Unlocking the Mystery of
Sharks (part #2)
In November of 1995, Dr. Samuel Gruber (Professor, Rosensteil School
of Marine and Atmospheric Sciences, University of Miami) visited the marine
biology classroom at New World School of the Arts High School (NWSA) taught
by Gail Tucker (AE 94) where he discussed his research with her students
and other classrooms online from across the country. That realtime chat
was saved off the Internet and has been archived here for your interest.
Some editing has been done to provide continuity and the sense of community
we felt at that time.
Note: Historically, the first Access Excellence Fellows used AOL as their
provider and had screen names reflecting their AE status. That has changed
of course as we've moved to other providers and become more individualized
in our screen names! The teacher named was keying in questions from his/her
class as the chat progressed.
STUDENT CHAT, NOVEMBER 20, 1995:
NWSA : Hi folks. Just got on line and am waiting for Dr. Gruber to get
parked and in.
AEDBronner : We're just exchanging information about ourselves.
NWSA : If your students want to start asking questions I can put them
into this text as he answers them.
NWSA : Where are you all from?
AEDBronner : Van Hornesville, NY.
NWSA : How's the weather in upstate New York?
AEFCarter : Somerset, Kentucky. Not a very big town.
AEDBronner : Cold and wet.
NWSA : We're having mild weather. You all bracing for a cold winter rainy
storm I understand.
AEDBronner : We are also a small town 6 hours north of NYC.
NWSA : Who else is on? I'm showing four.
AEFCarter : Somerset is 75 miles away from Lexington, Kentucky.
NWSA : Hi Sandy, are you there or just holding?
NWSA : Anyone get a chance to look at the shark chat from last week that
I downloaded last night?
AEFCarter : No, not yet.
AEDBronner : Not yet. Hope to get to it tonight, barring surprises on
the telephone wires.
AESWardell : Last weekend it was snowing heaviy and this weekend it was
AEDBronner : Hope your weather comes this way by this weekend
AESWardell : We havent looked at the shark chat yet.
AEFCarter : The weather here is slightly cold, but that will be good
for the Thanksgiving weekend.
NWSA : Dr. Gruber just arrived. Any questions?
AESWardell : Can sharks smell?
AEDBronner : How long do sharks live?
Dr. Gruber : My studies on the lemon shark suggest at least 75 years.
One of the smaller species called the spiny dogfish males do not mature
until 30 years of age. After they have been caught, killed and rot on the
deck they smell pretty bad! But their sense of smell is par excellence supposedly....That's
one of the GREAT MYTHS about sharks--they have been called swimming noses.
But the fact is that any old catfish has a better sense of smell than the
biggest great white shark!
AEFCarter : Is it true that sharks at the beach rub against you, and
won't eat unhealthy people? Dr. Gruber : An old poem called them gentleman
maneaters--because they don't eat women--but keep in mind that this is not
true and nor is it true that sharks discriminate between healthy/ill people.
But ill people won't be swimming will they?
Dr. Gruber : Death however becomes attractive to tiger sharks; they will
scavenge dead prey. Sharks do seem to prefer men over women--but this is
not statistically related to number of men versus women who swim in the
oceans. The reason for this difference is unclear. The problem about shark
attacks in general is that they cannot be studied scientifically. What if
most of these drove Toyota cars? Does that mean that this is related to
shark attacks? Maybe they are mostly from Japan? or attacks in Japanese
waters? We can only make correlations--not discover all the causes why sharks
AEFCarter : How many species of sharks will attack a human?
AEDBronner : If you punch a shark in the nose will it go away?
Dr. Gruber : Yes, it probably will. And if it is close enough to do that
I would punch and kick and try to make it go away. Only the nurse shark
is less aggressive. Coming in that close is a clear warning.
AESWardell : How much do you know about the Mega Mouth shark, since it
was only discovered in 1976. [ Will be forwarded to Dr. Gruber ]
AEFCarter : What is the Dr.'s favorite species of shark and why?
Dr. Gruber : My 2nd favorite shark is the "land" shark from
Saturday Night Live! My favorite is the lemon shark because I've been working
with them for over 30 years. You either like em or hate em if you work with
someone for 30 years!
AEDBronner : Is it true that sharks will eat anything if hungry? Even
Dr. Gruber : Is it true that sharks don't like eating humans?
Dr. Gruber : Why do males take so long to mature? [to be forwarded--omit
AEGTucker : Do sharks mistake humans for seals?
DR. Gruber: No seals here in our Florida waters, yet we have shark attacks,
so this is not true. 3-4 species of shark consider any human legitimate
prey--great white, bull shark, tiger shark, oceanic white tip--all consider
you good to eat and could consume you completely--possibly also great hammerhead
but I doubt it. Other 70 species are not out to eat you--other reasons for
bites. Can't talk and so on, so communicates with jaws, teeth and body postures--rotates
around you to say "get out of my space" then attacks or bites
when you don't move. Typical posture: Fins down, heads up and hunches back
and swims toward you with a very exaggerated stroke--in a display (In his
own hilarious display, Dr. Gruber demonstrated this to the class!)
