Research Article:
Shark Senses and Ecology

Dr. Robert Hueter is interested in studying shark ecology: how sharks have adapted to their environment and what impact humans have had on the sharks. During the JASON Project, he hopes to perform experiments to learn more about the sensory systems of sharks. He also wants to tag and track sharks so that he can compare the shark populations in various parts of Florida. Finally, he wants to help you understand the impact that humans have on sharks.

Sensory Systems of Sharks

Did you know that sharks have six senses? In addition to the five senses that humans possess (hearing, olfaction [smell], taste, vision, and mechanoreception [touch]), sharks also receive sensory input through a sense called electroreception, a sensitivity to electric fields. To a shark, the marine environment is buzzing with information picked up by all these senses.

The shark's sensory system, one of the most sophisticated in the animal world, has raised it to the top of the food chain in its habitat. Sharks are often called apex predators-predators at the very top of the marine food chain. Depending on the strength of the stimuli, the shark can detect and locate its prey up to a distance of several kilometers.

During this Investigation, you will perform Experiments to help you learn about sharks' sensory systems. But first, let's take a closer look at all six of the shark's senses.


Sharks have no obvious external ears. But this doesn't mean they don't hear very well. In fact, sharks have well-developed inner ears that respond with high sensitivity to sounds of low frequency.

Scientists have performed many Experiments and accumulated much evidence that sharks can hear and respond to sounds transmitted through the water. The results of these experiments show that sharks respond to and are attracted by irregularly pulsed sounds of very low frequency (20 to 300 Hertz, or cycles per second). Sounds in this low- frequency range are precisely the kinds of sounds made by fish that are swimming erratically. Such fish are often young, old, or sick, and therefore easily captured.

Olfaction (smell)

Fishermen have long known that sharks rely on a keen sense of smell to locate food. Chum-a bloody, oily blend of fish juices and parts, is commonly used to attract sharks. The chum provides a trail of scent that may attract sharks from kilometers away.

In the past 30 years, scientists have performed many studies to determine how sensitive sharks are to certain types of chemicals in the water. Research on lemon sharks has determined that they can detect the presence of as little as 1 part of tuna extract in 25 million parts of seawater. That's equivalent to about 10 drops of extract evenly dispersed through the water in an average-sized home swimming pool. Other studies, on blacktip and gray reef sharks, have concluded that these species respond to grouper fish extract in concentrations as low as 1 part per 10 billion-1 drop in a quarter-acre lagoon 6 1/2 ft deep!


Believe it or not, sharks do taste their food-and they like some foods better than others. The shark's mouth and throat are lined with papillae, small mounds visible to the naked eye. These papillae contain numerous taste buds. If these taste buds are not satisfied, sharks often reject the food after tasting it.


The shark's eye is quite similar to the human eye, with a few exceptions. Like humans-but unlike most fish-the shark can open and close its pupil in response to varying amounts of light. But since the cornea has no focusing power underwater, sharks have a thick, round lens to focus images in the eyes. In the lemon shark, for example, this lens is seven times more powerful than the human lens!

Sharks can be trained to respond to visual cues or targets. Scientists have conducted many studies using targets with different colors, shapes, designs, and degrees of brightness:

On the basis of such research, scientists have determined that humans in the water may provide certain visual cues that could invite or provoke a shark attack. Because they see contrasts well, sharks may be attracted by uneven tanning or bright-colored clothing. Sharks may also be attracted by shiny jewelry that looks like the scales of the fish they generally hunt.

Mechanoreception (touch)

In the water, a shark's sense of touch is stimulated by direct contact or water movement. Sharks use even the faintest movements and vibrations in the water to detect the presence and location of moving objects in their vicinity. This sense of "distant touch" comes from a system of mechanoreceptors distributed over the shark's body. Some of these receptors-the pit organs-are tiny, independent receptors. Others are organized into a branching canal system located beneath the skin of the head and body, with periodic openings to the surface. The most prominent branch of this canal system is the lateral line, which runs along each side of the shark's body. Specialized sensitive nerve or hair cells are responsible for sensing even the smallest vibrations in these pit and canal organs.


The shark's "sixth sense" is electroreception, the ability to sense weak electric fields. Although humans build devices that detect electric fields, we do not feel the presence of weak fields around us. Sharks not only sense these fields but also rely on them to locate prey and, perhaps, navigate through the ocean.

The receptors responsible for detecting these weak electric fields are concentrated in the shark's head at the base of tiny, jelly-filled canals over the shark's snout and lower jaw and around the eyes. Dark pores on the skin's surface mark the external openings to these sensory structures, which are called ampullae of Lorenzini. The ampullae contain nerve cells that respond to very faint electric stimuli.

