Por que os tubarões martelo têm a cabeça no formato de martelo?

domingo, novembro 29, 2009

Why the hammerhead shark got its hammer

15:05 27 November 2009 by Shanta Barley

It's one of evolution's most eccentric creations: a head shaped like a hammer. Now, a study suggests that the hammerhead shark may have evolved its oddly shaped snout to boost the animal's vision and hunting prowess.

For over a century, scientists have speculated why hammerheads evolved such an odd shape and whether having eyes so far apart would enhance their vision. In 1942 a leading authority on sharks, Gordon Walls, suggested the position of the shark's eyes prevented it from having binocular vision. But others have argued exactly the opposite, saying the animals must have enhanced eyesight.

Improved vision (Image Brian Skerry/NGS/Getty)

Now, hammerhead sharks have had their first eye examination, and it has laid the debate to rest. Sharks with wider heads have better binocular vision – all the better to track fast-moving prey like squid with far more accuracy than sharks with close-set eyes.

The research also shows that hammerheads – among other sharks – have a 360-degree view of the world in the vertical plane, allowing them to simultaneously see prey above and below them.

Read more here/Leia mais aqui.

Journal reference: The Journal of Experimental Biology, vol 212, p 4010

First published online November 27, 2009
Journal of Experimental Biology 212, 4010-4018 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.032615

Enhanced visual fields in hammerhead sharks

D. M. McComb1,*, T. C. Tricas2 and S. M. Kajiura1

1 Biological Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA
2 Department of Zoology and Hawaii Institute of Marine Biology, University of Hawaii at Manoa, Honolulu, HI 96822, USA

* Author for correspondence (dmccomb@fau.edu)

Accepted 9 September 2009

Several factors that influence the evolution of the unusual head morphology of hammerhead sharks (family Sphyrnidae) are proposed but few are empirically tested. In this study we tested the ‘enhanced binocular field’ hypothesis (that proposes enhanced frontal binocularity) by comparison of the visual fields of three hammerhead species: the bonnethead shark, Sphyrna tiburo, the scalloped hammerhead shark, Sphyrna lewini, and the winghead shark, Eusphyra blochii, with that of two carcharhinid species: the lemon shark, Negaprion brevirostris, and the blacknose shark, Carcharhinus acronotus. Additionally, eye rotation and head yaw were quantified to determine if species compensate for large blind areas anterior to the head. The winghead shark possessed the largest anterior binocular overlap (48 deg.) and was nearly four times larger than that of the lemon (10 deg.) and blacknose (11 deg.) sharks. The binocular overlap in the scalloped hammerhead sharks (34 deg.) was greater than the bonnethead sharks (13 deg.) and carcharhinid species; however, the bonnethead shark did not differ from the carcharhinids. These results indicate that binocular overlap has increased with lateral head expansion in hammerhead sharks. The hammerhead species did not demonstrate greater eye rotation in the anterior or posterior direction. However, both the scalloped hammerhead and bonnethead sharks exhibited greater head yaw during swimming (16.9 deg. and 15.6 deg., respectively) than the lemon (15.1 deg.) and blacknose (15.0 deg.) sharks, indicating a behavioral compensation for the anterior blind area. This study illustrates the larger binocular overlap in hammerhead species relative to their carcharhinid sister taxa and is consistent with the ‘enhanced binocular field’ hypothesis.

Key words: binocular vision, elasmobranch, eye, pupil dilation, Sphyrnidae, winghead shark


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