A new study involving hammerhead sharks from Käne'ohe Bay indicates the weird-looking creatures have outstanding binocular eyesight and a 360-degree field of vision.

Binocular vision, which occurs when the view fields of two eyes overlap, gives the sharks excellent depth perception, making it easier for them to pinpoint squid, fish and other prey against the one-dimensional backdrop of an expansive blue ocean and to avoid becoming prey for larger sharks, said Mikki McComb, a researcher at Florida Atlantic University in Boca Raton.

"Being able to see what's happening all around you is certainly an advantage," she said.

Scientists have debated whether the wide-spaced placement of the hammerhead's eyes improves its vision or makes it more difficult for the shark to look forward.

"To be honest, when we first started the project, I thought no way the hammerhead would have binocular vision," McComb said.

Her study examined three hammerhead shark species, ranging from the bonnethead, with its semicircular head that resembles a shovel, to the winghead hammerhead, whose long and narrow lateral head blades measure half the length of its body.

In between is the scalloped hammerhead found in Käne'ohe Bay.

Six juvenile scalloped hammerheads were caught by McComb and associates and taken to the Hawai'i Institute of Marine Biology on Coconut Island, where they were anesthetized and strapped to a stationary board in an aerated tank.

Using an electroretinogram technique, researchers measured the sharks' field of view by sweeping a weak beam of light in horizontal and vertical arcs around each eye and recording the eye's electrical activity.

McComb said the tests took 60 to 90 minutes, after which the sharks were returned to the ocean.

The researchers also examined winghead hammerheads in Australia and bonnethead hammerheads, and "normal-looking" lemon and blacknose sharks from Florida.

The results showed the winghead hammerhead — the shark with the widest head — had the best binocular vision, with a 48-degree overlap of visual fields, nearly four times that of the "normal" sharks.

The scalloped hammerheads showed a 34-degree overlap, and the bonnethead was at 13 degrees, not much better than the lemon and blacknose sharks.

When researchers factored in eye rotation and head movements, the forward binocular overlaps stretched to 69 degrees for the scalloped hammerheads and 52 degrees for the bonnetheads.

"We found that hammerheads not only have frontal vision but binocular vision and overlaps that are even larger than in normal sharks," McComb said. "And the binocular overlap increases with the lateral expansion of the head."

The study also found the sharks compensate for blind spots by rotating their eyes and moving their heads while swimming.

"When they move their head and move their eyes, they can actually see 360 degrees around their head," McComb said.

Hammerheads are one of the most recently evolved groups of sharks, and McComb said the study suggests the benefit of enhanced vision may have influenced evolution of the shark's unusual head.

Still unanswered, she said, are questions about whether the hammerhead's lateral head blades improve its sense of smell and electrosensory capabilities, and aid swimming by producing hydrodynamic lift.

Teaming up with McComb for the research were Stephen Kajiura of Florida Atlantic University and Timothy Tricas of the Hawai'i Institute of Marine Biology.

Their research is published in the Nov. 27 Journal of Experimental Biology.