THE SIX SENSES
 Fish hear, see, taste, touch, smell and even sense electricity in
different and more acute ways than humans. Fish can sense light,
chemicals, vibrations and even electricity, and they have developed
senses that humans to not yet possess.
SIGHT
Within vertebrates, the eye seems to be the most constant feature.
The eye is used in conjugation with the other senses to find food,
shelter, mates, and to avoid predators. The eye of a fish contains a
retina, an optic nerve, a lens, a cornea, an iris and three pairs of
oculomotor muscles. The eye of a fish is generally soft. However,
there are many adaptive changes due to slight changes in habitat, such
as depth. Many fish can see in color and in both air and water. The
size of a fish's eye varies according to the depth and quantity of
light present. Generally the size of the eye of a fish increases with
depth as the quantity of light present decreases. Some fish contain
even a Tapetum Lucidum; an eye structure composed of glowing guanine
crystals that amplify light. At the end of the photic zone, eye size
drops off due to complete lack of light. The eye of the fish has
evolved to be completely sphere as to enable vision underwater due to
higher refractive index. Fish do not have the capabilities to dilate
or contract their pupils as the lens bulges through the iris due to
the fact that fish focus by moving the lens in and out instead of
stretching. Fish, aside from several species of sharks, do not posses
eyelids. A nictitating membrane cleans the surface of the cornea, and
an adipose layer composed of fat protects the eyeball.
HEARING AND TOUCH
Though hidden, fish do posses ears and a system of touch. These ears
are located within their bodies along with a lateral line system that
helps them feel. The internal ears are used to sense sound vibration
transmitted from the water to the fish's body and into its ear.
Though fish lack an outer ear and a cochlea, the inner ear is still
used to sense balance. This fluid-filled inner ear is a very delicate
organ divided into two sections: the parasuperior (top half) and the
utriculus (lower half). The parasuperior is responsible for balance.
Divided into three semicircular canals, the fish uses the fluid and
sensory hairs to detect rotation and acceleration. The utriculus
contains two relatively large stone-like structures called otoliths,
which vibrate with sound waves, stimulating interpretation of these
waves.
Fish touch using a technique similar to that of the technique used
for hearing. Fish possess neuromast; clusters of hairs linked to a
thick fluid called the cupala. Many of these neuromast come in direct
contact with the water allowing the fish to detect objects moving in
the water a short distance away. Not all neuromast however come in
contact with the water. Some are arranged linearly to form lateral
lines to give the fish an actual sense of touch.
SMELL AND TASTE
Fish use chemoreception to smell and taste as humans do. In many fish,
these senses are even further developed than those of humans, and many
species are dependant on the to detect prey. Most fish have two
nostrils, one on either side of their head not attached to the throat,
but contain an olfactory organ used to detect chemicals. The size of
this organ is normally proportional to the fish's sense of smell; the
larger the nostril the better the sense of smell. Many higher species
of fish, such as sharks, can sense chemicals in the water with a
concentration as low as one part per billion!
Most fish also possess the ability to taste. As humans do, fish
possess taste buds. However, fish possess many more, those of which
are much more accurate. Taste buds line not only the tongue, but the
entire mouth as well. Some species of fish, such as the catfish,
possess special structures called barbels. These barbels are whiskers
that have taste structures allowing fish the dig for food, tasting it
before it even reaches their mouths.
ELECTRORECEPTION
Fish have developed a sixth sense to aid them in detecting electrical
potential or voltage. To do this, fish use canals filled with a gel
containing sensory cells. These canals combine to make up the
electroreceptive system called the Ampullae of Lorenzin. These
sensors are extremely sensitive in higher species of fish. Some fish
may even sense the heartbeat of another fish up to 500 miles away!
Sharks use this technique to detect prey, as well as to sense the
earth's magnetic field for navigation.
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