A Mysterious Disease Is Infecting Northeast Clam Beds

Farming—even shellfish farming—is like war: a constant struggle against invading hordes. On clam plots in the tidal waters of Cape Cod, the battle lines seem clearly drawn. Inches below the sand, thousands of clams filter ocean water and grow big enough to eat in about two years. Above them, crabs sidle by hunting young, soft clams. Moon snails troop past like pale-purple golf balls, drilling into older clams and making pencil-tip holes so neat they appear to be countersunk. The slow-motion conflict plays amid waving fronds of seaweed or, if the tide is low, on wet sand flecked with tiny mud snails.

Cape Cod’s $5 million shellfish aquaculture business has bloomed in recent years, in part because the natural threats to quahogs, or hard clams, seemed to be minor and manageable. But that was before a mysterious new threat—code-named QPX, for “quahog parasite unknown”—began killing clams by the thousands.

QPX seemed to emerge from nowhere when it struck the clam plots of Provincetown, Mass., in 1993. Although the disease poses no threat to humans, it killed nine out of every 10 clams in some plots and all but shut down clam farming in Provincetown. Since then, the disease has appeared across Cape Cod, in Duxbury, Mass., as well as in Virginia, New Jersey, New York, and Canada.

Shortly after the outbreak, Roxanna Smolowitz, a veterinarian at the Marine Biological Laboratory in Woods Hole, examined the sick clams under a microscope and identified QPX as the culprit. Although she could diagnose infected clams, she couldn’t be sure that clams without symptoms were disease-free.

Now, a sensitive new genetic test developed by Rebecca Gast, a biologist at Woods Hole Oceanographic Institution, can detect QPX cells in clams, seawater, and sediment. The new test means Gast and her colleagues can study how QPX exists in its environment—something they could only guess about before. They’re learning that QPX may normally spend its time peacefully decomposing dead seaweed, and that crippling the clam industry may simply be a side effect. Still, the virulent infections highlight a general problem that other aquaculture industries may one day face.

A bigger problem than red tides

“Red tide got the big attention this year,” said Gast. But months after a red tide, or harmful algal bloom, has faded, shellfish clear the algal toxins and can be safely sold at market. “In terms of livelihood,” she said, “QPX can be a bigger problem for shellfishermen—because it kills the clams.”

QPX is an obscure, single-celled relative of slime mold that has both animal and fungal characteristics. It secretes a thick layer of mucus to ward off the clam’s immune response. Other species related to QPX cause diseases in squid, octopus, abalone, and sea slugs.

Infected clams are weak, and their shells hang slightly open. Irritating sand lodges between the shells, and the sick clams work their way up to the seafloor. By this stage, the mucus is sometimes obvious on the edges of the shells, Smolowitz said, and the sound of the clams trying to close up is “like a million people grinding their teeth.” The force of the grinding often chips the shell margins. Inside the clam are more telltale symptoms: tan nodules set apart from the healthy, pale-peach clam flesh.

Smolowitz has noticed that infections typically start at the base of the clam’s siphon. She and Gast think that’s because of the way a clam’s digestive system works. As the siphon sucks in seawater, chunks of rejected food particles collect at its base. That material (called pseudofeces because it is periodically excreted but never goes through the gut) may act like an incubation chamber for the QPX. When QPX does enter the clam’s gut, it gets digested and can’t cause an infection.

Infectious ‘Trojan horses’

Before Gast developed her new test, people knew only that QPX spread directly from clam to clam, like flu in humans. Although QPX also infects wild clams, direct infection is especially dangerous for densely planted clam plots. “Just like a crowded city is more prone to have sick people than a less-crowded city,” said Bill Walton, an aquaculture specialist with the WHOI Sea Grant Program and the Cape Cod Cooperative Extension.

The direct-infection model supported the idea that QPX was moving around the Eastern Seaboard in batches of tiny “seed clams” that farmers buy in bulk from suppliers. Gast’s preliminary testing suggests QPX persists in many coastal waters, clinging to dead seaweed or riding in the sludge that collects on the backs of snails.

Smolowitz and Gast think clams may also get infected when QPX reaches a critical threshold in the water. So their next step is to find out how much QPX it takes to make clams sick, and whether different strains of clams have different levels of resistance.

Smolowitz thinks that clams brought to the Cape from southern waters might have functioned as Trojan horses. These less hardy southern clams contracted the disease at a low threshold, and as they died, they may have raised QPX levels in the water high enough to make the native clams sick, too.

One of Smolowitz’s experiments has already suggested as much. She grew clams from five parts of the Eastern Seaboard in both warm and cold waters. When southern clams grew in cold, Cape Cod-like waters, they succumbed to QPX more often.

The idea makes sense: Clams accustomed to warm water might be weakened when forced to grow in chilly water. Or as winter approaches, their immune systems might shut down before the QPX organisms become inactive, giving the disease easy access, Smolowitz said.

Searching for ways to prevent the disease

With QPX turning up in so many locations, Gast said, “we’re not going to be able to cure it. Keeping it below the level of infectivity is going to be important.”

Some fishermen are turning to age-old tactics like crop rotation. Alternating clams with oysters might hold down the levels of disease in both crops.

Another approach uses the details of physiology that Smolowitz and others have painstakingly identified. QPX dies almost immediately when plunged in fresh water, so a rinse of seed clams could help make them safe.

Other ideas include keeping plots cleared of seaweed, since it harbors QPX. Removing the nets that shellfishermen use to keep predators away from young clams may also keep seaweed from building up. And Walton is still experimenting to find an optimal density for growing clams—enough to turn a profit, but not so close together that the clams become stressed and weak.

Aquaculture is the fastest-growing segment of the fishing industry as wild stocks decline and the world appetite for seafood grows. Farming instead of fishing makes as much economic sense as ranching instead of hunting. But QPX is a reminder that the same pitfalls—overcrowding, reliance on a single crop, and susceptibility to disease—are waiting to be solved in the oceans as well.

“There’s a joke about aquaculture, that you find out all the ways to kill something, and then you stop doing them,” Walton said. Shellfishermen have already learned how to keep crabs, snails, and seagulls from robbing their plots. But for the moment, QPX remains on the to-do list.

WHOI Sea Grant (NOAA National Sea Grant College Program), and the Northeastern Regional Aquaculture Center funded Gast’s and Smolowitz’s research. Tom Marcotti, Town of Barnstable shellfish warden, and Elizabeth Cushman, a WHOI Summer Student Fellow, contributed to the study.