Science / Medicine : Big Brains Come With the Big Teeth : Sharks: Despite a reputation as stupid, these predators are far more complex than scientists once thought.
In a world that hates predators, sharks probably get the worst press of all. Lions, tigers and wolves may be portrayed as bloodthirsty, but no one calls them stupid. Sharks have long been dismissed as dunderheads of the deep: “Lame-brained oceanic noses,” as one researcher put it.
Because sharks are such ancient life forms--they cruised the seas before the time of the dinosaurs--for many years, scientists considered them too primitive to have much in their heads than urges to smell and eat.
But a growing body of research on sharks and their relatives portrays these creatures as behaving in ways far more sophisticated and complex than was thought possible. “Some of their behaviors are far more reminiscent of birds and mammals” than of fish, says Scott Michael, a University of Nebraska shark specialist.
Perhaps most significant and surprising, scientists are documenting elaborate social behaviors among these fish, including never-before-witnessed mating rituals that seem to be based on electrical signals.
Shark researchers have also discovered a “third eye” in some deep-water sharks.
“There’s something big and significant going on between their ears,” Miami University shark expert Sam Gruber insists.
Compared to other fishes, Gruber points out, “shark brains are huge.” Their brain-to-bodyweight ratio is more comparable to that found in birds and mammals than to other fishes. But because sharks are so difficult to study--they are dangerous, far-ranging and usually inhabit murky waters--scientists only recently have accumulated enough data to even hint at their behavioral and sensory complexity.
In one of the most surprising findings, University of Maryland shark researcher EugenieClark last fall discovered a new sense organ, located on top of certain sharks’ heads. Diving among the six-gilled sharks of Bermuda and Grand Cayman Island and using specially pressurized gear to reach 4,000 feet, Clark saw what appeared to be “a big dark spot” on top of one shark’s head. What seemed to be an individual marking turned out to be present on all the six-gilled sharks she saw at close range on the dive. On closer inspection, the spot turned out to be “a transparent window on top of the shark’s head.”
Clark, known as the “shark lady” from National Geographic TV specials, is certain this window is a pineal organ. The organ is a sort of light-gathering “third eye” known in some prehistoric fishes, the lantern fishes, and at least one living reptile, the tuatara of New Zealand. Its precise function in the six-gilled shark is still uncertain, though sensing light at the deep depths to which they dive is most likely.
Sharks are literally covered in sense organs. Over the last two decades, researchers have found chemical receptor cells embedded inside sharks’ teeth, in their throats, around their heads, and in pore-like openings on the skin. With no fewer than four separate sensory systems to detect chemicals in the water, sharks are able to detect amino acids in concentrations as low as one part per billion.
Past researchers have documented that sharks also have good vision and can see in color; they have directional hearing, and although they cannot hear notes much above middle C, they can hear sounds below the threshold of human hearing.
But perhaps the most astounding sense possessed by sharks is their ability to sense electric fields, a fact first discovered two decades ago by a Netherlands researcher, Ad Kalmijn. But until last spring, scientists never suspected that sharks and their relatives might use this ability in a social context.
Sharks, skates and rays--a group of closely related fishes collectively known as elasmobranchs--can detect fields so weak they cannot be measured by standard laboratory equipment. All live organisms, immersed in water, have a weak bioelectric field, a current generated between biological membranes and the surrounding water.
Elasmobranch fishes, as well as Australia’s platypus and the related spiny anteater, use electroreception to locate prey. Some scientists, including Kalmijn, suspect that sharks, skates and rays, sensing and interpreting the much larger voltage potentials created by salt-water currents moving through the earth’s magnetic field, use this information to navigate.
Now it appears that electroreception may also play a key role in the mating systems of sharks and the 1,000 other fishes in the same taxonomic group. The finding was so unexpected, said Washington University investigator Tim Tricas, that “it really knocked our flippers off.”
Co-investigator Scott Michael knew he had “stumbled onto a gold mine” when he first visited the mating grounds of round stingrays in a quiet bay near Sonora, Mexico. Some of the small, flattened sharks piled atop one another, five animals thick, in all-female groups of 40 to 50. Other females lay buried in the sand in mysterious groups of up to 40, their rounded fins overlapping. And others, he discovered, lay buried in the sand singly--a fact Michael discovered when he saw a male ray abruptly turn in its path and excavate another ray. Then began the violent stingray courtship: the male biting the female’s side fins, the female stinging him with her spined tail.
To test whether the male had found the female using the electroreceptive sense, last spring Michael and Tricas constructed a dummy from plastic equipped with electrodes to generate a field to attract males. In the majority of cases in which a male swam anywhere near the buried dummy, the male turned and excavated the plastic model just as he would a live female.
But to the researchers’ astonishment, the males were not the only rays attracted to the dummy. One female swam within one meter of the dummy ray, then suddenly turned, approached, and quickly buried herself right beside it.
“The females are clearly using electroreception for some sort of social cues,” said Tricas. It seems, offers Michael, that “reading” one another’s’ electric fields is a key to an elaborate strategy: receptive females may be advertising their availability by congregating in large, highly visible, unburied piles; but unreceptive females, perhaps already pregnant, may use electroreception to locate other buried females, to hide from amorous males in buried aggregations.
With so many fins overlapping, it would be difficult for a male to find a single pectoral fin to grasp. But for the persistent males, to whom any chance to mate with a female--even an unreceptive one--is desirable, electroreception allows them to find singly buried females.
Tricas thinks sharks and rays might purposely vary the information content of their electric fields to send different messages to one another. Field strength intensifies when the fishes open their mouths. Literally, “heavy breathing” could enhance a female’s attraction. The researchers hope to test this hypothesis in Mexico this spring.