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Shark imageUnlocking the Mystery of Sharks (part #1)

Presenter: Dr. Samuel Gruber
Host: Gail Tucker

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 follows.


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 came up].

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 shark.

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 sleep?

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 perfectly well!

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 net.

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 how often?

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 flower beetles.

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 that much.

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 time.

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 or less.

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 nurse?

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 patterns?

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 shark products).

E-Mail Question: How will the overharvesting of sharks affect the environment?

Dr. Gruber: Deleteriously. Sharks play a significant role in the food chain.

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 and discarded!

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 of you.

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.


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