Hammerhead sharks are some of the Ocean's most distinctive residents. 'Everyone wants to understand why they have this strange head shape,' says Michelle McComb from Florida Atlantic University. One possible reason is the shark's vision. 'Perhaps their visual field has been enhanced by their weird head shape,' says McComb, giving the sharks excellent stereovision and depth perception. However, according to McComb, there were two schools of thought on this theory. In 1942, G. Walls speculated that the sharks couldn't possibly have binocular vision because their eyes were stuck out on the sides of their heads. However, in 1984, Leonard Campagno suggested that the sharks would have excellent depth perception because their eyes are so widely separated. 'In fact one of the things they say on TV shows is that hammerheads have better vision than other sharks,' says McComb, 'but no one had ever tested this'. Teaming up with Stephen Kajiura and Timothy Tricas, the trio decided to find out how wide a hammerhead's field of view is and whether they could have binocular vision and publish their results on November 27 2009 in the Journal of Experimental Biology at http://jeb.biologists.org.
Hammerheads come in all shapes and sizes so McComb and Kajiura, opted to work with species with heads ranging from the narrowest to the widest. Fishing for juvenile scalloped hammerheads off Hawaii and bonnethead sharks in the waters around Florida, the team successfully landed the fish and quickly transported them back to local labs to test the fish's eyesight.
The team tested the field of view in each shark's eyes by sweeping a weak light in horizontal and vertical arcs around each eye and recorded the eye's electrical activity. Comparing the hammerheads with pointy nosed species, the team found that the scalloped hammerheads had the largest monocular visual field, at an amazing 182 deg., and the bonnethead had a 176 deg.
|Contact: Kathryn Knight|
The Company of Biologists