Unlocking the Mystery of
Sharks (part #1)
In November of 1995, Dr. Samuel Gruber (University of Miami, Rosensteil
School of Marine and Atmospheric Sciences) appeared in live-chat format
for the AE community. At that time we all used AOL as our online service
provider. In his chats with us Dr. Gruber responded to questions e-mailed
ahead of time, and discussed some of his research. An internationally recognized
scientist who has worked with Elasmobranchs throughout his professional
career, he has also shown his commitment to secondary school education by
providing four day experiences in shark telemetry for high school students
at the Bimini Marine Station where he does his field research. He selects
students from those recommended by individual teachers in the Dade County
Public School District, Miami, Florida. This contact, which includes swimming
with the sharks in the shallows of the Bahamas, excites and intrigues students
and often prompts them to pursue careers in science. Some minor editing
has been done on the chats--some questions/answers have been moved to improve
continuity. This chat was held in the evening for teachers only, the second
chat was held during the school day to encourage student participation and
LIVE TEACHER CHAT, NOVEMBER 14, 1995:
AEGTucker : Hello all. I'm here with Dr. Samuel Gruber and we're ready
to chat! Dr. Gruber is hosting me at his home and I'm acting as communicator.
You'll enjoy some of his comments.
AERMcKinne : Great. Welcome, Dr. Gruber.
AEVLWard : Hi Gail and Dr. Gruber, pleased to meet you.
Dr. Gruber : How are you? We've been waiting and working on some of the
forwarded questions [the answers are inserted below as related questions
AERMcKinne : My students were wondering about Guiness Book of Records
questions for sharks.
E-Mail Question: How many species are there?
Dr. Gruber: About 370 living species.
E-Mail Question: What species live in the Atlantic Ocean?
Dr. Gruber: Too many to name!
AERMcKinne: Largest sized shark caught?
Dr. Gruber : The biggest shark? The whale shark--as big as a tractor
trailer! 40-45 feet long. Whale sharks used to be harpooned and then carted
around as oddities to charge admission to see them. The basking shark at
about 10 meters has been hunted to the brink of extinction in many places.
E-Mail Question: What are the smallest sharks?
Dr. Gruber: Dwarf shark; pelagic; about 12 inches at maturity; a squalloid
AERMcKinne : What is the farthest migratory distance they travel?
Dr. Gruber : Blue sharks--continuously travel the great ocean gyres and
make their ways around the oceans tens of thousands of km.
AERMcKinne: The largest predatory feeder?
Dr. Gruber: Largest predatory feeder? Fossil: Carcharodon megalodon,
up to 50 feet; Today: tossup between the Great white, the six-gill (deep
sea) and the sleeper shark (giant squaloid deepsea shark).
E-Mail Question: What is the most common species?
Dr. Gruber: Probably the spiny dogfish, Squalus acantheus.
AERMcKinne: How smart are they?
AESWardell: Do sharks see color?
AEGTucker : Sonny has spent 15 years training sharks in vision expts.
Dr. Gruber: They could be reliably classically conditioned using the
exact method and stimuli used with rabbits and cats. Namely, movement of
the nictitating membrane paired with flashes of light. Sharks learn this
task 10 times faster than rabbits and cats.
Dr. Gruber : Also, they run a simple two-choice maze; after learning
the brightness discrimination, one particular lemon shark was taken out
of the apparatus for over one year and when returned got the discrimination
on the first trial! Finally, if you look at the relation between brain and
body weight, you find that the sharks are up there with mammals and birds!
AERMcKinne : I believe it! I was amazed when the same black tip shark
stalked my boat for 3 hrs.
Dr. Gruber : Big brains and fast learning does not always mean "intelligence".
In fact we have not even been able to define intelligence for humans....So
WHAT DO YOU REALLY MEAN BY YOUR QUESTION?????
AERMcKinne : Smart ones can really be dumb too! Intelligence can be a
deep question for analysis.
AEVLWard: Is it true that sharks have no eyelids and that they never
AERMcKinne : Is this also true of dolphins, that they never sleep. They
do have eyelids though.
Dr. Gruber: Re EYELIDS: Fish have no eyelids. Sharks have among the most
elaborate eyelids and ocular adnexa in the animal kingdom. There are sharks
that can close their eye in a way similar to ours. Others have a third eyelid--nictitating
membrane. But some do not have eyelids, instead of closing eyes, they roll
their eyes upwards and backwards to protect the delicate transparent cornea.
