Samuel H. 'doc' Gruber: Shark Science Pioneer

Doc Gruber began studying sharks in 1961, perhaps before any other scientist had done full-time research on a living shark. During his long career, he founded the Bimini Biological Field Station (Shark Lab), the Shark Specialist Group of the International Union for the Conservation of Nature, (IUCN), a United Nations organization based in Switzerland, and the American Elasmobranch Society. He has published over 200 scientific papers, and his research is still ongoing today. 

His decision to study sharks was as unplanned as it was final.

Doc in 1957 with black grouper
As a young man growing up in Florida, he loved to dive, and often went off for weekends of scuba diving and spear fishing on a 30 metre schooner called the Blue Goose. The ship had belonged to Hermann Göring, commander-in-chief of the German Luftwaffe (Air Force) under the Nazis, and it had found its way to Miami when it was liberated at the end of World War II. A weekend of diving fun on the Blue Goose cost only seven dollars, and at that time, there were still big fish!

On one of these outings in 1958, Gruber had speared a grouper hiding in a submarine cave, and was emerging with it into open water when he saw a hammerhead shark approaching. It was the largest shark he had ever seen, and as it glided towards him, it seemed to be the size of a submarine! Sure that he was about to die, he plunged back into the cave with his fish, and found himself in the same position that the grouper had just been in, as he looked out. Watching in awe as the momentous shark passed him, he was seized with the desire to know what sort of an animal it was.

When he returned to the university, he asked his professor what was known about sharks, and found that no one knew much at all. So he decided then and there to become a marine biologist and study them.

His research begins

With the idea of becoming a medical doctor, Gruber was attending Emory University at the time, and was majoring in pre-medical studies. He had been especially intrigued by the study of comparative anatomy in which he had dissected a shark, a giant salamander, and a cat. That summer he was taking courses at the University of Miami, and had asked if he could assist the comparative anatomy laboratory dissecting the animals.

Now, inspired by his riveting meeting with the hammerhead shark, he transferred to the University of Miami, earned his under-graduate degree in zoology and chemistry, and applied to graduate school to study sharks.

Gruber in 1963 studying sharks' eyes
In 1960, the University of Miami’s Marine School had hired Dr. Warren Wisby as professor and researcher in marine animal behaviour, with an emphasis on sharks. As a student of the famous professor Arthur Davis Hasler, Wisby was best known for having discovered the actual mechanism of homing in salmon. By marking hatchling salmon, and going back to their streams when they returned to spawn, Wisby had found that they came back to the exact stream in which they had hatched.

So Gruber’s timing was perfect. He was given a research assistantship, and didn’t have to pay for tuition. In fact he was paid the huge sum of 103 dollars a month as a graduate student there!

Wisby told him that the Navy had given them a grant to study shark senses. When aircraft went down at sea, it happened at times that sharks attacked the flyers in the water. In those days flyers wore two types of suits—high visibility suits called International Orange, and the standard khaki flying suits. According to a Navy report, the flyers wearing International Orange suits were attacked to a man, while the ones wearing khaki suits were left alone.

As a result, the Navy had started calling those orange suits yum yum yellow.

Wisby directed Gruber to look at the literature and report back on the possibility that sharks have colour vision. So he examined all the old reports. They were mostly in German, and they concluded that sharks could not see colours because they lacked the cone-shaped photoreceptors in the eye’s retina that permit colour vision in humans and other animals.

Gruber and Wisby collecting shark eyes to study at a fishing tournament
The Duplexity Theory of vision was introduced in the 1860s by a German scientist named Max Schultze, and states that rods and cones in the retinas of an animal that possesses both, have two functions. Rods are used in night vision when there is little light, while the cones take over during the day, providing the ability to see colours, fine details, and to discern rapidly flashing lights.

Some animals, such as squirrels and iguanas, that are active in daytime, have no rods in their eyes, and nocturnal animals or those adapted to the darkness of caves or the deep sea, have no cones. Therefore, the lack of cones found by early researchers in the retinas of sharks, suggested that they were unable to distinguish colours.

But Wisby questioned the old conclusions. He asked Gruber to go out and actually collect sharks’ eyes, and see what he could find.

Collecting eyes at a shark tournament
So Gruber went to shark tournaments and collected the eyes of every species of large shark caught off the of the eastern seaboard of the US. He put a catheter into their hearts while they were still beating—the animals were brain-dead—and perfused gluteraldehyde, a preservative chemical, through their arteries, to fix their eyes for future study under the electron microscope.
On one occasion, he was notified that a young great white shark was caught and was being held for him. This was the chance of a lifetime for the young graduate student, as white sharks were very rare. He ran out in a boat to where the fisherman was waiting with a barely living 54 lb. specimen, successfully perfused the shark, and collected the eyes.