Dr. Gruber : People usually don't see it before they attack. There are
many motivations, fear because someone grabs them (sic!). Small sharks bite
more often for this reason. About 25% of attacks are food motivated. Keep
in mind, of 100 attacks reported in a year, 25 are fatal. Likelihood of
attack is therefore very remote.
Dr. Gruber : Not much data on whether sharks attack still swimmers or
moving or whatever. Spear fisherman get attacked more often than scuba divers.
Probably related to fish caught and blood in the water.
AEFCarter : I would attack a spear fisherman, too.
AEFCarter : Has the Dr. Gruber ever come face to face with any dangerous
sharks? [ YES INDEED--REGULARLY! ]
AEFCarter : I heard that sharks mistake human flesh for other fish, that's
why they bite.
AEDBronner : How did you study the sharks? Did you "swim with the
Dr. Gruber : Swimming with sharks? My first research with sharks was
100% lab based. I learned to "Talk" to sharks by training them,
using electrodes on their corneas to study their retinal responses and did
microscopy of retinas. Subsequently studied their bioenergetics in the lab.
Moved on in last 10 years to field studies. I don't study sharks by swimming
with them--THAT'S MY RECREATION! My field studies involve indirect observation
using remote tracking devices.
AEFCarter : Approximately how many species of sharks exist? Answer: 370.
Are many in real danger of becoming extinct [See teacher chat]?
AEFCarter : Why sharks? Why did you choose sharks? [See teacher chat]
Short answer--began studying shark protective clothing with a Navy project--led
to visual processing studies, color vision, etc.
AEDBronner : Do sharks see colors? If so, are there colors that agitate
them? [ See Teacher Chat. ]
AEDBronner : Why is it, that when they eat,their eye rolls back white?
Student question: What are the eye covers when sharks attack their prey?
Dr. Gruber : Varies. Certain sharks have a lid and close their eyes,
great whites roll their eyes back to protect them from the prey and fragments.
AEGTucker: A student here questioned how they could "hit" their
prey if they close their eyes as they "close in" Dr. G. demonstrated
estimated pathways with eyes closed to students, and snapped up a target
(prey)! [ more in the Teacher Chat ]
Dr. Gruber : Verbal response: It is only at the last possible second
that the eyes change--they survive that way and manage to capture prey that
move. We basically do the same kind of higher math to avoid traffic as does
a pilot that lands a plane. Imagine the subconscious calculations that occur--the
shark can "aim" itself just as effectively.
AEDBronner : Where in the water will attacks most likely occur?
AEFCarter : I heard that someone in Florida got bit in a little over
knee- deep water.
Dr. Gruber : Most likely attacks are in warm waters/tropics. The more
sharks with the ability to attack and the more people in the water the more
likely that they will meet. Sort of like the gas laws and the probability
of collisions with more sharks/people and in the tropics attacks are enhanced.
Dr. Gruber : How long can sharks go without eating? Food motivation.
Great myths about sharks #2--"eating machines" After five years
of study of feeding behavior of lemon sharks I found that they eat about
once every 36 to 48 hrs; only about 2% of their body weight every day. Only
about 6% of body weight in six days. So a 100 lb shark eats only 6 pounds
of food in six days. They are on a very strict schedule of feeding.
AEDBronner : Isn't the whale shark larger than the great white?
Dr. Gruber: Yes.
AEFCarter : Someone here swam with a Lemon shark.
Dr. Gruber : Hunger motivation versus aggressive attack. Some scientists
have written that sharks eat without hunger. Extremes exist in nature: Preying
mantis will eat until its digestive tract is packed and food comes out (its
mouth). Sharks will eat about 2.7% feeding in one day and stop eating even
with food available (this was discovered in the laboratory setting). So,
unlike the mantis they do not overeat.
AESWardell : How do sharks mate?
AESWardell : Do female sharks stop eating before having pups.
Dr. Gruber: Females and pups? That is a theory, but in my experience
I can catch a full term pregnant female on a bait--suggesting they do not
stop feeding until they are ready to deliver.
Dr. Gruber : Later on--24 hours later they might come back and eat those
AESWardell : How fast do shark swim?
Dr. Gruber : Speed....Not known at max. No one knows for sure even for
Dr. Gruber : Sleeping....Seems to me that animals with large brains relative
to body size require activity to cease for a while because the brain is
so active that it depletes brain chemicals, neurotransmitters. Sharks do
have relatively large brains (proportionate to their body size), but evidence
re never stop moving (not true--another myth). Deep sea sharks must move
because they sink otherwise--negatively buoyant.
AEGTucker : A builtin spinal program puts shark on autopilot and Dr.
Gruber has seen decerebrate sharks swim in perfectly normal fashion--suggesting
that they can swim while "asleep". Reef sharks rest on the bottom
and are there longer than they swim. Lemon sharks spend about 25% of their
time on the bottom, without forward progress--uses tracking devices on sharks
to determine this.
