Scientists identify new clam parasite

Organism may be factor in harvesting decline

June 02, 2000|By Ted Shelsby and Heather Dewar | Ted Shelsby and Heather Dewar,SUN STAFF

Marine scientists have identified a new parasite in the Chesapeake Bay that may be a major factor in the sharp decline of the state's soft-shell clam population.

The microscopic creature, which scientists dubbed Perkinsus Chesapeaki on Tuesday, is closely related to Perkinsus marinus, or Dermo, a pest that has devastated Chesapeake Bay oysters for the past 20 years.

Dermo and Perkinsus Chesapeaki were found in clams taken from 10 sites in the upper bay between 1990 and 1998. Scientists studying the new parasite say they aren't sure whether it can kill soft-shell clams, the variety that Maryland clam dredgers rely on for their catch. But the parasite may be the reason clams tend to die in hot, dry summers, and why the harvest is less than one-fifteenth of what it was a decade ago.

"We really don't know how much harm this organism causes, but we have seen some infestations that completely cover some of the animals' major organs," said researcher Mohamed Faisal of the Virginia Institute of Marine Science, a member of the team studying the parasite.

Soft-shell clams, often sold as "steamers" at restaurants, are the only commercially important clam species found in the Chesapeake Bay.

The creature prefers cold water and can't survive south of the upper bay. Few clams were harvested here until the invention of water-powered clam dredges in the 1950s. The harvest peaked at 659,000 bushels in 1969, but last year, it was 12,300 bushels, according to the Maryland Department of Natural Resources.

Watermen and scientists have long known that the clam population drops during drought years, when water temperatures rise and salinity is high. But they do not understand why, said Chris Judy, DNR shellfish division director.

The new research findings suggest that water temperature "may be encouraging the spread and severity of the disease," Judy said.

Lead researcher Shawn M. McLaughlin, an Oxford-based research biologist for the National Oceanic and Atmospheric Administration, stressed that it's too soon to say whether Perkinsus Chesapeaki is the reason for the oysters' decline.

It doesn't seem that the newly named parasite produces a poison, McLaughlin said. Nor does it appear that the organism can make people sick. Cooking kills most aquatic disease organisms, McLaughlin said, and soft-shell clams are rarely served raw.

McLaughlin said a parasite that resembled Dermo was spotted in clams as early as the 1950s. But until now, scientists had not identified the parasite as a new species, nor shown that Dermo can infect clams.

"We were surprised to see that Dermo survives on clams along with the new parasite," said Shaban I. Kotob, a research assistant professor for the Virginia Institute of Marine Science.

Don Comis, a spokesman for the federal Agricultural Research Service in Beltsville, said scientists never expected to see two different parasites attacking clams.

Research that identified the new parasite was a collaborative project of the National Oceanic Atmospheric Administration, Virginia Institute of Marine Science and the U.S. Department of Agriculture's Agricultural Research Service.

McLaughlin said the initial discovery of Perkinsus Chesapeaki was at a clam harvesting section of the Chester River near Rock Hall. It has been found at nine other sites, including Gibson Island on the Western Shore, and near the mouth of the Wye River on the Eastern Shore. She said additional water surveys need to be done to determine the extent of the Perkinsus Chesapeaki presence in the bay.

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