Page 20 - Encyclopedia of Aquarium and Pond Fish, 3rd Edition

18 INTRODUCTION TO FISHKEEPING

Swim bladder Oxygen is taken up over
the gill surface, and
Kidney Spinal cord waste carbon dioxide is
released into the water
Vertebral column Brain
Nostrils

Gills are highly
efficient breathing
Urinary Gills
bladder Heart organs, able to extract
Urogenital Liver Stomach (present Water, carrying Deoxygenated water 80 percent of the
opening Gall in some fish) dissolved oxygen, leaves the gill chamber oxygen dissolved in the
Gonad bladder enters the mouth via the gill flap
Anus (operculum) water around them.
Fish have many organs—such as the brain, stomach,
liver, and kidneys—in common with humans. Others, like Digestion and respiration
the gills and swim bladder, are not present in our bodies.
The digestive system of a fish is typical of vertebrates;
as with terrestrial species, herbivorous fish tend to have
Water position and buoyancy longer intestines than carnivores, because plant matter is
All fish rely on their fins—especially the pectoral and ventral tough, fibrous, and difficult to break down. The respiratory
fins—to control their position in the water and prevent them system, however, is unique to fish. Most fish extract oxygen
from being swept away by currents. In fish that live in fast- from the water, rather than the air, using gills, which are
flowing mountain streams, the fins can be fused together; located on the sides of the head behind the eyes, hidden
the result is a suction cup that anchors the fish in place. under flaps known as opercula.
This adaption is seen in the hillstream loaches of the family Gills are bony rods to which are attached fleshy filaments,
Balitoridae. Position in the water is also influenced by the rich in thin-walled blood capillaries. Water enters the fish’s
swim bladder—an elongated gas-filled organ situated beneath open mouth, which then closes.The water is forced over
the vertebral column. To achieve neutral buoyancy (when the the filaments and out again through the opercula. Oxygen
fish neither rises nor sinks), the swim bladder must occupy dissolved in the water is taken up into the bloodstream
about 8 percent of the fish’s body volume. The amount through the filaments, which usually have fine secondary
of gas in the bladder can be adjusted in two ways: the fish can flaps (or lamellae) to maximize the surface area available for
gulp down air, which enters the swim bladder via the foregut, gas exchange. Astonishingly, the total surface area of the gills
or gas can be released into the bladder from blood vessels. can be more than 10 times the fish’s outer body area. Within

Fish deter predators in a variety of ways. Some species use
Spines on inflated clever camouflage to break up their outline against the
Porcupine fish fish deter attack colorful reef background; others are armed with
venomous spines or can inflate their bodies,
making themselves too large to swallow.

False eye confuses Venomous
predators spines

Threadfin
Butterflyfish
Volitans
Black bar Lionfish
masks body shape

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