Dr. Gruber : As far as SLEEPING is concerned: there is even a shark known
as the "sleeping" shark (in Japanese). And even if a shark were
swimming, it could be on "autopilot" so to speak as Gray showed
in the 30's that a spinal shark (brain separated from spinal cord) can swim
Dr. Gruber : A shark swimming in the middle of the ocean COULD be asleep.
We really don't know.
AERRussell : I've been told by different experts sharks always have to
move or die. Others say that isn't so. So?
Dr. Gruber : Short answer--no they do not have to be moving constantly.
e-Mail QUESTION: Do all species/or some species of shark have to keep
moving in order to respire or is this a myth?
ANSWER: No, all species do not have to keep moving in order to respire.
In fact, more species can rest on the bottom than not. There are two methods
that sharks use to extract oxygen from water. By using their pharynx as
a buccal pump some species are able to pump water over their gills. Once
the animal begins to move at about 1-2 body lengths per second, they simply
open their mouth and do what is called "ramjet" ventillation.
Some species such as the mako, great hammerhead, silky and other open ocean
sharks are obligate ramjet ventillators, but as I said, many species have
no problem pumping water over their gills.
AERMcKinne : Do all or most sharks migrate? Is it seasonal for temp.
adaptation or food seeking or reproduction?
Dr. Gruber: MIGRATION....No not all. Many are highly site-attached, especially
when young. e.g.: young lemon sharks carry out their activities for the
first two years of life in a space only 40x400 m. Yet, when they mature
they do undertake long migrations. Other species have a homerange for their
whole life that would never exceed a few tens of km (e.g. small cat sharks).
The reasons: yes to all of those. Some seasonal--connected with sea water
temp. Also migratory runs where certain species (e.g., blacktips) will move
down the eastern coast into Florida Bay to mate and reproduce. Others are
essentially continuously in migration (open ocean sharks--blue sharks, oceanic
white tips, silky, Cuban night sharks, threshers and scalloped hammerhead.
Some oceanic whitetips have been observed to follow migrating schools of
giant tuna. Also associated with pilot whale migrations. In fact a lot of
marine mammals and sharks cue on the migration of tunas and feed together
with or on the tuna. That's why we have tunafree dolphin these days, because
dolphin and sharks are both associated with tuna and are caught in the same
Dr. Gruber : I think we should have SHARK FREE tuna as well!
AERMcKinne : Do the males protect territory?
AESWardell : Do female sharks protect their young?
E-Mail Question: Do they leave their mother at birth, or swim along for
awhile (how long?)
Dr. Gruber: There is no maternal care, and a lemon shark mother might
eat her baby if she gets a chance. Do they spend time with their mother
learning the tricks of the "shark trade"? No, they learn on their
own, they are genetically programmed.
E-Mail Question: If they give birth to fully-developed baby sharks, what
do the babies eat after they're born?
Dr. Gruber: Lemon sharks: 80% fish, 20% inverts, and at one stage of
their young life they eat lots of octopus; when they get older their diet
shifts to larger fishes such as stingrays, other sharks, and large reef
fish. Tiger sharks eat nearly anything--they love sea birds and sea turtles
for instance which they can cut up shell and all.
E-Mail Question: Are the babies born with teeth?
Dr. Gruber: Those babies that are fed nutritive eggs in utero have teeth.
The others have a membrane covering their teeth before birth.
AERRussell : Was there a time when the army put people in yellow suits
thinking it was protective, and it wasn't?
Dr. Gruber : Re PROTECTIVE SUITS.....This is how I got started in shark
research! The navy asked my professor whether sharks could see color. They
observed that in some air-sea crashes, pilots wearing the bright orange
suits were attacked by sharks while those in the green suits were spared.
This seemed to suggest that sharks had color vision. They even called the
yellow survival suits "yum yum yellow". So-this launched me on
a 15 year study of the shark visual system. My conclusion was that sharks
had respectably good vision from the standpoint of brightness discrimination
and temporal discrimination (flicker fusion capability e.g.). the lemon
shark had poor but existing color vision. The retinas of all sharks that
I looked at had receptors associated with color vision--that is cone receptors.
