Fish with Melanoma - Our Enduring Environmental Legacy

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

We've all heard the horror stories. Melanoma is one of the most dangerous kinds of skin cancer, killing around 50,000 people worldwide every year. If caught early enough, it can be cured, but once it invades past the skin, it's deadly. On the advice of doctors, we try to protect ourselves, donning floppy hats and coat upon coat of SPF 50 sunblock. We pick over our bodies in the mirror regularly, looking for dark, irregularly-shaped spots. The recent rise in the incidence of skin cancer, though, is our own fault. It is the result of our environmental hubris, a combination of a chemically-depleted ozone layer and our pathological obsession with a tanned physical appearance. Now, we're becoming increasingly aware that our choices don't just impact our own species. The rest of life has to deal with our poor decisions, and studies are just now determining the wide-ranging consequences of our actions.

Unable to slather on sunscreen, the creatures on our planet are much more limited in their ability to deal with the sun's radiation. Some, like the red seabream, are able to tan much like we do, increasing the melanin content in their skin to defend against damaging rays. But most animals are not so lucky, and are ill-equipped to deal with drastic changes in UV radiation. Yet drastic changes in UV radiation are exactly what occurred in the late 20th century, when chemicals we used as refrigerants and in aerosol sprays quickly depleted one of the most UV-protecting molecules, ozone, from our atmosphere. From 1972 to 1992, places like Australia saw a 20% increase in UV radiation levels, and colder areas like Antarctica saw ozone decreases of 50 percent or more, creating large ozone holes which allow more than double the normal level of UV radiation to pass through.

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In the late 1970s, scientists began to realize that certain chemicals we were producing, called chlorofluorocarbons, or CFCs, were making their way into the stratosphere. These chemicals release chlorine atoms which, when combined with cold temperatures, begin a destructive chain reaction that turns UV-blocking ozone into oxygen. By 1987, there was so little protective ozone in the stratosphere over Antarctica that global lawmakers decided CFCs were too dangerous to go unchecked. They established the Montreal Protocol, which set strict limits on the use of CFCs. In the 25 years since, the ozone layer has rebounded some, but it is still 50 to 70 years away from returning to pre-1980s levels. Now, the ozone layer is under a new threat: climate change. Scientists predict that rising carbon dioxide levels will lead to more ozone holes, as carbon dioxide and other greenhouse gasses trap heat at the surface, chilling the stratosphere, and allowing atmospheric chlorine atoms to wreak havoc.

We are only now beginning to fully document the consequences of ozone depletion. In people, the loss of ozone at the end of the 20th century was directly connected to a 16 to 60 percent increase in the incidence of skin cancer. But while we carefully documented the effects on our own species, little research has looked for health effects on other animals. Now, Australian scientists have found an entire population of fish plagued with the deadliest form of skin cancer: melanoma.

The team of researchers from Newcastle University began looking for skin cancer in the commercially and culturally important species of coral trout off Australia in 2010 when a different team of scientists studying sharks first noticed lesions. Because these other scientists from The Australian Institute of Marine Sciences were catching trout to study predator-prey dynamics, Michael Sweet and his colleagues were able to screen over a hundred coral trout (Plectropomus leopardus) for melanoma between August 2010 and February 2012. They examined lesions histologically, to determine the exact type and severity of the cancer. Lastly, they tested lesions for bacteria and viruses, to rule out a microbial cause.

A whopping 15% of the fish surveyed had melanoma. "Studying disease in wild fish populations is very time-consuming and costly so it's hard to say how long the disease has been around," explains lead author Michael Sweet. "What we do know is that it is now widespread in the coral trout population. We found evidence of cancer in the common coral trout, the bar-cheeked coral trout, and the blue spotted coral trout."

While 15% sounds high, Sweet and his colleagues believe it's only a minimum estimate. "Once the cancer spreads further you would expect the fish to become quite sick, becoming less active and possibly feeding less, hence less likely to be caught. This suggests the actual percentage affected by the cancer is likely to be higher than observed in this study."

This isn't the first melanoma to be found in fish, as individual cases have been identified in a wide variety of species, from catfish to nurse sharks. Never before, however, has melanoma been found population-wide. "To the best of our knowledge, cancer of any sort has never been shown in a wild marine fish population before, making this a first for science," said Sweet

While it is a first, Sweet and his colleagues don't think coral trout are unique. "We would not be surprised to find [melanoma] in other species as well," he said, "including some of the smaller reef species." So far, skin cancer in fish has likely been overlooked due to the high cost of evaluating fish for disease as well as the low likelihood of sick and weakened fish landing in fishermen's or scientists' hands.

Extensive laboratory analyses ruled out microbial agents as the driver of the disease, and since the fish were caught far from shore in a marine protected area, it's unlikely that pollution factored in, either. The samples were also directly compared to UV-induced melanomas in laboratory fish, which are used as a model for human disease; the ones in coral trout looked identical to the lab-created cancers. "This combination of evidence leads us to suspect UV as the casual agent."

If UV is the cause, then it's really our fault. "The occurrence of this disease in today’s day and age and not before can be linked to the changes we are experiencing in our climate and the ozone hole," explained Sweet. "It is highly likely there will be higher prevalence around areas which have these ‘ozone holes’." While the Montreal Protocol has helped reverse some of the worst damage, Sweet is careful to note that we're not out of the woods yet. "An increase in smaller ozone holes (other than the two large ones of the Arctic and the Antarctic) is thought to be occurring, and this has been related by other researchers to be due to climate change."

The overall effect of skin cancer in fish populations could be devastating. In laboratory fish, melanoma cuts the lifespan of Xiphophorus species from four years to only six months, and makes them more susceptible to small changes in their environment like fluctuations in temperature. "It is unclear whether future changes in the ocean environment or climate will similarly exacerbate the effect of melanomas in wild P. leopardus populations," write the authors, "but clearly further research is urgently needed to understand the distribution, prevalence, ecological and fisheries significance of this syndrome."

Since lawmakers are hesitant to restrict greenhouse gasses and other pollutants, we're stuck with whatever happens, for now - especially, as Sweet notes, when it comes to disease. "Without addressing the underlying issues, sadly, there is likely no feasible or practical cure for skin cancer in wild fish populations." If melanoma is found in other species, too, the consequences will only magnify.

With little natural protection against UV rays, fish and most other species are at our mercy when it comes to radiation-induced disease. Skin cancer only adds to a growing list of pathological consequences to our poor ecological choices - a list which includes devastating diseases like chytridiomycosis and avian malaria. Until we change the way we treat the world around us, that list will continue to grow, while the abundance and vitality of our planet's biodiversity shrinks.

 
Citation: “Evidence of melanoma in wild marine fish populations.” M J Sweet, N Kirkham, M Bendall, L Currey, J C Bythell, M Heupel. PLOS ONE. August 2012. DOI: 10.1371/journal.pone.0041989.g005

Histological sections from the paper; photos of coral trout by Michelle Heupel