Page 21 - Encyclopedia of Aquarium and Pond Fish, 3rd Edition

WHAT IS A FISH? 19

the mouth, straining devices called gill rakers prevent
food and debris from passing over and damaging the gills.
Some fish also gulp atmospheric oxygen using the swim SENSES AND COMMUNICATION
bladder as a basic “lung.” These species are usually the natural Although fish brains are poorly developed compared to those of
inhabitants of muddy pools, where dissolved oxygen may mammals, fish possess acute and often highly specialized senses
be in short supply. and means of communication.
Barbels—structures on the lips
Color and pattern that resemble elongated whiskers—
Almost all fish use color to aid camouflage or to are common in fish that live
in water where visibility is poor.
attract mates. Some are colored with inconspicuous Barbels may contain touch and
browns and greens to blend in with the background and taste organs that help the fish
navigate and find food.
escape the attention of predators; others—such as the
flatfish—change their pattern to match their background. The lateral line runs down
The brilliant colors displayed by many tropical species are each side of the fish’s body. It
comprises a row of pores opening
also a form of camouflage; bold vertical stripes, for example, into a channel that leads to the
break up the outline of a body and make it hard to see. And head. The channel is filled with
dark stripes through the eyes often continue through the a viscous jelly, which detects
vibrations in the water.
iris, making the eye almost invisible. Some fish have “false”
eyespots (also called ocelli) on their tails; predators will Fish eyes are similar to those of
attack what they believe to be the head, giving the prey other vertebrates and can see
colors. Vision is particularly
a few moments to escape. sharp in fish that live close to
Fish that have a solid, dark color tend to have lots the surface. The Four-Eyes (see
of pigment in their skin, while species that appear silvery pp.156–157) can see in both air
and water at the same time.
have little skin pigment but rely more on the iridescence of
the scales. This reflectiveness is caused by the presence of the Electrical fields produced by
chemical guanine—a waste product from the blood. Many fish mormyrids (see p.186), like this
Peter’s Elephant-Nose, enable fish
have transparent bodies as fry and develop color only with age. to sense their environment. Some
experiments suggest that the
electrical signals may also be
The coordinated movement of a shoal of fish increases used in communication.
the chances of survival of each individual. Many eyes
are more effective than one at detecting danger, while Some species use sound to
swimming in close formation makes it harder for an communicate with each other,
individual to be targeted by a predator. such as the Croaking Gourami
(see p.110). Their “drumrolls”
are produced by the action of
muscles beating against the
swim bladder.

US_016-019_What_is_fish_v2.indd 19 29/08/18 4:08 PM