Maybe it's time fish use sunblock. A new study has found the first skin cancers in wild fish, specifically in coral, bar-cheeked, and blue spotted trout swimming on Australia's Great Barrier Reef. The lesions and dark patches are a scalier version of what melanomas look like on humans, but it's unclear whether they make the animals unsafe to eat.
Scientists know how to give fish skin cancer in the lab. They breed a swordtail and a platyfish, both common pets, to create offspring that are more sensitive to UV light. The reason is genetic. Platyfish have a so-called tumor gene and a regulator to control it, whereas swordtails have neither. When the two mate, their offspring will sometimes inherit only the tumor gene without the regulator, leading to increased incidence of various cancers. These offspring crosses have been used to study skin cancer in humans. But researchers were unsure whether fish got the disease in the wild.
The first hint that they did came when a group of marine biologists at the Australian Institute of Marine Science in Townsville, who were studying sharks on the country's Great Barrier Reef, noticed that the coral, bar-cheeked, and blue spotted trout being eaten by the sharks had black patches on their skin. At first, the researchers thought the patches might be caused by a fungus. But when they sent tissue samples from the fish to colleagues at Newcastle University in the United Kingdom, the U.K. scientists found no evidence of microbial infection. What's more, the Australian group found no pollutants in the highly protected Great Barrier Reef that would have caused such discoloration. That left one main suspect: skin cancer.
The team tested its hunch by examining skin cells from the lesions under the microscope. Unlike normal fish skin cells, they were tightly clustered and produced pigment in areas where there usually is none. What's more, the lesions were identical to those from the offspring of sword tail-platyfish crosses. It's therefore highly likely that the three species of trout on the Great Barrier Reef are suffering from skin cancer, the team reports online today in PLoS ONE.
The researchers blame the disease on a couple of factors. The Great Barrier Reef lies directly below the largest hole in the ozone layer, which means the region receives significantly more UV radiation than other place on Earth. In addition, they believe that the three fish species may be crossbreeding with each other, resulting in offspring that are more prone—due to the loss or mutation of certain genes—to UV-induced skin cancer. The team wants to run more genetic tests to determine if the inheritance patterns in crossbreeding coral trout might be the same as they are in the offspring of swordtail-platyfish matings.
It's not clear what impact the skin cancer is having on the fish, says Michael Sweet, a microbiologist at Newcastle University and co-author of the study. "It's possible that the more advanced the melanoma gets, the more likely the fish are to slow down and the more likely they are to get munched" by sharks, he says. It's also unclear whether the quality of diseased fish pose a danger to humans who eat them, he says, but because they are also bred commercially, he is doubtful that diseased ones will ever make it to market.
For now, the researchers have confirmed only skin cancer in fish from the southern end of the Great Barrier Reef, but it could be more widespread, says Sweet.
Christopher Lowe, a marine biologist at California State University, Long Beach, is impressed by the process of elimination approach the authors took to reach their conclusion. "People have been working on coral trout for a long time, and it's interesting that this relatively recent observation might have a genetic component," he says. Still, he cautions that although UV radiation seems the likely culprit, it is still possible that an undetected pollutant might also be a contributing factor.