Careful experimentation has demonstrated that sharks use their keen electroreceptive sense to locate prey undetectable by other senses. For example, sharks use this sense to detect the heartbeat of a fish buried under the sand. Because the bioelectric field created by the animal's heartbeat carries only over short distances, the shark's electroreception is effective only for finding objects that are very close to the shark's head.

Hammerhead sharks have long fascinated shark biologists. While scientists are still not sure why these sharks developed such unusual heads, one possibility is that the flattened head shape was an adaptation that provided more effective electroreception, by increasing the electroreceptive surface. The head shape may also have been an adaptation that improved the shark's directional sense of smell by spreading out the nasal chamber and changing the way water flowed past the head. It is interesting to note that the favorite food of hammerhead sharks appears to be stingrays, which typically bury themselves in the sand. One can imagine these sharks sweeping their great heads over the sea bottom, like metal detectors over sand, searching for a tasty treat.

Occasionally, sharks are misled by their electroreceptive sense to investigate objects that are not suitable prey. Such objects might include metal bars or wires that give off electric current in seawater. The sharks are unable to distinguish between natural signals and those produced by artificial objects. Therefore, they might attack metal objects on a diver's cage or boat. While it might appear that the sharks are attacking the people within the cage or boat, they are in fact attacking the metal and may not even be aware of the presence of the people.

Tagging and Tracking Sharks

In addition to the sensory system of sharks, Dr. Hueter is also interested in the distribution patterns (where certain types of sharks are found) and the abundance (how many sharks there are) in certain locations around Florida. In particular, he wants to compare the distribution and abundance of older, younger, and different species of sharks at the Florida Keys coral reefs and the Florida Straits with those in Florida Bay, the site of nurseries and pupping grounds.

To study the distribution and abundance of sharks, Dr. Hueter and his colleagues use tagging and tracking methods as well as general observations.


The process of tagging sharks begins when scientists catch a shark using gill nets, a longline, or a rod and reel. Once they catch a shark, the scientists write down several pieces of information about it, including its weight, length, and location. They then attach an identification tag to the shark. This tag generally contains an address for contacting the scientists and a code (generally a number) that matches the file in which the information about the shark is stored. When the shark is caught again, scientists use the code to locate the information gathered at the previous tagging. They then gather new information and add it to the file. This helps scientists learn a lot about the shark, such as how much the shark grew in a certain time period and where it traveled between catches.

Dr. Hueter used the tagging method just described to tag shark pups in Florida Bay in August 1995. Most other scientists use similar methods. Another tagging method uses the archival tag, a computer-based recorder that has sensors to measure light intensity, water clarity, water temperature, salinity, depth, and the shark's internal temperature. The archival tag stores these measurements in an electronic memory in the tag until the scientists can retrieve the data. The archival tag can be attached to the shark to record information every day for a period of years.


Tracking (or telemetry) is another method Dr. Hueter and his colleagues use to learn about sharks. Tracking begins the same way as tagging. First, scientists must catch a shark. But instead of attaching a simple tag, scientists attach an external ultrasonic transmitter to the shark, using a tag stick and dart tag. The transmitter is attached to a tag stick by two rubber bands. The transmitter is also attached to the dart tag at the end of the tag stick by a plastic streamer. Scientists use the tag stick to insert the dart tag beneath the shark's skin. They then pull the tag stick away from the shark. The transmitter that is attached to the dart tag by the plastic streamer rolls loose from the rubber bands holding it to the tag stick and stays with the shark.

The transmitter sends data about the shark and its location back to the scientists. The data include information about the shark itself (e.g., body temperature) as well as information about the shark's swimming patterns (e.g., depth and speed).

The transmitter uses ultrasonic signals, because radio waves do not travel well in sea water. To pick up these ultrasonic signals, the scientists attach a device called a hydrophone to the boat. The hydrophone rotates and scans underwater to determine the direction of the transmitter. The hydrophone is connected to an ultrasonic receiver that converts the ultrasonic signals to sounds that the human ear can detect. The scientists listen to these signals through headphones attached to the receiver.

Human Impact on Sharks

As you learned earlier, sharks are at the top of the ocean food chain. Their role as apex predator of marine systems has been unchallenged since the age of dinosaurs. But today new predators-humans- threaten the sharks' position. Humans hunt sharks for food and for byproducts such as sharkskin. In addition, humans have changed the shark's environment by contaminating the oceans.

In one of the Experiments in this Investigation, you'll see how contaminants such as mercury affect sharks and other marine inhabitants. These contaminants have start by affecting plants and small animals at the low end of the food chain. As larger fish eat the smaller fish, the contaminants accumulate in organisms higher up the food chain. Because sharks are the apex predators, they often experience the greatest effects from human contaminants. They therefore serve as indicators of contamination in the ocean.


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Revised: 17 Oct 1995