Can sharks see colour?

Year after year, Gruber worked in a histology lab comparing the retinas of the many species of sharks to see whether they had both rods and cones. And, amazingly, every species he studied had them. Some species seemed to have better retinas than others, but they all had rods and cones. The great white shark had five rods to one cone, which was an especially high ratio.

It seemed that the earlier scientists had studied the cold-water, bottom-loving sharks of the northern seas off Europe, and those species had very few cone cells because they were adapted to dark conditions. Those inaccurate early findings had resulted in many false ideas about sharks taking root. The idea that they had an excellent sense of smell had spread because they came quickly to a scent, and so the concept of a shark as a swimming nose, with poor eyesight, was born a century ago.

The forebrain of a shark, called the telencephalon, is considered one of the most important parts of the brain, like our cerebral hemispheres. And in the shark, the telencephalon was thought to be the centre that analysed scents, because that was how it looked to the early researchers. They did not know how the forebrain worked, and they had never looked at how sharks really behaved, or tried to do neural examinations, yet their primitive ideas about sharks had persisted.

diagram of the brain of a shark, showing the telencephalon
Wisby was pleased with Gruber’s discovery, but pointed out that just because the sharks had cone cells, didn’t mean that they could see colours. “What do the rods and cones mean for sharks?” Wisby asked, and directed Gruber to experiment to learn whether sharks see colours, and investigate their other visual capabilities.

As the eyes of a human or animal adapt to darkness, rods take over the function of vision, providing high sensitivity to light, but no colour. The switching over between the cones and rods is something that can be measured, and it was this line of research that Gruber began to pursue in 1962 for his master, and later his doctorate degrees.

He used the lemon shark as a model, and chose three methods to test the Duplexity Theory of vision in sharks.

Image of the rods and cones in a shark's retina
To assess the rod and cone activity in the retina, he put contact lens electrodes on anaesthetized lemon sharks, and recorded the electrical responses of the retina when illuminated. He used electrophysiology to record electroretinograms, which are similar to the electrocardiogram of doctors, in order to examine the electrical activity of the associated neurons.

His second method involved a behavioural study, following the Pavlovian method of testing called classical or respondent conditioning, and the third was the Skinnerian method, which is called operant conditioning.

Gruber’s results were unambiguous. The three methods gave the same results—there was no doubt that sharks could see colour. The rods functioned as expected in the dark, and the cones were most active in the light-adapted state. He found that as sharks adjusted to darkness, the sensitivity of their eyes became greater and greater, and reached the maximum dark-adaptation after about an hour, achieving a million-fold increase in sensitivity!

They adapted better than humans because unlike us, they possess the tapetum lucidum. This is the mirror-like membrane at the back of the eye which produces eye-shine in some animals. Light entering the eye passes through the retina, and is reflected, as if by a mirror, back from this membrane, potentially doubling the eye’s sensitivity.


The learning rate of a shark

Another interesting finding that emerged from Gruber’s research during his doctoral studies was the speed at which sharks learn.

He was working on Pavlovian training conditioning, doing an experiment in which a shark would see a flash of light, and then receive a mild electric shock. After a certain number of trials, when the shark saw the light, it would have learned to anticipate the shock, and have a reaction. This is called a conditioned response.

The reaction that Gruber planned to use for the experiment was the pause in the shark’s heart-rate resulting from the fear of the coming shock. Fear causes the heart to skip a beat, then accelerate, so at the moment that the shark realized that it was about to receive another shock, its heart paused, and this reaction could be measured. The number of trials it took for the animal to learn the association between the light and the shock, gave a measure of its ability to learn.

While Gruber was flashing the light, and giving the shock, he was looking at the readout on the oscilloscope, rather than at the shark, which was trussed up underwater, with an electrode in its cardiac chamber, looking out into the room through a big Plexiglas bubble.

Then one day, he happened to look at the shark at the moment in which it anticipated the shock, and saw that it winked—the nictitating membrane of the shark’s eye closed. This provided another conditioned response to the light, which meant that there was no need for the heart monitoring—all Gruber had to do was observe. Due to the need for the heart monitoring, the sharks had been unable to survive long enough for him to get them trained, so his discovery was crucial to the success of this important experiment.
The nictitating membrane can be seen here as the lemon shark in the experiment winks.

Now Gruber used a World War II infra-red sniper scope to observe the shark in total darkness, and found that after about ten trials, or repetitions, the shark would wink in expectation of the shock. It was a conditioned response that he got from a shark in only about three minutes!

