NEWPORT, Ore. - Scientists have identified a prolific parasite thatpreys on mud shrimp - a native species of West Coast estuaries - andthreatens to decimate mud shrimp populations, raising concern for thefragile, complex ecosystems of these coastal inlets.
This bopyrid isopod, known as Orthione griffenis (or Griffen'sisopod) is a form of aquatic crustacean that enters the shrimp gillchamber under the carapace. It destroys the shrimp's ability toreproduce by sucking their blood or nutrients. Oregon StateUniversity's John Chapman, an invasive species expert, thinks theparasite is a non-native species, probably introduced to West Coastwaters through ballast water released from ships.
"If we're right," Chapman said, "this may be the most significantballast water introduction of a non-native species yet discovered onthe West Coast."
The zebra mussel, the most well-known invasive aquatic species inthe United States, has thus far been restricted to the East Coast andGreat Lakes, where it has caused millions of dollars in damages todams, ships and structures.
A professor of fisheries and wildlife at OSU, Chapman heads theBiological Invasions Program at the university's Hatfield MarineScience Center in Newport. He and his colleagues already have found theparasite in Yaquina Bay, Alsea Bay, Siletz Bay and Tillamook Bay inOregon, as well as in Willapa Bay in Washington. There also are reportsof the isopod as far south as Santa Barbara, Calif., and as far northas British Columbia, he added.
Chapman says the parasite's impact on mud shrimp populations isdifficult to estimate. In 1999 and 2001, Ted DeWitt, an HMSC ecologistwith the Environmental Protection Agency, conducted mud shrimp surveysin Yaquina Bay. This summer, Chapman and colleagues are working withLincoln County natural resource crews on new surveys that will givethem 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," saidChapman, who added that all of the mud shrimp populations they'veinvestigated this summer have been infested with the parasite.
The researchers estimate an overall parasite infestation rate ashigh as 45 percent and believe that 80 percent or more of thebreeding-sized adults may be infested. Once infested, reproduction -almost without exception - is halted.
Humans use mud shrimp primarily as fishing bait, but they arevaluable prey for birds, fish, and other animals in estuaries. The mudshrimp are a dominant species in many Oregon estuaries, comprising thegreatest biomass in many intertidal mudflats. Mud shrimp feeding mayfilter 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 theestuary," Chapman said, "but because they are so abundant and filter somuch of the water, we have to be concerned. Mud shrimp also areimportant in the sediment dynamics of estuaries and their loss couldconceivably lead to greater erosion. There also are signs that wheremud shrimp are disappearing, populations of sand shrimp increase. Bothspecies cause problems for oyster growers, but sand shrimp may beworse."
Chapman and Brett Dumbauld, a U.S. Department of Agricultureecologist working out of OSU's Hatfield Marine Science Center, recentlyexamined 42 female mud shrimp from Yaquina Bay during the winterbreeding season and found that only eight of them had eggs. The restwere infested with the parasite. Only one of the eight females that hadeggs was infested - and she had just 15 eggs. Normally, females produce1,800 to 11,000 eggs.
The message, Chapman said, is that infestation cuts offreproduction. He is working to find out why. "Mud shrimp don't begin toreproduce until their carapaces are about 20 millimeters long, andthese parasites seem to have targeted them by that time," he said.
Chapman said this parasite is huge compared to previously discoveredspecies. At eight-tenths of an inch, Griffen's isopod is the largestbopyrid isopod ever seen on the West Coast.
"Over the past 140 years, virtually every new parasitic isopodspecies discovered has been smaller than previously known species," hesaid. "In comparison, this recently discovered species is a monster.Because of its great abundance, large size and it occurrence also inJapan, we are sure this is an introduced species, and not an overlookednative 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, Upogebiapugettensis, but doesn't seem to affect sand shrimp. Though the twospecies appear to be similar, sand shrimp are burrowing animals thatget their nutrients from the sand, while mud shrimp draw water downinto their mud tubes and filter it through their feeding baskets.
Chapman said this parasite wasn't fully identified until this pastwinter. Since then, he and other scientists have been scrambling tolearn more about it. They have been able to trace its appearance on theWest Coast back about 20 years by combing through references inscientific literature, examining samples and/or photos of mud shrimpused in other studies, and working with biological museums. Itsnumbers, however, have been very small.
Something caused the population to "take off" over the last fewyears, Chapman said, and scientists aren't sure why. Unusual oceanconditions, characterized by changes in upwelling, may have played arole by maximizing their growth and reproduction at a time when otherspecies have suffered. Or more of them may have been introduced to WestCoast 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 EnvironmentalStudies at Ehime University found a Japanese bopryid isopod that ismorphologically 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 itis and what impact it may have.
Dumbauld has been working with oyster growers in Willapa Bay foryears studying the impact of mud shrimp on the industry. Some localpopulations of the shrimp there have almost completely disappeared, andthe researchers suspect the parasite is to blame. The few remainingWillapa Bay mud shrimp are heavily infested.
Chapman said it doesn't appear that the isopod invasion willdisappear soon. Water samples from the bay are full of the parasites attheir early stages of life.
Female Orthione griffenis brood their young and may release as manyas 60,000 epicarid offspring. These epicarids migrate out of theestuaries and into the ocean, where ey attach themselves to copepodsand parasitize them before moulting into their final dispersal phase,known as "cryptoniscans." These cryptoniscans return to estuaries andapparently seek out mud shrimp as final hosts. Their abundance in theseawater covering mud shrimp communities is surprising, Chapman said.
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