Even sharks can get a tan. Sharks' skin turns from dark brown to black as the pigment melanin increases in direct response to radiation. In other fish, the exposure can lead to skin cancer. Sharks, however, just seem to tan.
What's their secret? The answer could hold the key to preventing skin disease in humans.
"As far as I'm aware, sharks appear very robust to skin damage and disease," said Michael Sweet, lead author of a study about shark tanning in the journal PLoS ONE. "There have been a lot of attempts to induce melanomas in sharks to no affect."
"I don't know what makes shark skin so special, but it definitely needs to be studied," added Sweet, a researcher in the School of Biology at Newcastle University's Newcastle Institute for Research on Sustainability.
Sweet and his team report on extensive melanoma in wild populations of the commercially important marine fish, coral trout. The scientists believe that the likely cause is environmental exposure to UV radiation. Climate change, which many studies tie to pollution and other human-driven factors, could be escalating incidence of the disease. Relatively high prevalence of skin cancer in fish, for example, has been recently reported at sites in the Great Barrier Reef Marine Park.
Sharks, on the other hand, simply change color.
In a separate study, Christopher Lowe, who runs the CSULB Sharklab at California State University Long Beach, and Gwen Goodmanlowe noticed the color shift in juvenile scalloped hammerhead sharks located in Kaneohe Bay, Hawaii. The sharks normally spend most of their time near the bottom of the bay, but when they were held in a shallow seawater pond with more sunlight exposure, their skin turned from light tan in color to a dark brown/black. Ultraviolet light intensity was 600 times greater than usual.
To determine if the sharks were actually sun tanning, the researchers exposed the sharks to irradiance treatments using filters attached to the sharks' pectoral fins. The scientists saw changes in melanin densities, but did not detect visible lesions or growths associated with skin cancer.
"It is possible that sun tanning is used when the shark pups leave the bay and enter the clear pelagic waters they occupy as adults," the researchers speculated.
Certain shark species might then also be able to tan, depending on their particular habitat and environmental needs. It remains unclear, though, how disturbances related to human activities could impact the natural process.
"I believe many other fish species will be suffering from cancers, and the more we look, the more we shall see," Sweet said. "Already, a few researchers have been sending me photos of other species that may be infected similar to those in my study."
He continued, "We need to do further work to confirm the UV link, and if this comes true, then I'm sure human activities are having a direct effect on these fish. As I'm sure you are aware, climate change has dramatically affected many ecosystems and many, many organisms."
In the meantime, other scientific teams continue to investigate why shark skin is so sturdy. In a study accepted for publication later this year in Food Chemistry, Phanat Kittiphattanabawon of Songkla University and colleagues analyzed blacktip sharks.
Kittiphattanabawon's group determined that antioxidant properties of the shark's skin were tremendous, such that it even inhibited cooked pork fat from turning rancid. Blacktip sharks are listed as near threatened by the International Union for Conservation of Nature. Many other shark species are also in jeopardy.
The hope is that the biochemical secrets of shark skin can be unlocked so that the beneficial properties can be replicated to help humans without involving killing sharks.