Page 18 - Encyclopedia of Aquarium and Pond Fish, 3rd Edition

16 INTRODUCTION TO FISHKEEPING

What is a fish?

Defining a fish is harder than it seems. To most people, Eye, with outer
iris and central Dorsal fin
it is an animal that lives and breathes in water. dark pupil Fin rays
Some fish, however, emerge onto land, breathe
air, and use their fins like legs. And many other Body scales
animals, including amphibians, mammals, and
invertebrates, live in water. To add further
Mouth
confusion, some creatures called fish—starfish,
Gill cover or
jellyfish, and cuttlefish—are not really fish at all. operculum
Pectoral fin Caudal
peduncle
Fish are vertebrates, which means that—like humans, Ventral or Urogenital
pelvic fin
but unlike starfish, jellyfish, and cuttlefish—they possess Common Goldfish and anal Anal fin Caudal or
openings
a backbone. Most are cold-blooded (ectothermic), so they tail fin
cannot raise their body temperature above that of their The body shape of a fish is determined by its skeleton.
Most fish have bony skeletons, but primitive species,
environment, unlike mammals and birds; this limits the notably sharks and rays, have skeletons of cartilage.
effective range of fish to warmer waters (though there are
many exceptions). All fish have gills, which they rely upon to which are thin, overlapping outgrowths of the
varying degrees to obtain oxygen, and most species have two outer skin that protect the body. They secrete
pairs of fins in place of arms and legs, as well as several other a slimy coating that protects them from parasites
fins on the body. The majority of fish are covered in scales, and bacteria and helps them slip through the
water. The presence of scales helps to Crescent
distinguish fish from amphibians—another
group of water-dwelling vertebrates.
BODY COVERINGS
The skin of a fish is usually covered by protective scales or bony Body and fins
plates. A fish has the same number of scales throughout its life; if There are many alternative body plans for fish.
some are lost, they will be replaced, but new scales are not added. A “typical” fish is designed to be streamlined so
Several basic types of body covering are recognized. that it can cut through the water with the least
effort. It has a spindle-shaped profile, though it
is somewhat wider to the front of the Round
midline, and its head joins the body without
a neck. Its eyes are flush with the head, and
only the fins extend beyond the body; even
the fins can be pressed flat against the sides
Ctenoid scales, such Cycloid scales, such Bony plates, as seen in
as those of a Queen as those of a goldfish, this catfish, offer better to minimize water resistance.
Angelfish, have a have a smooth rear protection than scales Fish rely on their fins for locomotion,
comblike rear edge. edge. Like ctenoid but restrict mobility.
Ctenoid and cycloid scales, they have a hard They start as folds in though they may have more specialized uses Deeply cleft
scales are found in surface layer over a the skin of fry; the folds in some species. Swimming through water,
the vast majority of deeper fibrous layer harden and develop into which is much more dense than air,
bony fish. made of collagen. bony plates.
Lacking scales or plates, Synodontis The shape of the caudal (tail) fin varies between
catfish rely on their thickened skin species, and greatly affects swimming ability. Fish
and plentiful mucus for protection. with deeply forked tails rank among the most
Many bottom-dwelling fish lack powerful swimmers. In some cases, the tail has
scales on their undersides. become enlarged naturally, or by selective breeding, Selectively
into a more decorative feature. bred twintail

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