Page 339 - (DK) Ocean - The Definitive Visual Guide

BONY FISHES 337

gas
gland intestine dorsal aorta Buoyancy
rete brightly colored
mirabile swim Most bony fishes have a gas-filled swim bladder second dorsal fin
bladder
that allows them to adjust their buoyancy,
enabling them to hover in midwater and keep
from sinking. This is especially useful to fish
that spend their lives in midwater. Many
bottom-living fish, such as flatfish, have a
poorly developed swim bladder or none at all.
SWIM-BLADDER FUNCTION To compensate for pressure changes as a fish swims toward or
A bony fish regulates its buoyancy by secreting away from the surface, it regulates the amount of gas in the swim
gas, usually oxygen, from a gas gland into bladder, usually by secreting gas into it through a gland. In some
its swim bladder. The gland is supplied with
blood (the source of the gas) by a network primitive fish, such as the herring, the swim bladder is connected
of capillaries called a rete mirabile. to the gut and is filled when the fish gulps air at the surface.
Many bony fishes can vibrate the swim bladder with special muscles to
anal fin
produce sounds. Cartilaginous fishes do not have a swim bladder. They gain
buoyancy to some extent with their large, oil-filled livers and lightweight MULTIPURPOSE FINS
bones. However, cartilaginous fishes must also use their large pectoral fins Triggerfish swim by undulating their
and tail to give them lift. Bony fishes with a swim bladder have been freed second dorsal and anal fins, maintaining
buoyancy with their swim bladder. Bright
from this necessity and, in many species, the fins have developed into fins may also function as visual signals
versatile appendages used for courtship, feeding, attack, or defense. in communication, including courtship.
FLEXIBLE APPENDAGES
A frogfish displays one of the many functions Senses
of bony fish fins. Since they are not required Bony fishes use vision, hearing, touch, taste, and smell. Vision is most
to give the fish lift, the warty frogfish’s
paired fins have evolved into flexible important in well-lit habitats. Coral reef fish have good color vision, and
appendages with which it they use colors and patterns for recognition, warning, deception, and
clambers over the sea bed. courtship. Color receptors in the eyes do not operate well in dim light.
Nocturnal fish and fish living in the twilight zone (see p.170) have large,
sensitive eyes, but little sensitivity to different colors. Dark-zone fish
often have only tiny eyes, but have a sharp sense of smell and use
pheromones for long-distance communication. Sound also carries well
underwater (see p.37) and some fish produce intense sounds with their
swim bladder. Bony fishes move in unison in shoals with the help of
their lateral-line sensory system,
for which there is no equivalent in
other vertebrates. Sense organs
arranged in a canal along the head
and sides of each fish pick up water
movements created by the other
fish. The wide field of view, due to
having eyes set on the sides of the
head, also helps precision shoaling.

SIGNALLING COLORS
Color is effective in communication on well-lit
coral reefs. The gaudiness of the Mandarin
Fish may warn predators that it is unpalatable.

LATERAL LINE
The lateral-line system can be seen in
many bony fishes, such as the pollack
shown here, as a white line along the
sides of the fish. The shape of the line
is a useful identification feature. OCEAN LIFE