But, I think the navy does not use the yumyum yellow, while other branches
do (Coast Guard does). Probably the more appropriate question is is it more
important to be seen by a rescuer than by a shark
Dr. Gruber : Unlike their reputation, we can say that sharks are extremely
social animals in general.
Dr. Gruber : Social Behavior: Only poorly studied. There have been 4-5
papers on this subject. In all cases, they were found to be extremely tolerant
of one another and no site defense has ever been observed.
Dr. Gruber : Their mating seems to take a different form from that of
elephant seals for example. Elephant seals will fight for a strip of beach
and for their harem.In contrast, many sharks will simultaneously attempt
to mate with a single female--just the opposite of the seals.
AERMcKinne : I believe some whales also have simultaneous mating of several
males on one female.
AERMcKinne : What environmental cues stimulate their breeding cycle and
Dr. Gruber : Sharks are perhaps the oldest vertebrate example of what
ecologists call a K- selected species. This means that the human life history
pattern and those of other large vertebrates are considered K-selective.
The characteristics are: slow growth, late maturity, low fecundity (#'s
of births), long life with repeated reproductive events throughout their
life. Humans are perfect examples. Also, parental care for a long period
characteries K-selective species. The opposite is r-selective and a good
example is the salmon: fast growth, short life cycle, thousands of eggs,
no parental care. Curiously sharks have all the classical signs of a K-selective
species except for parental care! So, no shark mothers do not protect their
babies and that is surprising. Incidentally, r and K are parameters of the
logistic growth curve that you have all seen in population studies with
AERMcKinne : Yes. I have done some work with K sel in lizards.
AERMcKinne : Do any elasmobranchs have parental care?
Dr. Gruber : No Elasmobranchs that we know of have parental care. In
fact many sharks lay eggs on the bottom and leave. However, you might say
that caring for your babies inside your body for periods of up to 22 mos.
could be considered maternal care and so this is how sharks handle their
K-selective requirement. For example the spiny dogfish carries its young
for almost 2 years which is as long as an elephant.
Dr. Gruber : I call that care!
E-Mail Question: How fast can a great white swim?
Dr. Gruber: No one knows!
AERMcKinne : How long do sharks live?
Dr. Gruber : Lemon sharks live for at least 78 years. I spent 6 years
learning how to tell the age of a lemon shark based upon its growth rings
on the vertebral centra. I verified the time between these growth rings
by injecting tetracycline into hundreds of sharks, releasing them and recapturing
them up to periods of 5 years. We found that these sharks lay down one growth
ring every lunar month--or about 29 days. The largest lemon shark we ever
caught had the equivalent of 78 years of growth rings.Incidentally the shark
does not have to be killed to observe it vertebrae. A small section can
be dissected out of the base of the tail and not really hurt the animal
AERMcKinne : What cues their mating?
Dr. Gruber: MATING CUES.... Of all the vertebrates the sharks are the
least known from the point of view of social behavior including courtship
and mating. We do know that in all cases fertilization is internal and males
are provided with special copulatory organs called claspers. We believe
that pheromones play a very important role in their mating behavior. But
this is based on anecdotal evidence. At least in the case of the lemon shark
and most other species, mating and parturition are highly seasonal. I wish
I could give you more facts but they simply don't exist.
AERMcKinne : Can the female retain viable sperm for long periods of time?
And how long? Dr. Gruber : Yes! Males transfer sperm to females in a unique
way. When the clasper is inserted through the cloaca into the oviduct a
pump mechanism fills sacs along the abdominal wall called siphon sacs--with
sea water at high pressure. When the sperm transfer takes place, the sea
water rushes out through the clasper and propels sperm packets called spermatozeugma
into the female. These are stored in the shell gland for periods of at least
two years. Thus in the few species studied, males are not absolutely necessary
for annual reproduction.
Dr. Gruber : Many pinniped species store sperm for extended periods of
AERMcKinne : So do reptiles.
Dr. Gruber : Hi folks, any questions or comments? I'll forward the entire
talk to all who log on if you sign on in a question.
E-Mail Question: Do sharks have a fairly successful reproductive capability?
Could they recover numbers as fast as say, the American alligator?
