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Suntanning in Sharks

Juvenile scalloped hammerhead sharks, which are typically light tan in color, were found to suntan when exposed to increases in solar radiation (Fig. 1). These sharks spend most of their time in the deeper, murkier portions of Kāneʻohe Bay, where little sunlight penetrates. However, hammerhead shark pups held in a shallow seawater pond at the Hawaiʻi Institute of Marine Biology subsequently darkened after being in the clear waters of this pond for several weeks, where UV levels are 600 times greater than those at the murky bay floor (Fig. 2). To determine whether sharks were darkening in response to increases in solar radiation, an opaque filter was placed over part of the pectoral fin of untanned sharks that were collected from the bay. Sharks were then placed in the pond for 21 days after which time the filters were removed.

Fig. 1. Dr. Lowe holding a dark colored juvenile hammerhead shark and Dr. Goodman-Lowe holding a light colored juvenile hammerhead shark
Fig. 1: Dr. Lowe and Dr. Goodman-Lowe show the color differences in juvenile scalloped hammerheads that were exposed to the sun and those that were not exposed. Photo taken in the mid-1990s.
Fig. 2. two juvenile scalloped hammerhead sharks swimming in an outdoor pond, one is dark colored and one is light colored
Fig. 2: Two juvenile scalloped hammerhead sharks swim side by side in a shallow outdoor pond at the Hawaiʻi Institute of Marine Biology. The UV levels in the pond are much greater than the levels found in the juvenile's usual, murky habitat. The darker colored shark in this photo (lower right) had been in the outdoor pond for some time, while the lighter colored shark (upper left) was more recently put in the pond for study.

Areas of skin from under the opaque filter were untanned, whereas all other skin exposed to direct sunlight was considerably darker, resulting in distinct tan lines (Fig. 3)

Fig. 3. A) two scalloped hammerhead sharks are held up side by side, B) histology of untanned scalloped hammerhead shark skin, C) histology of tanned scalloped hammerhead shark skin
Fig. 3: A) The physical differences of sharks that had been allowed to tan in the shallow pond are visually noticeable, and when Dr. Lowe and Dr. Goodman-Lowe (1996) examined the skin of the animals, they noticed that B) the skin structure and melanin content of untanned sharks differed from C) that of the tanned sharks.

Our experiments demonstrated that the sharks were truly suntanning and that the response was, in fact, induced by the increase in solar radiation, particularly UV. These sharks increased the melanin content in their skin by 14% over 21 days, and up to 28% over 215 days. This study represents the first documented case of suntanning in lower vertebrates (Fig. 4). To our knowledge, the only other animals that have been documented to suntan are a few mammals (including humans!).

Fig. 4. Nature 1996 cover featuring juvenile hammerhead sharks
Fig. 4: When Dr. Lowe and Dr. Goodman-Lowe discovered juvenile scalloped hammerhead sharks were able to suntan, their finished publication made the cover of the scientific journal, Nature, in 1996.

Because the pups may use the murky waters of the bay as a refuge to avoid being eaten by adults, it is most likely that this suntanning ability better serves them after they leave the bay. As adults, hammerhead sharks spend more time in clear pelagic waters where UV penetrates to much deeper depths.

Other species of elasmobranch have shown comparable changes in pigmentation after long-term exposure to direct sunlight; however, suntanning has not be directly confirmed.