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Are hammerheads an experiment in evolution gone wrong?

November 7, 2013
BY DONNELL TATE , Lahaina News

If the hammerhead shark (Mano Kikikihi) did not exist, it is hard to imagine that anyone would ever dream up such a bizarre and unusual form.

The distinctive "flying wedge" shaped head, with its eyes and nostrils pushed out to the tip of the wedge, suggest some weird mutation or perhaps an experiment in evolution gone wrong.

Of course, this is not the case at all. In fact, the hammerhead family is a most remarkable group of sharks perfectly adapted to their own particular way of life.

Article Photos

Three hammerhead shark species are found in Hawaiian waters.

Hammerheads are found in almost all of the tropical and warmer temperate waters of the world. Eight different species are known, and three of these are found in Hawaiian waters: the scalloped, the smooth, and the great hammerhead shark.

Hammerheads have quite a range in size depending on the species. Scalloped hammerheads are generally 5-10 feet long and weigh about 175 to 225 pounds. Smooth hammerheads can grow a couple of feet longer, but the great hammerhead is by far the largest. These carnivores can grow up to 18 feet long and weigh as much as 800 pounds.

So, let's first deal with the question of exactly why hammerheads have such a bizarre modification to their heads.

The first is that it acts as a lift when swimming - much like an airplane wing (bearing in mind that sharks are negative buoyant and need to generate lift as they swim). Research indicates that while hammerheads have better maneuvering capabilities than other sharks, it's not likely due to the cephalofoil. They're more flexible and can therefore turn and pivot more easily with greater speed.

Second, the broad head spreads the eyes, making binocular vision more efficient.

Third, their "hammer-shaped" head is thought to have evolved to maximize the area of sensory organs. The many sensory capabilities of the shark, such as smell (the nostrils are wide apart), and chemical, electrical and pressure senses are increased by spreading them along the leading edge.

To me, this seems a highly likely function of the broad head, since hammerheads move their heads in sweeping arcs when hunting, not unlike biological metal detectors. This allows hammerheads to scan significantly larger areas of the bottom when hunting than other shark species.

The most likely explanation is that the cephalofoil increases the hammerhead's ability to sense prey. All sharks have electrical sensors in their nose and heads called ampullae of Lorenzini, named for researcher Stephan Lorenzini. These sensors can detect weak electric emissions from other sea life.

There is even a suggestion that some hammerheads are particularly sensitive to the Earth's magnetic lines of force, since they congregate over basaltic sea mounts, and the shape of their head is some how connected to this sense.

Hammerheads are certainly highly electro-receptive, being able to detect the presence of fish buried in sand. The hammerhead is particularly fond of stingrays and is often observed swimming close to sandy bottoms obviously hunting for this prey.

Another theory is that the hammerhead uses its cephalofoil to aid in trapping prey. Once a stingray is located, the hammerhead pins the ray to the ground with its cephalofoil and starts eating. This theory has been observed in the wild, but it's probably a learned technique and not the central reason for the wide cephalofoil.

Have you ever wondered how such a bizarre body form evolved? Where was the missing link between normal sharks and these broad-headed wonders?

Well, the missing links do exist, with several species within the family, all found in the Americas, having flattened but not hugely laterally extended heads.

These are variously called bonnetheads, scoopheads or shovelheads, showing nicely the intermediate "grades" between a normal shark shape as it evolved into the unmistakable hammerhead.

 
 
 

 

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