One session consisted of 10 sets of 10 trials, and after about 80 trials, the shark was responding 100 percent of the time. The next day it took only 3 or 4 trials to get the shark to respond, and it responded 100% of the time after 40 trials. On the third day, it had a 100% response from the start. So it took only about 10 trials for the first response, 80 trials to get the 100% response, and only three days to the state in which the shark was conditioned long term.

By chance, psychologists had been studying Pavlovian conditioning in cats and rabbits using the same paradigm. They used the flash of light as the initial stimulus, and followed it with a puff of air onto the cornea, instead of an electric shock. As in Gruber's experiment, the conditioned response was the movement of the nictitating membrane, so the experiment was a terrestrial version of precisely what Gruber had been doing with sharks.

What was surprising was that the sharks had learned the relationship between the light and the shock ten times faster than the cats and rabbits! These had taken about 800 trials to achieve 100% conditioning, while the sharks had taken only 80 trials!

Gruber working on the experiment in 1966
Further, the sharks remembered what they had learned for over a year, which was far longer than anyone had expected.

Gruber discovered many more interesting things in the course of his research. He found that sharks have different “IQs” and different personalities, and that some are left-handed and some are right-handed. He also found that they have an occlusable tapetum, a further refinement of their vision, as well as a mobile pupil in a variety of shapes, which only sharks and rays, and no other fish, have.


Arthur A. Myrberg, the youngest full professor at the University, took Wisby’s place in 1964, and mentored Gruber during the last years of his master and doctorate degrees. While Gruber and his student, Joel Cohen, were studying shark vision, Myrberg and his students worked on their hearing, and others (not at the University of Miami), were working on olfaction, taste, and electoreception.
In 1976, the Navy asked for a summary of all of the work it had funded about the sensory systems of sharks, and they put their findings together in a major book, which was published in the government printing offices. Gruber’s discoveries about shark vision filled a large chapter.


With his findings published, Gruber no longer wanted to work in a dark and damp laboratory, and longed to understand the role sharks play in the marine environment. He dreamed of studying their behaviour in a similar, systematic way, in the wild, and decided that he would find a way to do it, using the lemon shark he had come to know so well, and wanted to understand more deeply.

By then he had become an assistant professor at the University of Miami Rosenstiel School of Marine and Atmospheric Sciences, teaching advanced courses in animal behaviour, tropical marine biology, and the physiology and behaviour of marine organisms.

He had done a post-doctoral study in 1971-72, at the Max-Planck Institute in Germany, under Nobel laureate Konrad Lorenz, the famous Austrian ethologist, just as Myrberg had done a decade before. Both men were interested in pursuing their interest in ethology, and as a side project, they had put together an observational study on a captive colony of bonnethead sharks, which was published in the journal Copeia in 1974. It remains the only ethogram, which is an inventory of the repertoire of behaviour patterns displayed by a species, that was ever published for sharks.

Then in 1976, they gathered together all of their information on shark behaviour for a symposium in New Orleans, and it was published in the journal American Zoologist.

Gruber and Myrberg developed a close personal friendship that was to last all their lives. They travelled together as they pursued their research in Europe and the Middle East. They dove with Cousteau in Monaco, travelled through Europe visiting friends and colleagues, and studied sharks in the Florida Keys and the Caribbean. And when Gruber got engaged, it was at Myrberg’s home.

Gruber left Navy research in 1976, and in 1977 he proposed a study to the National Science Foundation, on the role lemon sharks in the tropical marine environment. His goal was to do an autecological study designed to examine many aspect of this species’ biology, with an emphasis on behavior and ecology. It was funded in 1978 and is still continuing today in 2014.

At last he was fulfilling his dream, of discovering what a shark is, and what it does in its life in the wild.

A watercolour portrait of a tiger shark (by the writer)


Gruber chose bio-energetics as way of understanding the biology of the lemon shark. Bio-energetics gives the researcher a mathematical method to describe the workings of a living system. Like all things in nature, a shark conforms to the laws of thermodynamics, that decree that in a system, the energy that comes out, no matter how it is channeled or converted, has to equal the energy that has gone in.

In other words, the energy that goes into a shark as food, will come out through growth, metabolism, waste, other biological products. The processes of metabolism—the nerves, digestion, muscles, respiration, and other biological processes—will likely burn half of the calories, and such materials as mucus, urine, and faeces can be burned up in a calorimeter to find how many calories were lost that way. The calories consumed must be partitioned within the body into only four unknowns, and the process can be analysed.

As an example, the common practice of dieting and exercising to reduce one’s weight, utilizes the principle of bioenergetics to achieve a goal. By decreasing the energy going into the body in the form of food, we can force it to use stored fat to make up the deficit, and thus lose weight.

