A growing body of research on sharks and their relatives portrays thes

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. Scientists are documenting elaborate social behaviors among these fish, including never-before-witnessed mating rituals that seem to be based on electrical signals.

Compared to other fishes, sharks have huge brains. 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, a researcher discovered a new sense organ, located on top of certain sharks’ heads. 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 no fewer than four separate sensory systems to detect chemicals in the water. 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. Elasmobranchs— sharks, skates, and rays—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 use electroreception to locate prey. Some scientists 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.

Electroreception may also play a key role in the mating system of sharks and the one thousand other fishes in the same taxonomic group. The females use electroreception for some sort of social cues. 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 aggregation. Field strength intensifies when the fishes open their mouths. Literally, “heavy breathing” could enhance a female’s attraction.

1. The complexity of the shark has been underestimated for which of the following reasons?

I. Sharks are a very ancient life form.
II. Sharks are dangerous.
III. Sharks rarely live in clear waters.

A. II only
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III


2. It can be inferred from the passage that female sharks hiding in groups from male sharks

A. could not be found if they were buried in the sand.
B. could be discovered by a male shark using his “third eye.”
C. would emit no bioelectrical signals.
D. would be easier to detect by electroreception than a female shark hiding alone.
E. would open their mouths as often as possible.


3. According to the passage, sharks may use their electroreceptive ability for all of the following EXCEPT to

A. locate prey.
B. navigate.
C. gather light.
D. locate other sharks.
E. locate breeding partners.


4. Which of the following is most relevant to the research described in the passage?

A. Certain birds communicate by emitting cries pitched two octaves above middle C.
B. Certain migrating birds determine their location by detecting variations in the strength of the earth’s magnetic field.
C. Migrating herds of wildebeest can reach their destination in spite of major changes in landscape from one year to the next.
D. Some migrating birds and insects appear to arrive at the same place on exactly the same day year after year.
E. Dogs are capable of hearing sounds at pitches inaudible to human ears.