Dr. Gruber: Very low fecundity. Fits in with their lifestyle. However
the artificial pressure of human fishing has disrupted that lifestyle. Certain
species take up to 30 years to mature and have very few young. The lemon
shark barely replaces itself in one year, so they could not recover as fast
as the alligator.
E-Mail Question: How much do sharks eat?
Dr. Gruber: Very little. On the average about 2% body weight per day
AEBCuller : Do sharks see color?
Dr. Gruber : We answered that a bit earlier so we'll say: some species
do; most species have the retinal equipment.
AEDBronner : You spoke earlier of anecdotal evidence of pheromones. What
is that evidence?
Dr. Gruber : PHEROMONES....Evidence comes primarily from aerial observations
which I make in the shallow waters of Bimini Lagoon. During the reproductive
period I often see formations of sharks that are separated by distances
that preclude vision. For example, 4 or 5 sharks can follow each other in
a circle that is 300 meters in diameter--and the circle is PERFECT! They
must be following each other in some sort of odor corridor. I've also seen
what I call a follow formation of up to 15 sharks in line. Some of the sharks
were tens of meters apart and could not have seen each other. Again suggesting
an odor corridor. I have attempted to analyze the urine of pregnant females
but so far the results are not impressive. I wish I could say more.
E-Mail Question: Egg-laying vs live birth? I'm familiar with dogfish,
but what is the status of birth in the great white, hammerhead, tiger, and
Dr. Gruber: Of all the vertebrates, sharks have the most variable developmental
strategies. There are a number of species that lay the familiar mermaid's
purse--eggcase. In this example, development is totally outside the mother
and such development is oviparous. The dogfish--is an example of ovoviparity--eggs
are laid within the body of the mother and develop in a way similar to snakes,
followed by live birth. There have only been two examples in the history
of science where pregnant great white mothers have been captured. In both
cases, the mother had numerous babies up to perhaps ten and as is the case
with all sharks in this group the babies stomachs were filled and distended
with yolk--because after producing eggs that hatch out within the mother,
the mother than produces dozens of nutritive eggs and the embryos grow via
a process called ovophagy. This strategy is taken to its extreme in the
related sand tiger shark where the fittest sibling kills and consumes its
brothers and sisters, then goes on to eat the nutritive eggs. This may seem
cruel and unusual but in fact it is a highly effective way to produce strong
babies! The hammerhead represents perhaps the highest evolution in reproductive
strategies. Here, a maternal-fetal connection called a pseudoplacenta is
formed and the fetus receives nutrition directly from the blood of the mother
in a way perfectly analogous to our own placental viviparity. However, the
hammerheads take it one step further--the umbilicus is covered with thousands
of minute villi with absorptive surfaces and materials pass from the mother's
uterine fluid into the fetus as well. The tiger shark is a real oddball
when it comes to the family in which it belongs. This family, called the
Carcharhinidae, includes the greatest number of shark species of any family--such
as the blue, lemon and bull shark. In every case, placental viviparity is
the mode of development but in this one case the tiger shark does not produce
a pseudoplacenta, but rather each embryo is covered in its own membranes.
Another interesting fact about tiger shark development is that they produce
up to 50 young which is extraordinarily high for a large shark. The nurse
shark is perhaps the most unusual of all. This is a shark which lays eggs
inside its oviducts and the eggs are covered with a horny shell similar
to a mermaid's purse, but for some mysterious evolutionary reason, instead
of laying these eggs the nurse shark retains them in her body. Incredibly,
these eggs could survive outside the body and grow to parturition. Are they
evolving away from egg laying or toward egg laying?
AEBCuller : Are there any fresh water species of sharks?
Dr. Gruber : Yes, several species enter fresh water and there is one
species that is said to spend its entire life cycle in fresh water. This
is the Ganges River shark. The bull shark is known to enter fresh water
rivers and lakes all throughout the tropics and poses a significant threat
or danger to humans under these conditions. Pancake sharks, i.e. sting rays
of the genus Potomotrygon and others are obligate freshwater flattened sharks.
AERMcKinne : Hammerhead migrate in a horizontal shoulder to shoulder
line a set feet apart? Do others do this?
AEDBronner : Is the follow formation part of hunting behavior or migratory
Dr. Gruber : I have only seen follow formation in a social context in
the daytime. I could not imagine lemon sharks using that kind of pattern
to hunt. Although I cannot say no. And they certainly do not migrate in
follow formation. It seems to be a specialized behavior in shallow water
mating situations, although no one has ever seen lemon sharks mate.
