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.
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
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.
- Scientists have studied lemon and nurse sharks to investigate the
ability of sharks to recognize differences between colors (white,
black, and yellow), shapes (rectangles, triangles, and circles) and
designs (horizontal and vertical stripes).
- Blacktip sharks have been used to investigate the ability of sharks
to distinguish forms. The blacktip shark can readily distinguish
between rectangles placed at 90-degree angles.
- Scientists have investigated the ability of lemon sharks to
distinguish between objects of varying brightness. Scientists set up
two doors, each of which was lit by optical fibers connected to a
light source. The lemon shark was trained with food
reinforcement to swim to the brighter of the two doors.
- Young lemon sharks have been trained to blink their eyelids when
they see a flashing light. Using this technique, scientists have
determined that these sharks can see a light ten times dimmer
than the dimmest light that a human can see. This means that
these sharks can hunt at night using starlight to see their prey.
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
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
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
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