In 2012 an angler pulled a brown bullhead out of Lake Memphremagog with what looked like tar smeared across its skin. He brought it to Peter Emerson, a fish biologist with the Vermont Fish and Wildlife Department, who had surveyed the lake since 2009. Emerson had never seen anything like it.
Within three years the black growths were on a quarter to 40 percent of the lake’s bullhead. This week researchers at the University of Vermont reported in Nature that the melanoma is contagious, spreading from one fish to another as living cancer cells.
It is the fourth type of transmissible cancer known to science, the first in a fish and the first in fresh water.
What Is A Transmissible Cancer?
Cancers found in a cluster usually have a common cause, a chemical carcinogen or an infection. Brown bullhead are a sentinel species for pollution because they develop liver tumors in water contaminated with polycyclic aromatic hydrocarbons, and walleye carry a retrovirus that causes skin sarcomas.
So, these are the causes the researchers hunted for, hard and for years. Vermont biologists tested the watershed for those hydrocarbons after Tropical Storm Irene tore through in 2011, thinking the flooding had stirred up a point source. But waters with far higher levels held clean fish. Tissue went to the bioinformatics core at the University of Vermont from about 2014 on, yet years of sequencing turned up no virus and no microbe of any kind.
So Julie Dragon, the associate professor of microbiology and molecular genetics who directs that core, went looking for a shared mutation instead. In an ordinary cancer each tumor is a close genetic relative of the animal it grew in, and a family tree of cancers and non-cancerous tissue puts every tumor next to its own host. The Memphremagog trees came out the other way around: all the tumors in one cluster and all the fish in another, with hundreds of thousands of mutations shared across cancers in different fish and absent from the fish carrying them.
“It’s a clonal cancer,” Dragon told me. “It’s not driven by a bacteria or a virus. It’s the infectious agent itself, and it’s moving from fish to fish.”
One Lake Out Of Ninety
In 2017 the team re-surveyed the lake, four sites in Vermont and three in Quebec. Prevalence was pretty similar at every one, meaning the disease had saturated a binational lake more than 30 miles long within five years of appearing. The genomes suggest it came from elsewhere: tumors from Memphremagog are closer kin to healthy bullhead in New Hampshire and Maine than to the fish they are growing in.
Nor is it confined to Vermont. “It looks like it’s widespread, including New Brunswick, Nova Scotia, Maine, New Hampshire, Massachusetts, Pennsylvania,” Emerson said. “And some even in Wisconsin, New York … they’re everywhere.” Dragon has also genotyped bullhead from other waters and found the same disease. “Everywhere”, in Emerson’s usage, means wide geographic spread, but not local abundance. His district holds 90 lakes. Bullhead live in nearly all of them. The cancer is in one.
There is more than one way to be rare. In 1981 Deborah Rabinowitz sorted rare species along three axes: geographic range, habitat specificity and local abundance. Eight combinations fall out of that scheme and only one describes a common species. This cancer sits in one of the other seven, with a range spanning half a continent, a narrow set of waters within it and locally large numbers, at least in Lake Memphremagog.
Ecologists know this pairing of scarcity and saturation well, and it points to how a species moves rather than what it needs. A cancer constrained by the suitability of its hosts would be uncommon in the lakes where it did appear. One that takes a third of the fish in every lake it reaches, then skips the next 89, is constrained by arrival alone. That puts the weight on how it moves between waters, and in a lake system the usual answer is people. That is how an invasive species behaves.
What It Does To A Fish
“It looks like a plague,” Emerson said. The growths turn up anywhere on the outside of the body, often inside the mouth and on the barbels, the whiskers a bottom-feeding catfish uses to find food. “There should be long barbels, but when you’ve got cancer, melanoma on them, they get short,” he said. “So that is affecting the way they feed.” Then, perhaps thinking I would worry about the welfare of the fishery, or of the fish themselves, he added: “This lake is full of food, though. So they’re not going to starve here.”
What the cancer does to one fish matters less than what it does to a lake full of them, and nature has already run that experiment twice, to opposite ends. Devil facial tumor disease, caused by one of the three other known types, drove Tasmanian devils, the largest carnivorous marsupials on Earth and found only on the Australian island of Tasmania, down by about 90 percent. The clam and mussel leukemias spread through populations that persist anyway. Bullhead are still plentiful and prevalence has stopped climbing, which points toward the second ending.
How Many More Transmissible Cancers Are Out There?
This tumor is only the fourth type of transmissible cancer known to science, which raises the question of how many more are out there. Three researchers, asked separately, gave me versions of the same answer.
Frédéric Thomas, who studies the evolutionary ecology of cancer at France’s CNRS, called the emergence of such a cancer “a very rare evolutionary transition.” Although there are only four known types of transmissible cancers, this does not mean transmissible cancer has only evolved four times. The devil cancer arose twice, independently. The bivalve leukemias have arisen at least ten times, in as many species. Even the count of types is probably low, Thomas said, because finding one takes paired genomic analysis of tumors and hosts, which “have rarely been performed systematically in wildlife.”
Andrew Storfer, an evolutionary geneticist at Washington State University, said the same thing more plainly. “I don’t think these are as rare as we think,” he told me by phone, and Dragon was even blunter: “People don’t pay a lot of attention to catfish.”
Dragon’s team is now finding tumored bullhead in photographs anglers post to a fishing app called Fishbrain. The surveillance system for a novel transmissible cancer is a social network for anglers. That is less makeshift than it sounds. Crowd-sourced images have become a recognized tool in biodiversity informatics, catching outbreaks, invasions and range shifts at a scale no field crew can match. For Dragon they speak to the question underneath all the others, whether these cancers are truly rare or only unwatched.
What Can Be Done?
What should be done, either to manage the disease in Lake Memphremagog or to prevent it from spreading to other inland waters remains an open question. Storfer, who works on the devil disease, told me even the worst case, such as the 90 percent declines observed in some populations of that species, may not be permanent. His group advises managers to do nothing, expecting devils to decline and then recover on their own.
The rules that stop anglers carrying bait and water out of Memphremagog were written for invasive species, years before anyone knew a cancer was in the lake. Emerson would like them revisited and knows how that goes. “I’m one small voice in there.”
I asked whether an angler who catches one of these fish would have any idea what he was holding. There are no signs at the boat launches, Emerson said. Then: “Nobody ever asked me that before.”
