Estimated reading time: 8 minutes
Jim O’Connell cares a lot about clams.
They are central to his livelihood as a shellfish farmer and an excuse to spend his days on the tidal flats of Wellfleet on Cape Cod, his version of paradise. He even shows signs of an emotional attachment to the clams—even those that have perished from the contagious cancer running through his farm. Sometimes he takes them home and adds them to his crushed-shell driveway.
“I wouldn’t want to be a clam,” said O’Connell, who stands in knee-deep in hip waders and a grey sweatshirt. “When they become infected with the disease, they’ll get sick in the substrate and then rise to the top. At that point, there’s nothing inside them.”
On this crisp-clear fall morning, however, not a single clam rests at the surface of O’Connell’s farm. No trouble in paradise—at least for today.
“These days I dig them up early, because the longer you leave them at the bottom, the more problems you have with neoplasia,” O’Connell said as he muscles his Virginia Harvester rake through the sand to collect a basket of hard-shelled clams. “I don’t get paid as much for them due to their smaller size. But at least they don’t die.”
The disease, formally known as hemocytic neoplasia, is a contagious, leukemia-like form of cancer that spreads among clams through seawater (the disease cannot spread to humans). The cancerous cells first enter the digestive system of healthy clams then infiltrate their vascular systems. Death almost always follows. Mortality rates within hard-clam aquaculture plots throughout Wellfleet Harbor have ranged anywhere from 30-to-60 percent over the past decade, making for a series of challenging farming years for local clam harvesters like O’Connell.
While it’s a unique problem affecting hard clams in Wellfleet—one of the largest hard-clam aquaculture production areas in the U.S.—it’s not a new one. Roxanna Smolowitz, a veterinarian and biology professor at Roger Williams University (RWU), first saw hard-clam neoplasia in the early 1990s while researching a marine parasite known as QPX, or Quahog Parasite Unknown, that was causing mass mortalities in the shellfish. Then, nearly ten years later, Smolowitz saw clams floating at the surface.
“We picked them up, brought them into the lab, and saw they had leukemia-like neoplasia,” she said.
The discovery prompted Smolowitz and her team at RWU to start investigating the disease. She secured funding from the Northeast Regional Aquaculture Center (NRAC) at the University of Maryland and WHOI Sea Grant, with support from Barnstable County in Massachusetts. She later enlisted the help of WHOI scientist Becky Gast, with whom she had worked on QPX.
Gast, whose lab at WHOI is roughly 70 miles southeast of Wellfleet Harbor, had heard about transmissible cancer in Tasmanian devils, but until Smolowitz reached out, she hadn’t known it to be a problem for hard clams. “I thought it was a fascinating project, and I was surprised at how many different bivalves are affected by the disease,” she said.
According to Smolowitz, neoplasia outbreaks are hard to contain, particularly since the clams live in close quarters.
“The animals are typically planted at very high densities, so it’s not difficult for the disease to spread from clam to clam,” Smolowitz said. It’s like a cold spreading throughout a kindergarten class. Once one child gets it, “they’ll all be sick within a week,” she said.
The disease, however, is only contagious among hard clams and does not cross species, which is why it’s not infectious or harmful to people. Josh Reitsma, a fisheries and aquaculture specialist with the Cape Cod Cooperative Extension/WHOI-Sea Grant, hopes to correct a common public misconception around this.
“There have been rumors that humans can get sick or even develop cancer from eating these clams, which is completely false,” Reitsma said.
The fact that there’s no public health threat may be common knowledge to scientists, but there’s still a huge mystery surrounding hard-clam neoplasia that has researchers stumped: What, exactly, causes it?
Finding the culprit
In the Gast lab, Arun Venugopalan, a marine virologist and WHOI Sea Grant post-doctoral investigator has been running diagnostics on the samples with the goal of trying to help determine if the cancer is viral or genetic. He says that if it’s a marine virus that is transforming normal cells into neoplastic ones, there could eventually be the option of selectively breeding a new, virus-free strain of clams.
