An international team of researchers examined a commercially-important marine fish, the coral trout (Plectropomus leopardus). †In two sites in the Great Barrier Reef Marine Park, they discovered 15 percent of the trout bore lesions like the melanoma lesions in laboratory experiments with Xiphophorus. (Those playfish or swordfish have long been used in cancer research.) Some of the coral trout had only small lesions. Others were nearly black with them.
Prior to this study, scientists assumed only the hybrid crosses of Xiphophorus bred in captivity could contract melanoma. Their wild relatives were not susceptible, even after being exposed to high doses of physical and chemical carcinogens.
The lesions on the Great Barrier Reef fish had not spread beyond the skin, but lead author Dr. Michael Sweet, University of Newcastle, cautions that does not mean melanoma is not harmful to fish. He said:
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.
Dr. Michelle Heupel, Australian Institute of Marine Science, suggests the fish may have interbred, with hybridization giving them a genetic predisposition to melanoma. Their immune systems may also be compromised because of their living in an area that is near their temperature threshhold. She said:
It may just be that theyíre living close to the edge there and this genetic composition makes them a little more susceptible. And thatís combined with UV exposure in the region. I really think itís the combination of all those factors working together rather than one thing independently.
The research team cites an urgent need for more studies to determine what factors are causing the melanoma, how many fish are being killed by it, and how widespread the syndrome actually is. The answers will have important implications, not just for coral trout but also for commercial and recreational fisheries.
The research can be found on PLOSone: M. Sweet, et al. (2012) Evidence of Melanoma in Wild Marine Fish Populations.
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Photo 1 by Richard Ling, via Wikimedia Commons; Photo 2 courtesy of PLOSone