the historic oysters with modern oysters of the same age from similar sites in the James River that had the same salinity level as Jamestown in the early 1600s.

Studies of mineral deposits in the 400-year-old shells of tiny crustaceans and marine protozoans from the Chesapeake Bay were used to determine that salinity levels around Jamestown were 10-15 parts per thousand higher than they are now.

In 1617, oyster reefs in the James River near Jamestown were large enough to be navigational hazards, indicated as small islands on a 17th century map drawn by Johannes Vingboons, a Dutch cartographer. Harding said they would have been visible at low tide.

Although oysters can live 10 to 20 years, Harding said most modern oysters die before they are two or three years old, mainly because of diseases, harvesting and habitat degradation.

"Restoring them is not simply a matter of putting more oysters in, leaving them alone and expecting the same results we had 400 years ago," she said. "You need to plan and accommodate for things that are out there now that were not there then. That's a step that has not always been acknowledged."
Results from the study will be published in the Journal of Shellfish Research in December. For further information, visit www.vims.edu/mollusc/research/mehoyJT.htm