Why the hammerhead shark got its hammer

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.

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.

Sharks at the optometrist

Michelle McComb of Florida Atlantic University in Boca Raton and colleagues compared the visual fields of three species of hammerhead – the winghead, the bonnethead and the scalloped hammerhead – with those of two other species of shark.

After implanting electrodes into the sharks' eyes, the researchers moved a beam of light across them until the eyes no longer demonstrated electrical activity. This allowed them to measure each eye's field of vision, which they summed to calculate each species' "binocular overlap".

"To our surprise, we found that the degree of overlap increased as the head of the hammerhead species widened," says McComb. The shark with the widest head, the winghead, had 48 degrees of binocular overlap; the others ranged from 10 to 32 degrees.

Eyes on the ball

That overlap helps hammerheads to perceive depth as they hunt, says Demian Chapman of the Institute for Ocean Conservation Science at Stony Brook University in New York. "I've seen hammerheads chasing stingrays in the Bahamas," he says. "The rays are fast and can turn rapidly, so if the hammerhead is to catch one, it has to be able to keep its eye on the ball."

Shark biologist Samuel Gruber of the Bimini Biological Field Station in Miami, Florida, says the paper has changed his view of the debate.

Previously, researchers have theorised that the hammerhead's head may improve its sense of smell, boost its ability to locate prey using electric fields, improve its manoeuvrability or help it to pin down struggling rays so it can bite off their wing-like fins.

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

Source: New Scientist

Hammerhead Shark Killed, Left On Dock

PALM BEACH SHORES, Fla. -- A Palm Beach County conservationist said a 9-foot female scalloped hammerhead shark was caught and left on a dock at Sailfish Marina in Palm Beach Shores.Jim Abernathy sent photos of the shark to WPBF 25 News. Dock attendants told him that the captain of a vessel had brought the shark to the dock and left it.Abernathy said that it's illegal to kill a fish that you don't plan to eat. Abernathy said fish that won't be eaten must be immediately released under Florida law.Abernathy said this shark may have been pregnant."(Hammerhead sharks are) being driven closer and closer to extinction because of commercial fishing to their fins, and also because of careless and wasteful practices like what we've seen today," said Abernathy.

Copyright 2010 by WPBF.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Hammerhead Shark

Shark Week Sensationalism that Sells

Sharks are being given a bad rap again when Discovery Channel ‘Sinks Teeth Into Shark Week 2010’ with a lineup of programs, many of which depict sharks as bloodthirsty man-eaters. We desperately need improved public perception to win the battle against plummeting shark populations, and irresponsible TV and media portrayal of sharks’, which perpetuates poor perceptions of them, only adds to their threat. The media have immense power to help us save our planet’s resources through responsible reporting, or not. They can choose to continue accelerating the demise of these much maligned animals, which will ultimately result in our own downfall as everything in nature is connected; or they can support us and help us save them so we might save ourselves. It is a shame on Discovery Channel to see them continuing to work against the good efforts we as shark conservationists around the globe fight so hard to achieve, changing fear and loathing of sharks to much needed understanding and admiration.

But instead of me continuing to express my utmost disdain and irritation at the shortsighted, ignorant, high ratings, big dollar driven commissioning editors, and the like, at Discovery Channel, I will leave it up to fellow conservationist and multi-award winning filmmaker and author, Chris Palmer. Chris has spent 25 years producing more than 300 hours for prime time television and the giant screen (IMAX) film industry, and if anyone’s opinion should be respected it is his: “Teeth of death,” “Shark feeding frenzy,” “The Worst Shark Attack Ever.” It is that time of year again, when the Discovery Channel brings out shows like these as part of its annual “Shark Week” programming. This week of bloody feeding frenzies and vicious shark attacks is part of a larger trend in nature programming. Instead of seeking to educate or to promote environmental conservation, these shows focus only on presenting graphic, sensationalized animal violence. Programs like those in Shark Week – while they might garner high ratings and attract advertiser dollars – all too often mislead the audience, exploit animals, and fail to promote conservation. It is easy to understand why Shark Week or other shows like “Untamed and Uncut”, “Man vs. Wild”, or “When Animals Attack” would attract viewers…”

Read more: http://www.huffingtonpost.com/chris-palmer/shark-week—-education-o_b_660876.html

Hammerhead Shark

The hammerhead sharks are a group of sharks in the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a "cephalofoil". Most hammerhead species are placed in the genus Sphyrna; some authorities place the winghead shark in its own genus, Eusphyra. Many, not necessarily mutually exclusive, functions have been proposed for the cephalofoil, including sensory reception, maneuvering, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in schools. Some of these schools can be found near the Cocos Islands by Costa Rica.


