1. Shark repellent
2. How dangerous?
3. Shark lovers
4. A social killer?
The winghead shark, Eusphyra blochii.
Proof that evolution has a sense of humor?
Scalloped hammerhead shark, Sphyrna lewini
©Stephen M. Kajiura
A bonnethead shark, seen from above, has
the smallest head of the eight hammerhead species. Copyright©
See the movie
(184KB): At close range, sharks detect electrical signals
from their prey. Watch the hammerhead scout the artificial electric
fields under the sand. The shark doesn't roll as it turns.
Thousands of scalloped hammerhead sharks swim
the waters of Cocos Island, in the Pacific Ocean off Costa Rica.
The explanation for the hammer's curiously broad head has been bugging Stephen Kajiura, associate professor of biological science at Florida Atlantic University, since he caught a case of hammerhead fever in graduate school a decade ago.
The head occurs on eight species of hammerheads
(technically called sphyrnids) and appears in fossils from at least
20 million years ago, so it obviously works. "They have this wide
range of morphology [shape] that allows you to ask an interesting
question," says Kajiura. "Why did this particular head shape evolve,
and why was it maintained and radiated into such a huge variety?"
In evolution, "radiation" is the process of evolution from one parent species
into several different species. One result of hammerhead radiation
is the oddball winghead shark, with a head half as wide as the length
of its body!
Kajiura says several possible explanations for the ham-head's head have been begging for a test:
Mobility: Maybe the flat head helps the shark move through the water, acting like a fin.
Quality: Maybe widely spaced sensors on the huge head helps the fish locate smells, sights, and/or the electric fields emitted by prey.
Maybe the head holds a greater amount of apparatus for detecting
smells and/or electric fields. (Hammerheads, like all sharks, use
electrosensory organs to locate electric fields given off by prey,
but only when closer than 30 centimeters.)
In a series of experiments, Kajiura has been testing these hypotheses. Here are some results to date:
Mobility: Does the ham-head use what Kajiura calls "this large planing surface on the anterior end to provide lift, or to bank and roll"? He says videos like the one on this page show that even in fast, sharp turns, the animal remains more horizontal than closely related sharks with more conventional heads. Instead of providing lift or propulsion, he says, "The head is acting more like a stabilizer, holding the body horizontal."
Quality: Does the wide head help the shark locate prey
more accurately? No. The electrical receptors do not seem to locate
prey better than receptors on other sharks. Nor do the eyes have
it. Wider eyes give better depth perception -- but only for objects
visible to both eyes. But hammerhead eyes aim sideways, so only
distant objects are visible to both eyes. And those objects are
usually obscured by murky water. Nor does the idea that the head
serves as an olfactory direction-finder have the scent of success,
Kajiura adds. The central location of the nostrils, together with
channels that bring water toward them, both argue against an unusual
ability to find the origin of a scent.
Quantity: Does that honking big head house more
sensory receptors? Perhaps. The electrical receptors do sweep a
broader area each time the shark passes across the seabed in search
of prey hiding under the sand. To test whether the nose beats related
sharks at detecting faint scents, Kajiura is piping water laced
with amino acids into shark nostrils in Hawaii. The results will
become subject for another scientific paper in his effort to nail
down the evolution of the hammerhead.
The picture is murky at present, Kajiura admits. "There's no really simple answer, you can't attribute the head to any one selective force. All these factors come into play, they work together, rather than in opposition," to account for the highly unusual, but highly successful, hammerhead.
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