Both of these sharks are hammerhead pups. But the one on the right has a tan. From what we have been able to find out, sun-tanning had never been seen in marine organisms before, say marine biologists Chris Lowe and Gwen Goodman-Lowe of the University of Hawaii. The husband-and- wife team discovered the phenomenon when they moved some pups from the murky waters of Kaneohe Bay to a clear pond at the university. Some sharks are known to change color, like chameleons, in response to their environment. But we didn’t think it was because of that, Lowe says, because the sand on the bottom of the pond was white, and the sharks were turning black, so they stood out more against the sand. To find out if the animals were really tanning, Lowe placed opaque plastic tags on the pectoral fins of sharks fresh from Kaneohe Bay. After three weeks he removed the patches. The skin under the tags was the same color as when we put the sharks in the pond, but the rest of the shark was dark, Lowe says (except for their white bellies). The sharks had tan lines. As to why sharks evolved the ability to tan (other fish can get sunburned, but none have been observed to tan), Lowe has no explanation.