Gruber described it this way, “It is possible to make an equation that balances in four unknowns. It is a simple thing to do mathematically and it reveals a great deal about the animal. It shows how they make their living, what they require for food, their metabolic needs, what they need to digest food, how much of what they eat is assimilated, how much is lost in waste products, how quickly they grow, and how much food it takes them to grow.

“That’s what you can tell about a shark’s life and what it takes to grow a shark in the environment and that’s what we did. It took us over ten years.”

Through a combination of laboratory research and studies in the field, Gruber and his colleagues and students focused so much research on the lemon shark that they discovered much about its life history characteristics, its population dynamics, its growth, reproduction, and genetics. He was determined to make sure that their experiments were realistic by always comparing laboratory results to what could be learned from sharks in the marine environment.


Initially Gruber studied lemon sharks in Coupon Bight in the Florida Keys. Using nets, he and his colleagues would reliably catch 100 to 120 juvenile lemon sharks there, each summer season, which they would work up and release. But in the early eighties, their numbers began to fall, and in three years he could not catch one shark there. All of the work he had done during all of those years was wasted.

He found out that it was due to overfishing. Fishermen had been catching the little baby sharks in the nursery for crab traps and had fished them all out. The mothers that were supposed to be coming back to the place they were born to give birth had been fished too, so the entire local population of sharks had disappeared.

Gruber knew that the sharks were in trouble again, because this had already happened to him in Florida. So he began doing his research at a small island in the Bahamas at the place where he would later establish the Bimini Biological Field Station. Four times a year, he went there for his field research using National Science Foundation research vessels which were at his disposal from the University of Miami.

Later, when he had the Bimini field station set up, they could do permanent work there without the need to go back and forth to Florida, which allowed them to expand their research.

The research

The Bimini Shark Lab that Gruber founded in the Bahamas

The main thrust of Gruber’s research was trophic ecology or autecology, the ecology of one species.

Bimini’s lagoon was like a marine lake, where the juvenile lemon sharks were obliged to remain, and each shark could be looked at year after year for six or seven years before it left the area. The location was ideal. There were two to three hundred sharks divided between three to four nursery areas, and he tagged nearly all of them, and focused hard on learning all about their lives as he tried to unravel the ecology of the lemon shark. How many sharks were there? How many lived? How many died? How many grew up to maturity? How fast did they grow? What would it take to grow a lemon shark up from a pup to an adult of 80 kg?

Each spring, when the lemon shark pups were born, Gruber and the students did a comprehensive tagging study. They built a large pen, which could hold up to 150 sharks. Then they caught the sharks in nets, tagged them with an electronic micro-tag, took a genetic sample, weighed them, measured them, sexed them, and put them in the giant pen. Every year they would catch, tag, and work up between 180 and 230 sharks. The sharks were released, and then the next year the same sharks that survived, could be caught and measured again. The year 2014 was their 20th year.

The shark pens are visible in the foreground
After more than a decade of research, they balanced the bio-energetic equation for a young, fast-growing, 2 kg lemon shark at 25 degrees centigrade, and discovered that the little shark was an energy conserver, and ate only about seven times its body weight in a year. By comparison, many fish, such as tuna, blow a lot of energy and have to eat a lot of food. And humans, in comparison with lemon sharks, eat an enormous amount.

With the sharks living free in a place where they could be watched from year to year, many experiments were possible. For example, they were able to study their movements, their relationship to temperatures, their food, their place in the ecological system, and their social networks. The nursery was a region of mangroves, and certain little snails called Batillaria were low on the food chain and were the keystone species. The next level was crabs, then fish, then sharks—sharks were on the fifth level.

The countless studies took years and years, doctoral dissertations, and masters theses, and thousands of students all coming to Bimini to study sharks. Gruber taught a University of Miami course there for 22 years, sometimes twice a year; now he teaches five university courses at the station. He had graduate students, built big holding pens at his research facility, and kept dozens of lemon sharks.

And as time passed and he learned of its abilities and capacities, the lemon shark became an animal of complete fascination. To Gruber, the lemon shark had gone from a predatory fish with fins and teeth to being more like a family member.

He said,

“As I went through my early career and I got married and we had children, then we got a house and we got cars, I realized that the lemon shark had provided a living for me in the human world, whereby I could become a functional and useful citizen and have a family. It was all because of the lemon shark. That’s why I get so nervous when I think that they are having problems underwater, not only with being overfished, but also with us handling them.

So I have not been able to remain objective in my feelings for them, although I have tried to remain objective in my research of them.”

Doc Gruber loves lemon sharks

(c) Ila France Porcher

Illustrations courtesy of Dr. Gruber, except where indicated.

This first of three parts, was originally published in Shark Tales in X-ray Dive Magazine, issue 64. See Part two next!


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