For now, Venugopalan has been running tests to determine the severity of an individual clam’s cancer. He extracts blood samples using a syringe (the most non-destructive method possible), places droplets on a slide, and examines them under a microscope. Since the cancerous cells are roughly 1.5 times larger than the normal cells, he can quickly quantify the number of each and from that, determine the severity.
“It’s the equivalent of being able to indicate things like ‘stage 3 or stage 4 cancer’ which we often hear with human diagnoses,” Venugopalan said.
He also performs genetic sequencing on DNA-RNA from the blood samples using a nanopore sequencer device, which looks like a tiny silver flash drive. This allows him to compare the genetic changes and mutations between the normal and neoplastic cells, which may yield hints about the underlying causes of the disease.
Smolowitz, too, conducts genetic analysis in her lab at RWU, comparing the genes of surviving clams to those that died to see if there’s a particular gene that is important for resistance.
“We can’t treat them with antibiotics or cancer drugs,” said Smolowitz. “But if we can understand what strains are more susceptible or less susceptible and then find a genetic component to the resistance, we’ve got a whole new ballgame.”
Two red Old Town canoes are filled with baskets of freshly-dredged clams. O’Connell, who is 65, gently guides one to shore with his right hand. He trudges slowly through the rising tide, mindful of decades-old knee and back injuries. Moving shoreward, he marvels at the pristine bay. In the autumn sunlight, the gently-lapping clear water reveals bright green hues of algae growing several feet below.
O’Connell takes a moment to reflect on why Wellfleet Harbor makes ideal clam habitat.
“We get a 75 percent tidal flush, twice a day, so the majority of the water is going in and out every day,” he said. “We also have all these great estuaries surrounding the bay, which make beautiful algae for the clams to eat.”
It’s an odd juxtaposition—such a healthy ocean ecosystem teaming with disease that spreads like wildfire. But the neoplasia keeps hanging around, and has forced O’Connell to not only harvest smaller clams, but also to be more mindful of the planting densities. In recent years, he’s been planting roughly 68,000 clams within a 100-foot row, but could be planting as many as 75,000 in the same plot.
O’Connell also says he spends an “inordinate amount of time” raking, squeegeeing, and sweeping his nets to keep them clean and free from biofouling, which can inhibit the clams’ ability to feed. This, he believes, gives them their best shot at survival amidst the contagion.
Lower yields and higher costs are a dismal recipe in any farming operation. But raising hard clams still makes financial sense for O’Connell. And despite the general lack of funding for hard-clam neoplasia research, he has the support of the small but growing circle of local scientists working on the problem. “The work the researchers are doing is hugely important,” he said. “The more effort put into trying to figure out what it is, the better.”
Johnny “Clam” Mankevetch, an assistant constable for the Wellfleet Shellfish Department, also sees the value in the research of this understudied disease. And while he doesn’t have hard stats to back it up, his harbor-wide view tells him that things today are perhaps not as bad as they were several years ago.
“It’s not like we’re running the victory flag up the flag pole,” Mankevetch said. “But the fact is, everyone who wants to farm clams here still does.”
Hard-clam aquaculture in Wellfleet presses on, as does the researchers’ quest to solve the cancer mystery. Working in tandem with the Smolowitz lab, Gast has helped determine that there are multiple cell lines involved in hard clam neoplasia, a discovery that Smolowitz feels could be significant.
“If there are multiple lines of neoplastic cells out there, that indicates that there’s some additional infectious agent present in the water,” said Smolowitz. “That hints at the idea that the disease could be viral.”
Back at O’Connell’s clam farm, it’s late morning. He hauls his loaded canoes out of the waist-deep water back to his white Ford pickup on the beach. He climbs into the back to load the brimming, 80-pound baskets into a blue, refrigerator-sized cooler that takes up half the bed. Today was a good day out on the flats, and despite the risk of a neoplasia outbreak constantly hanging over his head, the healthy haul helps him stay optimistic.
“You know what? I haven’t read a newspaper since halfway through the first Gulf War,” O’Connell said. “I don’t watch TV, and I don’t listen to the news. And I don’t mean to belittle people, but they read stuff and think it’s all going south. Me—I have a little more faith than that.”
Funding for this research is provided by WHOI Sea Grant and the Northeast Regional Aquaculture Center (NRAC).