The nine known species range from 0.9 to 6 m (3.0 to 20 ft) long. The average hammerhead shark weighs about 500 pounds, but some may grow up to 1000. All the species have a projection of their face on all sides of the head that gives it a resemblance to a flattened hammer.

It was determined recently that the hammer-like shape of the head evolved to enhance the animal's vision. The positioning of the eyes give the shark good binocular vision, as well as 360-degree vision in the vertical plane, meaning they can see above and below them at all times. The shape of the head was previously thought to help the shark find food, aiding in close-quarters maneuverability and allowing sharp turning movement without losing stability. However, it was found that the unusual structure of its vertebrae allowed it to make the turns correctly, more often than its head. The hammer would also shift and provide lift.

Hammerheads are one of the most negatively buoyant of sharks. Like all sharks, hammerheads have electroreceptory sensory pores called ampullae of Lorenzini. By distributing the receptors over a wider area, hammerheads can sweep for prey more effectively. These sharks have been able to detect an electrical signal of half a billionth of a volt. The hammer also allows the nostrils to be placed farther apart, increasing its ability to detect chemical gradients and localize the source.

Hammerheads have disproportionately small mouths and seem to do a lot of bottom-hunting. They are also known to form schools during the day, sometimes in groups of over 100. In the evening, like other sharks, they become solitary hunters. Hammerheads are notably one of the few animals that acquire a tan from prolonged exposure to sunlight. Tanning occurs when a hammerhead is in shallow waters or close to the surface for long periods.

Source: Wikipedia Shark Portal

Scalloped Hammerhead Shark

Along with the seven other known species of hammerhead sharks, the scalloped hammerhead has a strange, flattened, T-shaped head. In this species, the front of the head has three notches, which produces the scalloped shape from which it takes its name.

The eyes are located at the sides of the head. Hunting near the seabed, the shark swings its head from side to side, looking for prey such as fish, other sharks, octopus, and crustaceans, and using sensory pits on its head to detect the electrical fields of buried prey such as rays. The head may also function as an airfoil, giving the shark lift and helping it to twist and turn as it chases its prey.

Scalloped hammerheads may be seen in large shoals of over a hundred individuals. They give birth to live young in shallow bays and estuaries, where the skin of the young darkens to give protection against sunlight.

Threats

The fins of the scalloped hammerhead are extremely valuable for use in shark fin soup and the scalloped hammerhead is taken both as a target species and as bycatch with pelagic longlines, fixed bottom longlines, nets and pelagic trawls. The meat, skin and oil are also utilized. The scalloped hammerhead has declined by more than 75 percent in the past 15 years along the eastern U.S. and is listed as “Endangered” on the IUCN Red List.

The decline of the scalloped hammerhead, and other great sharks in the Atlantic, has led to an increase in the cownose ray population which resulted in a collapse of the century-old North Carolina bay scallop industry. Oceana’s Predators as Prey report details this cascading effect and other important impacts the loss of sharks will have on the marine ecosystem.

Source: Oceana.org

New week, new shark information

This week I'll be talking about the Hammerhead Shark.
The great and the scalloped hammerhead are listed on the World Conservation Union's (IUCN) 2008 Red List as endangered, whereas the smalleye hammerhead is listed as vulnerable. The status given to these sharks is as a result of over-fishing and demand for their fins, an expensive delicacy. Among others, scientists expressed their concern about the plight of the scalloped hammerhead at the American Association for the Advancement of Science annual meeting in Boston. The young swim mostly in shallow waters along shores all over the world to avoid predators.