AERMcKinne : What cancer research has been done with shark cartilage?
Dr. Gruber: Some teachers were talking about shark cartilage being used
in cancer research. What is the current status of these studies? The question
of shark cartilage raises the hackles on the back of my neck! This is because
today shark cartilage is being touted as a cancer cure and nothing could
be further from the truth. However, there is a biological basis for believing
that a protein or peptide found in shark cartilage could be instrumental
in the death of cancerous tumors. Cartilage is an avascular substance and
as such is opposite from bone. Both are skeletal materials, but cartilage
actively excludes blood vessels by a process called anti-angiogenesis. Since
tumors are metabolically active they recruit blood vessels by the opposite
process. If the anti-angiogenic substance could be isolated and brought
to the tumor, then the tumor would starve and die. But the leap from a pure
substance at a tumor site to ground up shark skeletons taken by mouth is
one of great faith. The poor cancer-sufferer gains a bit of calcium. The
poor shark is dead! And people have made money off of these sad occurrences.
That's why I'm so angry when I hear about shark cartilage and cancer. However,
work continues at the Mote Marine Laboratory in Sarasota under the guidance
of Dr. Carl Leur. The Mote Marine Laboratory has a home page on the WEB
and you can learn more about it there.
AEBCuller : What reference would you recomend as a library source?
AEGTucker : Dr. Gruber provided a few book suggestions and references
y'all might find interesting and helpful.
AERMcKinne : Thanks.
E-Mail Question: Is there a good, readable book on sharks available that
we could use as an all-purpose reference in the high school classroom? Dr.
Gruber recommends: Sandy Moss, Naturalist's Guide to Sharks (advanced but
readable); e-mail address: SMOSS@UMassD.edu Paul Budker, Transl. The Life
of Sharks (lower level college) Samuel Gruber, Ed. Am. Littoral Soc. Discovering
Sharks (popular text) Victor Springer, Sharks in Question. Smithsonian.
E-Mail Question: FROM HAWAII: Being surrounded by ocean, sharks are always
of interest to our students. We have recently seen an increase in the number
of shark attacks in Hawaii (usually tiger sharks). Because of the public
outcry for our Department of Land and Natural Resources to do something
about the attacks the state has initiated a Shark Task Force. The task force
hunts and kills tiger sharks in areas where shark attacks have occurred.
This has caused a lot of controversy as the scientific community does not
see many benefits of such a program. I wonder if there has been any similar
programs of shark control in the Atlantic and if so what are/were the results?
Dr. Gruber: Controversial and complex question: Contact Brad Wetherbee
at Univ. Hawaii, Manoa; he will give you the answers to your question. He
is a former student of Dr. Gruber's.
E-Mail Question: Recently, there have been several articles on the plight
of sharks (overfishing, etc.,). Is the outlook better for sharks now or
are they still being overfished? .
Dr. Gruber: Much better today than 5 years ago; because nations are beginning
to manage shark resources and it looks like CITES [Convention International
On Trade in Endangered Species; UN agreement ] will control the trade in
E-Mail Question: How will the overharvesting of sharks affect the environment?
Dr. Gruber: Deleteriously. Sharks play a significant role in the food
E-Mail Question: What is the "real" status of the shark relative
to both species and populations of species? Are there certain species whose
numbers are getting especially low?
Dr. Gruber: Yes. Many of the pelagic species have been decimated by drifting
gill nets and long lines. For every swordfish taken, three sharks are killed
Dr. Gruber : Hold on one minute folks while I check out the AOL lobby.
Dr. Gruber : Back again.
Dr. Gruber : Hi all, any comments questions before we close off?
AERRussell : I learned a lot, thank you. And thanks Gail for all the
notice so I could schedule and remember to get here.
AEVLWard : This has been an incredible hour...thanks so much to BOTH
AEGTucker : Thanks for joining us.
AEBCuller : Sorry I did not get in earlier thanks.
AERMcKinne : This has been most interesting and informative. Thanks you
Dr. Gruber and Gail for all. :)
AEDBronner : Absolutely fascinating. Hopefully will be online with the
kids later. Thanks so much for all the info.