Fish getting skin cancer from UV radiation, scientists say
Approximately 15 percent of coral trout in Australia's Great Barrier Reef had cancerous lesions on their scales. In that regard, they resemble Australians who live on land - 2 in 3 people who live Down Under will be diagnosed with skin cancer before the age of 70, the highest rate in the world. It's probably no coincidence that Australia is under the Earth's biggest hole in the ozone layer.
Researchers hadn't set out to look for signs of cancer in fish.
Scientists from the Australian Institute of Marine Science were near the Great Barrier Reef conducting a survey of shark prey, predominantly coral trout. They kept seeing strange dark patches on the normally bright orange fish, and for help they turned to another research team from the University of Newcastle in England that was studying coral disease in the area.
The research team's first guess was that the patches were caused by an infection, said Michael Sweet, a coral disease expert. "We can check for microbial pathogens quite easily. So we designed an experiment, screened for them, and couldn't find anything," Sweet said. "So we had to look deeper."
Sweet and his colleagues cut the fish tissue into slices and put them under a microscope. "We basically stumbled onto these tumor formations," he said. Then they compared them with samples from fish that had been given melanoma - the most dangerous type of skin cancer - as part of a laboratory experiment. They looked nearly identical.
The findings were published online Wednesday by the journal PLoS One.
Even though this is the first published account of skin cancer in wild fish, it is unlikely the phenomenon is new, said Michelle Heupel, a researcher at the Australian Institute of Marine Science who wrote the study with Sweet and others.
"When I talk to people who have been fishing for a long time, they tell me they've seen this since back in the 1980s," she said.
The researchers were unable to determine why the incidence of melanoma was so high in these fish. Sweet said it was probably not a coincidence that the cancer occurred in the Great Barrier Reef, which sits under the outer reaches of the ozone hole centered over Antarctica. That greatly increases the area's exposure to harmful ultraviolet radiation, which can lead to cancer-causing mutations in DNA.
Team members also suspect that the Great Barrier Reef's location at the edge of the coral trout's range increases its vulnerability to cancer.
"They are at the extreme of their habitat," Sweet said. "They are struggling to cope, which means they will be more susceptible to more diseases."
Aside from their skin lesions, the fish that were captured by researchers did not show any signs of illness. But Nancy Knowlton, a marine ecologist at the Smithsonian Institution in Washington, said it remained possible that a significant number of fish were becoming sick from the cancer.
"Once the melanoma gets invasive, the fish probably get compromised," making them more vulnerable to sharks, said Knowlton, who was not involved in the study. "Somebody with advanced melanoma is being taken care of by doctors and loved ones, but fish will be eliminated fairly quickly."
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