NEWPORT, Ore. - Scientists have identified a prolific parasite that preys on mud shrimp - a native species of West Coast estuaries - and threatens to decimate mud shrimp populations, raising concern for the fragile, complex ecosystems of these coastal inlets.
This bopyrid isopod, known as Orthione griffenis (or Griffen's isopod) is a form of aquatic crustacean that enters the shrimp gill chamber under the carapace. It destroys the shrimp's ability to reproduce by sucking their blood or nutrients. Oregon State University's John Chapman, an invasive species expert, thinks the parasite is a non-native species, probably introduced to West Coast waters through ballast water released from ships.
"If we're right," Chapman said, "this may be the most significant ballast water introduction of a non-native species yet discovered on the West Coast."
The zebra mussel, the most well-known invasive aquatic species in the United States, has thus far been restricted to the East Coast and Great Lakes, where it has caused millions of dollars in damages to dams, ships and structures.
A professor of fisheries and wildlife at OSU, Chapman heads the Biological Invasions Program at the university's Hatfield Marine Science Center in Newport. He and his colleagues already have found the parasite in Yaquina Bay, Alsea Bay, Siletz Bay and Tillamook Bay in Oregon, as well as in Willapa Bay in Washington. There also are reports of the isopod as far south as Santa Barbara, Calif., and as far north as British Columbia, he added.
Chapman says the parasite's impact on mud shrimp populations is difficult to estimate. In 1999 and 2001, Ted DeWitt, an HMSC ecologist with the Environmental Protection Agency, conducted mud shrimp surveys in Yaquina Bay. This summer, Chapman and colleagues are working with Lincoln County natural resource crews on new surveys that will give them an idea of the early impact of the parasite on mud shrimp numbers, though the effect of limited reproduction may take time to complete.
"Nothing we know already provides reason to be optimistic," said Chapman, who added that all of the mud shrimp populations they've investigated this summer have been infested with the parasite.
The researchers estimate an overall parasite infestation rate as high as 45 percent and believe that 80 percent or more of the breeding-sized adults may be infested. Once infested, reproduction - almost without exception - is halted.
Humans use mud shrimp primarily as fishing bait, but they are valuable prey for birds, fish, and other animals in estuaries. The mud shrimp are a dominant species in many Oregon estuaries, comprising the greatest biomass in many intertidal mudflats. Mud shrimp feeding may filter as much as 80 percent of the water per day in some estuaries.
Chapman says removing a dominant species from any ecosystem can have large impacts.
"It's hard to guess what the removal of mud shrimp would mean to the estuary," Chapman said, "but because they are so abundant and filter so much of the water, we have to be concerned. Mud shrimp also are important in the sediment dynamics of estuaries and their loss could conceivably lead to greater erosion. There also are signs that where mud shrimp are disappearing, populations of sand shrimp increase. Both species cause problems for oyster growers, but sand shrimp may be worse."
Chapman and Brett Dumbauld, a U.S. Department of Agriculture ecologist working out of OSU's Hatfield Marine Science Center, recently examined 42 female mud shrimp from Yaquina Bay during the winter breeding season and found that only eight of them had eggs. The rest were infested with the parasite. Only one of the eight females that had eggs was infested - and she had just 15 eggs. Normally, females produce 1,800 to 11,000 eggs.
The message, Chapman said, is that infestation cuts off reproduction. He is working to find out why. "Mud shrimp don't begin to reproduce until their carapaces are about 20 millimeters long, and these parasites seem to have targeted them by that time," he said.
Chapman said this parasite is huge compared to previously discovered species. At eight-tenths of an inch, Griffen's isopod is the largest bopyrid isopod ever seen on the West Coast.
"Over the past 140 years, virtually every new parasitic isopod species discovered has been smaller than previously known species," he said. "In comparison, this recently discovered species is a monster. Because of its great abundance, large size and it occurrence also in Japan, we are sure this is an introduced species, and not an overlooked native species.
"If you had a water buffalo in your back yard," he said, wryly, "you would notice it."
The parasitic isopod primarily targets the mud shrimp, Upogebia pugettensis, but doesn't seem to affect sand shrimp. Though the two species appear to be similar, sand shrimp are burrowing animals that get their nutrients from the sand, while mud shrimp draw water down into their mud tubes and filter it through their feeding baskets.
Chapman said this parasite wasn't fully identified until this past winter. Since then, he and other scientists have been scrambling to learn more about it. They have been able to trace its appearance on the West Coast back about 20 years by combing through references in scientific literature, examining samples and/or photos of mud shrimp used in other studies, and working with biological museums. Its numbers, however, have been very small.
Something caused the population to "take off" over the last few years, Chapman said, and scientists aren't sure why. Unusual ocean conditions, characterized by changes in upwelling, may have played a role by maximizing their growth and reproduction at a time when other species have suffered. Or more of them may have been introduced to West Coast waters in recent years.
Chapman said the origins of the parasitic isopod may be in Asia. Japanese taxonomist Gyo Itani of the Center for Marine Environmental Studies at Ehime University found a Japanese bopryid isopod that is morphologically identical to Orthione griffenis.
How the parasite arrived at the West Coast is a matter of guesswork. More important, Chapman said, is trying to understand how prevalent it is and what impact it may have.
Dumbauld has been working with oyster growers in Willapa Bay for years studying the impact of mud shrimp on the industry. Some local populations of the shrimp there have almost completely disappeared, and the researchers suspect the parasite is to blame. The few remaining Willapa Bay mud shrimp are heavily infested.
Chapman said it doesn't appear that the isopod invasion will disappear soon. Water samples from the bay are full of the parasites at their early stages of life.
Female Orthione griffenis brood their young and may release as many as 60,000 epicarid offspring. These epicarids migrate out of the estuaries and into the ocean, where ey attach themselves to copepods and parasitize them before moulting into their final dispersal phase, known as "cryptoniscans." These cryptoniscans return to estuaries and apparently seek out mud shrimp as final hosts. Their abundance in the seawater covering mud shrimp communities is surprising, Chapman said.
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