Hammerhead Sharks Have Stereo Vision

The unique head structures of these creatures allow them to see 360 degrees.

Ever since hammerhead sharks were first observed over 200 years ago, their distinctive heads have been scrutinized, leading to widespread speculation about their function.

Now, a study published in the Journal of Experimental Biology claims to have proven one of those theories: The hammer shape gives the sharks astounding eyesight.

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Different hypotheses painted the shark's unusual lateral head extensions, or cephalofoils, as everything from fins for improved steering to antennae for increased electroreception, a trait that allows hammerheads to detect minute electrical currents generated by twitching fish muscles.

Other theories surrounding the cephalofoils involved the shark's vision, sense of smell and even its ability to properly bite into preferred prey.

Definite answers were hard to come by, but all that changed in 2008, thanks to a U.S. National Science Foundation collaborative grant aimed at finally solving the puzzle.

"No one has systematically attacked every one of these hypotheses regarding hammerhead morphology before," Dr. Philip J. Motta told Discovery News. "And that's what this project is doing."

Motta's University of Southern Florida research team has largely ruled out the hypothesis that head shape relates to feeding, leaving a few theories still on the table.

Enter the team of D. Michelle McComb, Timothy C. Tricas and Stephen Kajiura. The Florida Atlantic University and Hawaii Institute of Marine Biology joint study set out to discover just how the hammerhead's uniquely placed eyes affect visual perception.

The hammerlike structure extends the shark's eyes away from its body, leading scientists to wonder whether each eye's field of vision overlapped with the other.

Did they benefit from binocular vision, or did each eye provide its own, separate monocular scan of a given area?

"Initially I believed that the hammerheads would lack the capacity for binocular vision just, because the eyes point straight out to the side," McComb told Discovery News. "It turns out that the eye position was the key. Nearly every hammerhead shark species has eyes that are canted slightly forward, which affords them the large binocular overlaps and enhanced stereo vision."

The result is a head shaped by evolution to allow for exceptional depth perception, an attribute that comes in handy during high-speed underwater pursuits.

McComb and colleagues collected scalloped hammerheads, bonnetheads and blacknose sharks, a sampling of the diversity among hammerheads in the family Sphyrnidae.

Back in the lab, they tested the different sharks' eyes with sweeping arcs of light, recording each eye's electrical activity to measure its visual field.

All of the sharks exhibited 360-degree vertical visual fields, allowing them to scan both lower and lesser depths at any given moment.

Even more impressive, the research team found that the bonnethead and scalloped hammerheads also have excellent rearview binocular vision -- a particularly good skill for smaller sharks that haven't worked their way to the top of the food chain.

The study may clarify questions surrounding the shark's evolution, but it hardly devalues the other theorized benefits of the cephalofoils, all of which could prove valid.

"It has been demonstrated that hammerheads likely have enhanced electrosensory capabilities," McComb said. "Not so much in greater sensitivity but rather due to a larger surface area of electrosensors, which allow hammerheads to sweep a larger area in the same time that a normal pointed-nosed shark could."

Meanwhile, researchers continue to gauge the possibility that hammerhead sharks benefit from a heightened sense of smell due to wider nostril spacing and improved hydrodynamic lift and steering.

"I think with (McComb, Tricas and Kajiura's study) we're getting closer to figuring out the reason for the head morphology," Motta said. "My gut theory is that it's sensory."

Robert Lamb is a writer for HowStuffWorks.com.