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Shark imageUnlocking the Mystery of Sharks (part #2)

Presenter: Dr. Samuel Gruber
Host: Gail Tucker

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 That realtime chat was saved off the Internet and has been archived here for your interest.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.


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 about 65.

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 attack.

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 metal?

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 this ]

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 sharks"?

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 babies!

AESWardell : How fast do shark swim?

Dr. Gruber : Speed....Not known at max. No one knows for sure even for great white.

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 shark.

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.


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