AEDBronner : How does this apply to the story that sharks need to swim
to keep water moving through the gills? [ see teacher chat ].
AEFCarter : How many miles do sharks travel a day, or in a week? [ see
teacher chat ].
AEDBronner : The hammerhead shark has amazing adaptations as does the
stingray. What other unusual adaptations are there in elasmobranchs?
Dr. Gruber : We need to get away from anthropomorphosing these animals
to really understand them.
Dr. Gruber : Morphology versus special senses, digestion, etc.
AEFCarter : Is it true that they have microscopic "teeth" on
their skin for protection? (See Teacher Chat)
AEDBronner : How large is the largest recorded shark? [ See Teacher Chat].
Dr. Gruber : Size Variations: Sharks--whale sharks--versus 8 inch dwarf
shark gives tremendous range of size in sharks. Shape: regular shape is
"fusiform"; thresher shark has a tail as long as its body and
uses the tail as a feeding device; 3-4 sharks will surround the school of
fish and whack it with their tail. This also makes them get caught in nets.
3 different species. Other sharks such as Zebra shark have tails as long
as their bodies. Deep sea sharks may have "lights"--photophores
with luciferin and luciferase in these specialized surface organs--Green
lantern shark. Big Eye Thresher--largest eye relative to body size for any
vertebrate. 100 pound big shark has eye larger than a softball.
AEDBronner : Do any other sharks exhibit any social behaviors?
Dr. Gruber : Mating in sharks. Almost nothing is known about mating.
Vertebrates other than these are more understood. About four species have
been observed mating.
Dr. Gruber : The most characteristic thing about sharks that we see in
shark fossils and in contemporary sharks is that on the male there is an
extra organ--the clasper--which is the most characteristic shark structure.
All of the cartilaginous fishes have internal fertilization. Aristotle thought
the clasper was used to "clasp" the females with this, but this
is not true. They use their teeth to gain purchase of female around the
gills and pectoral fin to control her during copulation. The clasper is
involved in sperm insertion into the cloaca of the female.
AEFCarter : Have many different species ever been tried to be bred?
AESWardell : Can sharks be useful in cancer research?
Dr. Gruber : See Teacher Chat
AEDBronner : Are ther any sharks that care for young in any way at all?
Dr. Gruber : Already provided a detailed discussion of parental care
in the Teacher Chat.
Dr. Gruber : Short answer: Almost all sharks protect their young--BUT
only during pregnancy and this may last 2.5 years! Very long gestation.
AESWardell : How do sharks communicate with each other?
Dr. Gruber : Communication--olfactory signals, shape and size and patterns,
body postures are visual cues.
Dr. Gruber : Numbers of Sharks: About 370 species of sharks. This is
a small group in terms of organisms. 320 million years ago in the seas,
sharks outnumbered bony fish 6 to 1. For every 6 sharks there was only one
bony fish. For every shark today there are 25 species of bony fish--only
make up 4% or so of all fishes. There are 8-9000 mammals for example. So
sharks are an old, small, but highly diverse group.
AEDBronner : Are sharks cannibalistic?
Dr. Gruber : Most sharks are cannibalistic--especially the lemon shark.
Impression is that this is a regular part of their life history pattern.
Student Question : Is it true that the size of liver is related to keeping
a shark afloat?
Dr. Gruber : Yes, it is the oily material, liver oils in their huge livers,
that makes them buoyant. Also: Sand tiger sharks gulp air by surfacing and
taking it in to assist with buoyancy. Oil is called squaline.
AEDBronner : Would a leopard shark go after a person?
Student Question: What is a leopard shark--many species with this name.
Dr. Gruber: In my terms it is actually a cool water species with tiny
teeth and does not eat anything but molluscs etc. Tiger shark is sometimes
called leopard shark and it goes after humans. Leopard shark is a Pacific
Student Question : Do sharks have teeth on their skin?
Dr. Gruber: Yes! Scales are called placoid scales and make the surface
sandpapery to the touch. Sharks are armored by these teeth--actual anatomic
form is a tooth; have a base and a nerve that serves them. and have enamel.
Shape has been studied by NASA for the space shuttle shape allows good hydronamics
and is same to consider for plates on shuttle. Sharks however do not like
to be touched and yet they do rub up against people and things. Sharkskin
was used as sandpaper by billiard ball makers working with ivory in the
old days. This "Shagreen" has been used almost exclusively on
the handles of Samurai swords.
AEGTucker: I remember having dogfish [probably Mustelus sp.] rub up against
my legs while night swimming off Cape Cod in Woods Hole; very sandpapery
skin and very delicate touch; non-aggressive.
AEGTucker : Thanks everyone. We are finishing this period here. Change
of class. Will save and download this to you all.
AESWardell : Thank you
AEDBronner : Thank You for all the information .
AEGTucker : See you.