Mud Shrimp Meets Invasive Parasite

High Drama For Northwest Estuaries

By Eric Wagner

At first glance, mud shrimp might seem an odd choice for a poster child. They are not the least bit photogenic. They burrow deep in the mud, sometimes more than two feet under the surface, and turn intertidal mudflats into a soupy muck. They have beady eyes.

And yet, mud shrimp have found themselves in the thick of a tangle involving invasive species ecology and the shellfish industry. Their predicament shows just what happens when biologists and business interests differ on the best management strategy for Northwest estuaries.

The problem is this: To some, mud shrimp are a vital component of a complex ecosystem. A non-native parasite threatens them, and their decline could lead to ecological collapse. To others, the shrimp are themselves pests that must be controlled by whatever means available. And while neither side can confidently predicate the ultimate fate of mud shrimp, both feel that time is running out.

Invasive parasite raises concerns

A researcher at Oregon State University's Hatfield Marine Science Center in Newport, John Chapman sometimes eats his lunch on the shore of Yaquina Bay. During a high tide, he says, he can watch green coastal waters flow into the bay from the ocean. Over the course of an hour or so, the water slowly clears as intertidal organisms filter out the nutrients it brings in. The change is remarkable: from murky to translucent in 60 minutes.

One of the creatures filtering the most water is the mud shrimp. In Oregon estuaries, mud shrimp filter as much as 80 percent of the bay water per day, estimates Ted Dewitt, an ecologist with the U.S. Environmental Protection Agency. They are also an important food source for birds, fish, and other animals. Now, their populations are declining markedly. In some west coast estuaries, notably Willapa Bay, Wash., they have all but disappeared.

Chapman thinks he knows one of the reasons why. He recently identified an invasive parasite that exclusively attacks mud shrimp. The offender is an isopod, Orthione griffenis. Isopods are invertebrates that come in a variety of forms, the most familiar being the sow bug scurrying around people's basements. Orthione griffenis lodges itself in the mud shrimp's gill chamber, sucking blood and other nutrients. While the isopod does not kill the shrimp outright, it causes significant weight loss and halts shrimp reproduction.

The isopod has been reported from Santa Barbara, Calif. to British Columbia. Chapman thinks it arrived via ballast water, probably on container ships sailing from Japan. Such ships take on millions of gallons of water before departure to aid stability and propulsion. However, they also take with them millions of tiny hitchhikers, from single-celled diatoms to larval mollusks and crustaceans. These are all dumped into new environments when the ship empties its ballast tanks after it reaches its destination. Usually, the animal and plant stowaways do not survive, much less establish.

But occasionally some do, and if they become pests, they are called invasive species. On the East Coast, the best known is the zebra mussel, which causes millions of dollars in damage every year to waterways. On the West Coast, scientists keep a wary eye on the progress of the green crab and mitten crab as they steadily creep up the coast from San Francisco Bay.

For its part, Orthione griffenis was first seen in 1983 in Coos Bay, Ore. Although it has since persisted in low numbers, its population recently skyrocketed. The increase corresponds with strong El Nino years, times when summer coastal waters are warmer and prevailing ocean currents come from the south. "We're finding its larvae in huge numbers in the plankton," Chapman says. "It's just staggering."

Chapman and Brett Dumbauld of the U.S. Department of Agriculture recently examined 42 reproductive female shrimp in Yaquina Bay. The parasite had infected 35 of them, and 34 had no eggs. The one with eggs had only 15. Mud shrimp typically produce between 1,800 and 11,000 eggs.

Even more alarming is the overall rate of infestation in mud shrimp populations, says Chapman. Up to 45 percent of all shrimp have the parasite in some estuaries, and 80 percent of breeding adults are infected.

The potential implications are dire, according to Chapman. "Introduced species are one of those organisms where all bets are off,” he says. "The wolf's out the door now, and we have to do something about it.”

