A fish pest previously unknown to science has been found infesting menhaden taken from the site of Maryland's most notorious fish kill: the 1997 Pfiesteria outbreak on the Pocomoke River.
U.S. Department of Agriculture fish pathologist Renata Reimschuessel found the unnamed attacker while using a high-powered microscope to look at Pocomoke River menhaden found with bloody sores. Alongside the newly discovered cells she also found the distinctive four-petaled spores of a previously known parasite, called kudoa, which sometimes infests menhaden and its relatives, alewives and Atlantic herrings.
Scientists say the finding is significant for three reasons:
It suggests that a two-stage process led to the 1997 outbreak on the Pocomoke, said David Goshorn, a Maryland Department of Natural Resources Pfiesteria expert. It could be that kudoa or its invasive partner first caused the fish to develop bloody lesions, and the large groups of injured fish triggered a toxic outbreak of Pfiesteria, Goshorn said.
It adds to a growing body of scientific research suggesting that schools of menhaden with lesions are not necessarily a sign of toxic Pfiesteria, thus forcing health and environmental officials to rely more heavily on other warning signs.
And Reimschuessel said it suggests that there may be some combination of factors that makes the Pocomoke vulnerable to aquatic pests. "It may be that conditions that are favorable to Pfiesteria could also affect the parasites, but that's not really known at this point," she said.
"It makes the Pocomoke an interesting place to try and understand," said University of Maryland researcher David Oldach, one of several scientists working with Reimschuessel to figure out the complicated chain of events that led to the Eastern Shore's bouts of Pfiesteria.
Discovered last year
Reimschuessel discovered the presence of kudoa and its unnamed partner over the winter, while looking at slides of menhaden taken from the Pocomoke in 1999. Kudoa turned up in 80 of the 114 fish she examined. And half of those with kudoa also were infected with the mysterious cell.
She went back to her collection of slides taken from lesioned fish during the 1997 Pocomoke outbreak and saw the same combination: flowerlike, 3-D kudoa spores and nearby, the dark-speckled fingerlike blobs of the other organism. It's possible that the unnamed cells are a previously unknown stage in kudoa's life cycle, or they may be something entirely different, Reimschuessel said.
"At the moment, we really don't know what they are," she said.
Kudoa and its unnamed partner were not responsible for killing fish or making people sick on the Pocomoke in 1997, Reimschuessel said. Other experts say Pfiesteria remains the most likely culprit there.
But one or both of the other creatures, and not Pfiesteria, probably caused the bloody sores that were originally blamed on Pfiesteria, Reimschuessel said.
"I certainly believe that Pfiesteria piscicida kills fish. That's been shown experimentally very nicely," she said. "But when you start saying that that organism causes the holes in fish, that's when you start running into problems."
For the past three years, scientists have been trying to figure outwhether Pfiesteria is capable of attacking fish on its own; whether it weakens the fish so that something else can kill them; or whether another organism comes first, making the fish vulnerable to Pfiesteria.
Goshorn said he thinks that there's no one answer; the chain of events probably varies from place to place and year to year.
Meanwhile, the scientific debate rages on, and each scenario has its whole-hearted partisans.
North Carolina State University researcher JoAnn Burkholder, a co-discoverer of Pfiesteria, has long asserted that the toxic organism, can do all the damage itself, causing fish to develop lesions in as little as 15 minutes. But Chesapeake Bay researchers say the fish they have sampled show signs of long-lasting lesions that developed over a week or more.
Researchers at the Virginia Institute of Marine Sciences blame the lesions on Aphanomyces invadans, a marine fungus responsible for fish kills in Asia.
Now Reimschuessel says the newly discovered, unnamed cell she found in Pocomoke fish seems to be "invasive" - able to attack otherwise healthy fish.
Survivors in Virginia
But there are signs that fish can survive kudoa alone.
It has been found in fish that had lesions, but survived them, in Virginia's James River. And earlier this month, something caused large numbers of menhaden in the Pocomoke to develop lesions. But the fish didn't die, and nine of 11 water tests taken at the time showed no traces of Pfiesteria, while the other two tests left open the possibility of low levels of Pfiesteria, DNR officials said.
Reimschuessel said she plans to test lesioned menhaden from the Pocomoke this summer for signs of kudoa and its newly discovered companion.
In May, scientists at the University of Maryland's Center for Environmental Research took some young, lesioned menhaden from the Pocomoke and raised them in the laboratory. The lesions healed in about a month, said research assistant John Jacobs.
"This is definitely a piece of the puzzle," Jacobs said, "but it's going to take more research to hone in on what's going on."