Hammerhead Sharks Have "Human" Vision

November 27, 2009

Hammerhead sharks' distinctive T-shaped heads give the predators human-like vision, according to a new study.

"There have always been ideas about why hammerheads have funny-looking heads, but nobody has systematically examined these hypotheses until relatively recently," said study team member Stephen Kajiura, a sensory biologist at Florida Atlantic University.

Based on electrical activity in the sharks' eyes, scientists think the shape gives hammerheads, like humans, excellent stereo vision and depth perception. Such traits may help the marine animals hunt speedy prey.

"Squid are jetting around in three dimensions," Kajiura said. "It's got to be a real challenge to catch a fast-moving prey like that."

(Watch an octopus battle a shark to the death.)

Shark Eyes

Kajiura and his team tested the visual fields of at least six sharks from each of three selected hammerhead species: bonnetheads from Florida, wingheads from Australia, and scalloped hammerheads from Hawaii. Globally, nine hammerhead species are known to exist.

(See shark facts, pictures, more.)

All the sharks used in the experiment were caught in their native waters, transported to local university labs for study, and later released back into the wild.

For the scalloped hammerhead sharks—which can reach lengths of 14 feet (4.3 meters)—the team collected juveniles.

In the labs, scientists swept weak lights in horizontal and vertical arcs around each shark's left and right eye and recorded electrical activity via electrodes embedded just beneath the sharks' corneas.

The team found that the overlap between what hammerheads could see with their left and right eyes is three times higher than the overlap in pointy-headed blacknose and lemon sharks.

In animals with forward-facing eyes, this visual overlap is what creates stereo vision and thus depth perception.

Blind Spot

However, hammerhead sharks pay a price for their wider field of view, said study leader Michelle McComb, also of Florida Atlantic University.

Because their eyes are spaced so far apart, hammerheads have larger blind spots directly in front of their heads than those of other sharks.

"There's actually been anecdotal claims by divers that they see little fish schooling right in front of the hammerheads' heads," McComb said.

"It's like the fish are swimming by and saying, Ha, ha, ha, you can't see me!"

The research is detailed in the November 27 issue of the Journal of Experimental Biology.

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