The plot thickens

If this were the standard story of an invasive species, it would have ended there, with wildlife managers pledging everything in their under-funded power to save the beleaguered mud shrimp from the parasitic isopod. But this is not the standard story. In fact, the same managers already had their eyes on the mud shrimp, and with decidedly different aims. For years, they were trying to make the mud shrimp go away.

In Washington State, shellfish aquaculture is a 76 million-dollar-per-year industry, and Willapa Bay is the largest producer of farmed shellfish in the U.S. To shellfish growers, mud shrimp are a pest. With their cousin the ghost shrimp, they belong to a family known as burrowing shrimp. When burrowing shrimp dig into the mud, they loosen the sediment, stirring it around. Consequently, free-swimming shellfish larvae are unable to settle on their preferred hard substrates. Existing shellfish sink into the muck and die.

Bill Dewey is a shellfish farmer and public affairs manager for Taylor Shellfish Farms, Inc., one of the largest commercial producers of shellfish on the West Coast. He says that Taylor Shellfish started working to control the shrimp in 1957, when burrowing shrimp populations grew to such levels that their burrows stippled the flats like "Swiss cheese."

"Burrowing shrimp as a whole have had a devastating effect on production in coastal estuaries,” says Dewey. "Controlling them is beneficial to the estuary because removing shrimp boosts diversity."

Since the 1960s, shellfish growers have battled mud shrimp using Carbaryl, an organic pesticide. It is also known by its commercial name, Sevin. Carbaryl controlled burrowing shrimp populations well enough, but it also had negative impacts on juvenile Dungeness crab, another important commercial species. Additionally, the Washington Toxics Coalition was concerned about Carbaryl's effects on water quality. It eventually led a legal challenge against a water quality permit issued to oyster growers in Willapa Bay and Grays Harbor. In 2003, the oyster growers agreed to phase out Carbaryl use by 2012.

Suddenly deprived of their primary weapon against burrowing shrimp, the growers are uneasy. The shellfish industry has to find a means of controlling the shrimp, Dewey says, or the industry will not survive.

Now, along comes Orthione griffenis, which may be a savior of sorts. "One of the things growers have been looking at is biological controls," Dewey said. "We're searching for a suite of tools to put in our toolbox, and this isopod appears to be doing its job without our doing anything to enhance it." The growers would be content, then, to take their chances with the isopod.

But Dumbauld, who has studied the shrimp for more than ten years, has a different take. "Shrimp are a valuable part of the estuary, whether the growers want to acknowledge it or not," he says. "They're a native species and the oysters aren't. Still, they're both important to an estuary's ecology, and it may be possible to manage both sustainably." Also, he notes that while the isopod affects individual mud shrimp, its effectiveness as a biocontrol agent may be limited.

This may be due to the shrimp's reproductive behavior. Although infected females in a particular estuary might not breed, larval inputs from nearby estuaries sustain shrimp populations. Microscopic shrimp larvae leave an estuary for the ocean, and nearshore currents carry them to other estuaries. To have a pronounced effect on overall shrimp populations, Dumbauld says, the isopod would have to wipe out breeding females at several locations.

And so the small drama will carry on. Chapman plans to study the isopod to fill in knowledge gaps about its biology and natural history. The growers will keep their fingers crossed that it does away with the shrimp. And the mud shrimp, the lone original denizens of the estuaries somewhat obscured in all of this, will either endure or not, as the debate continues on their role and priority in the estuary.

Eric Wagner is a graduate student in biology at the University of Washington in Seattle.

Images

Top: The parasitic isopod Orthione griffenis infests native mud shrimp. The parasite causes the bulge in the mud shrimp's shell. Photo: Courtesy of Brett Dumbauld

Middle: An exposed parasitic isopod. Scientists think it was introduced via ballast water, probably from a ship from Japan. Photo: Courtesy of Brett Dumbauld

Bottom: Kodi Jones, a high school student, surveys an Oregon estuary during low tide. He is looking for the invasive isopod. Photo: Courtesy of Lincoln County and Hatfield Marine Science Center