Hammerhead shark mystery solved

From BBC Earth News
By Jody Bourton

Why do hammerhead sharks have such a famously strange-shaped head? One hypothesis is that having eyes on either side of such a wide 'hammer' allows the sharks to see better. But even this idea divides scientific opinion, as researchers argue over whether the hammerhead design makes it more or less difficult to see. The mystery may now be solved by a study showing that a hammerhead gives sharks outstanding binocular vision and an ability to see through 360 degrees. The finding is published in the Journal of Experimental Biology.

Debate over why hammerheads are shaped as they are goes back centuries, and arguments over their visual capabilities goes back decades, says Dr Michelle McComb from Florida Atlantic University in Boca Raton, Florida, US. For example, in 1948, zoologist Gordon Walls, a leading authority on vertebrate eye evolution, suggested that the position of a hammerhead shark's eye precluded it from having binocular vision. Yet in 1984, leading shark expert Leonard Campagno countered by suggesting that the distance between a hammerhead's eyes would actually give it excellent binocular vision. Binocular vision occurs when the fields of two eyes overlap, allowing the accurate perception of depth and distance. It is especially important for predators which need to judge the distance to their prey. However, despite its apparent importance, "frontal vision in hammerhead sharks has been speculated about for decades but never tested," says Dr McComb.

So she and colleagues Professor Timothy Tricas from the University of Hawaii in Manoa, US and Stephen Kajiura, also from Florida Atlantic University, decided to do exactly that. They placed a variety of shark species, each with a different shaped head, into an aquarium tank.They then placed sensors on the shark's skin to measure its brain activity, specifically testing whether the animal would react to beams of light shone from different locations around the tank. By doing so, they could measure each shark's field of vision. "This study confirmed that hammerhead sharks have anterior binocular vision," says Dr McComb. That means they can see directly ahead while swimming and can accurately judge distance, particularly to any prey they hunt.

What's more, the researchers show that the degree of overlap between the two eyes increases with head width. In sharks with a usual head, such as the lemon shark (Negaprion brevirostris), the field of vision of each eye overlaps by just 10 degrees. Scalloped hammerheads (Sphyrna lewini) have a relatively wide head, and their eyes overlap by 32 degrees. However, the eyes of a winghead shark (Eusphyra blochii), which Dr McComb describes as a 'swimming boomerang' because its head width is almost half its body length, overlap by 48 degrees. "As the hammerhead head has expanded, the degree of binocular overlap has increased with it," Dr McComb explains.

The results surprised the researchers. "I believed hammerheads would not have binocular vision, because their eyes were pointing out on the sides of the head," admits Dr McComb."However, it turns out that the positioning of the eyes was really the key." The eyes of hammerhead sharks are tilted slightly forward, she says, allowing the field of vision of each to significantly overlap. "This study has confirmed that vision may have played a role in the evolution of one of the ocean's most bizarre inhabitants," Dr McComb says. "This has been a scientific question which has persisted since hammerheads were first described over 200 years ago."

The shape of the hammerhead brings further benefits, the researchers discovered. By moving their head sideways as they swim, the sharks can see much of what is behind them. More extraordinary is that the position of the eyes allows the sharks to see through 360 degrees in the vertical plane, meaning the animals can see above and below them at all times. As well as improving their ability to catch prey, "this may be beneficial to smaller sharks that are potential prey to larger sharks," says Dr McComb.