Every year, the Vermont Fish & Wildlife Department conducts a fish census in Lake Memphremagog, which straddles northern Vermont and the Canadian province of Quebec. In 2013, the team was startled by some of the brown bullhead catfish they were finding in the lake. The fish were marred by unsettling dark streaks.
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"It looks like black tar on the skin — not very pretty," says Peter Emerson, a fish biologist with the department who has participated in the census since 2009. "You can tell it doesn't help the fish cause they're inside the mouth, they're on the fins." Sometimes the dark marks are on the lips or the gills or even covering the fish's eyes.
Just over a quarter of the catfish had the black blotches. Anglers had noticed them the year prior, but before that there had been nothing. "They arrived very suddenly," Emerson says.
In a paper published on Wednesday in Nature, Emerson and his colleagues report that these inky blemishes are a kind of transmissible skin cancer — one that appears to spread from catfish to catfish.
"It is the infectious agent itself," says Julie Dragon, a data scientist at the University of Vermont and a co-author on the study. "Those cells can somehow escape the host, maybe live in the environment — the aquatic environment — and then infect a new host."
Beyond this lake, Dragon believes these catfish may hold clues about how tumors spread throughout our own bodies. "We still don't really understand this event where a cell leaves the tumor, moves through the bloodstream, gets into a new organ," she says. In the catfish, "it might have some of the same processes going on that allow these cells to invade a new host, avoid the immune system and then establish themselves."
A fishy mystery
Shortly after the sinister lesions first surfaced, the Vermont Fish & Wildlife Department worked with researchers to determine that they were melanomas.
They found this to be odd since catfish don't usually spend much time at the surface where they would be exposed to the potentially damaging UV rays of the sun. Emerson and his colleagues thought maybe the melanoma was being caused by a chemical or pollutant leaching into the lake. (After all, the Coventry Landfill sits nearby.) So they looked for toxins in the bodies of the fish. fishes' bodies.
"But it turns out, as we did the studies, they don't have much of that at all," says Emerson.
This is when the team reached out to Dragon. She started by looking to see whether there might be a virus responsible for the cancers, but she found no evidence.
"Over time, genome sequencing got cheaper and easier to do," she says, "so we recommended whole genome sequencing of the fish and the tumors."
A tumor usually erupts because something goes awry in the cells of the host organism. This means that a tumor tends to resemble its host genetically.
This wasn't the case for these catfish, however. "These tumors were very different from the host fish that they came from," says Dragon, "which is not what we expected or what anyone would expect."
Instead, the tumors' genetics resembled one another quite closely, suggesting to Dragon that these cancers were clones that arose in a fish some time ago and have been spreading from catfish to catfish ever since.
A cancer with "a life of its own"
Transmissible cancer has been identified in only a handful of animals so far, including Tasmanian devils, dogs, certain molluscs — and now fish, it seems.
Dragon and her colleagues aren't sure how the catfish are infecting one another, but they suspect it may happen during spawning. First, they don't observe the melanoma in young fish.
Second, "they aggregate into these big clusters, flop around on top of each other, and they have these spines behind their pectoral fins that are pretty sharp," says Dragon. "And we think that they actually scratch accidentally each other, and that is one potential avenue for the cancer cells to invade a new host."
This is in part what Dragon will consider next, alongside whether there's something environmental or viral depressing the catfish immune systems. Today, 30% on average have the melanoma throughout the lake on both the U.S. and Canadian sides, though Dragon says, "they do appear to be able to live with these tumors for some time."
"Normally, cancers are sort of a dead end," says Vincent Lynch, an evolutionary biologist at the University at Buffalo who wasn't involved in the research. "When the host dies — when the person with the cancer dies — so does the cancer."
This means that it would be advantageous to the cancer to be able to escape and invade another host to stay alive. "The cancer wants to survive," says Lynch. "Now it's got a life of its own."
He adds that these catfish may be useful in the lab to study how cancers metastasize and migrate within our own bodies. "It could tell us something about how metastases evolve and originate within an individual," he says. "If you could stop that from happening, then you can actually stop people from dying of cancer."
This is likely a ways off, though.
In the meantime, the researchers will be studying catfish melanomas that have surfaced in lakes and ponds elsewhere. For instance, this particular melanoma may have first been documented by Henry David Thoreau who observed a catfish in a tributary of the Concord River in Massachusetts "with an inky-black kind of leprosy, like a crustaceous lichen."
The authors of this study argue that it's worth considering whether transmissible cancers are perhaps more common than we once thought.
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