There are almost as many myths and misconceptions about sharks as there are
species. NOVA asked Dr. Samuel "Sonny" Gruber, arguably the world's leading
shark researcher, to unmuddy the waters about this most maligned of fish. A
professor of marine biology at the University of Miami's Rosenstiel School of
Marine and Atmospheric Science, Gruber has been called the "guru of sharks." He
has more than 100 published scientific papers to his credit, and his work on
the behavior and physiology of lemon sharks in Bimini constitutes the most
extensive studies on any shark species.
NOVA: Are sharks really the vicious killers they're made out to be?
Gruber: Most sharks are far too small to be vicious killers of men and women,
and many sharks live in the deep sea where no humans swim. The fact remains
that around 100 shark attacks do occur each year, but elephants are bigger
killers; they do in 200-plus mahoots and trainers each year. Talk about vicious—dogs kill thousands. And get this: Soda machines killed more people last
year than sharks did. People get angry at the machines when they steal the
$0.75 the person honestly inserts for a cool, refreshing soda. But if no soda
comes out, people often viciously attack the machine, trying to make it cough
up the soda or the cash. The machines viciously retaliate by falling on and
crushing their tormentors.
Whitetip reef shark cruises past crown-of-thorns starfish.
NOVA: Do sharks have enemies?
Gruber: The greatest enemy of sharks is man. He kills and hacks off the fins of
30 to 100 million sharks each year. That is, in three to five years fishers
kill the equivalent of the entire population of the U.S. Humans are not natural
predators of sharks, as we are terrestrial hunters and didn't really fish for
sharks until we became civilized. Now we cut off fins and throw the living
creature back to die on the bottom of the sea. Is that civilized?
It's true, some shark species are adapted to feed on other sharks; a prime
example is the bull shark. In theory, any large shark could consume a smaller
one, though this is not usually the case. The most numerous natural enemies of
sharks are the various parasites that can kill a shark if they get out of
NOVA: Are sharks stupid compared to mammals and birds?
Gruber: I wrote a paper with Neil Schneiderman in 1975 demonstrating that lemon
sharks learn faster and retain a conditioned response longer than a cat or
rabbit. Sharks and their large-brained allies the rays have brain/body weight
ratios in the range of birds and mammals. Personally, I don't think they're as
intelligent as birds or mammals—whatever intelligence is. Still, if you've
ever dealt with a chicken you'd think sharks were genius-level
Silky sharking comin' at ye.
NOVA: Can sharks really find prey without seeing or smelling them?
Gruber: Absolutely. A shark is like the Starship Enterprise, bristling with
sensors. The most amazing is its electrical sense. Measurements demonstrate
that a shark can detect electric fields of a magnitude similar to a 12-volt car
battery placed in the middle of the Atlantic Ocean, with the positive pole
connected to Boston Harbor and the negative pole placed in the harbor at
Plymouth, England. The miniscule current flowing across the ocean—1/1,000,000,000 volt per square centimeter—would be at a level detectable by
sharks. Translated to hunting, a shark can sense a prey in turbid water or
buried beneath the seafloor by electrical sense alone. Hearing provides another
cue. Sounds of struggling fish attract sharks. Their senses are truly a marvel
A Lemon shark patrols the shallows.
NOVA: Can sharks, as I've read, really detect the smell of as little as one
drop of fish extract in a quarter-acre lagoon six and a half feet deep?
Gruber: I've seen lemon sharks go wild over a single drop of blood in a
2,000-gallon tank. In the underwater world of noses, catfish and eels may take
the cake, and salmon, of course, find their home stream after a year or more at
sea strictly by smell. Wondrous! But sharks are keen predators and, because
they are always searching for food, show up first if there is fish scent in the
water. That's why chumming is so effective for sharks. It comes down to the
concentration of the stimulating chemical at the nose's receptor cell that
determines if an animal will detect a smell. That level is parts per million in
sharks and parts per billion in catfish.
Whitetip reef shark at
NOVA: Can sharks see in the dark?
Gruber: Yes, and so can we and cats, too. But sharks are especially good at it.
In tests I did for my doctorate, I trained sharks to respond to minimum light
levels and compared these to human subjects. Sharks were about ten times more
sensitive than a human tested in the exact same apparatus. I think sharks can
see by starlight on a clear, moonless night.