BIOLOGY OF FISHES
gut, the swimbladder functions mainly to provide buoyancy.
In the spiny-ray fishes, the swimbladder has no connection with the gut, and any change in the amount of gas in the
swimbladder is a slow process of secretion or resorption. If
the fish is pulled rapidly to the surface from a depth of20 m
or more, the gas in the swimbladder cannot be adjusted to
the sudden decrease in external pressure. The result is the
gross expansion of the gas and therefore of the swimbladder, such that it may rupture or push the everted stomach
into the mouth. Anglers know that if they hook a large
rockcod in deep water and can bring it halfway up to the surface, the battle is won, as the expanding gas in the
swimbladder makes it very difficult for the fish to swim back
down to the bottom.
To get around the problem of rapid gas expansion with
sudden changes in depth, fishes have either replaced the
gas-filled swimbladder with a partly or wholly fat-filled
swimbladder, or they have lost their swim bladder completely. In many species of lanternfishes (Myctophidae),
the swim bladder gas is progressively replaced with lipids
(fatty oils that are less dense than sea water) as the fish ages.
The swimbladder of the coelacanth (Latimeria) is completely filled with lipid (mainly low-density wax esters), and
the body muscles are also rich in wax esters. The oilfish
(Ruvettus) is notorious for the purgative properties of its
flesh, which is due to the abundance of wax esters in the
body muscles. Latimeria and Ruvettus have thus evolved a
similar solution to the problem of buoyancy in their up-anddown world. These two quite unrelated species occupy the
same habitat at the Comores, and they apparently make
considerable depth changes in their search for the same
prey.
Sharks (and other chondrichthyes) lack a swimbladder,
but they also produce large amounts of lipid to provide
buoyancy. The lipid is stored in the liver as oil (mainly
squalene, a very low-density oil that incidentally also makes
an excellent lubricant for fine machinery). The livers of
some sharks are enormous - up to 25 per cent of the total
weight!
The swimbladder is used by some fishes to produce or
amplify sounds. By contracting muscles attached to the gasfilled swim bladder , vibrations are produced that can be
heard as growls, grunts, hoots and drumming noises. The
muscles are either wholly attached to the swimbladder (intrinsic muscles) or attached by one end to the swimbladder
and by the other to the skull, vertebral column or ribs (extrinsic muscles). Toadfishes (Batrachoididae) have a welldeveloped swimbladder with large intrinsic muscles. The
North American oyster toadfish (Opsanus tau) is known for
its repertoire of loud hoots, grunts, yelps and boat-whistle
sounds. These noises are used in courtship and in defence of
territory. The cave bass (Dinoperca petersi) has a large,
thick-walled swimbladder that is almost completely enveloped by three pairs of intrinsic muscles. Divers familiar
16
with this species report that it is often seen at the entrance to
its cave and makes a loud drumming noise when approached.
Migrations
Migrations are regular mass movements of animals from
one place to another, usually for purposes of breeding or
feeding. Many mesopelagic fishes (e.g. myctophids) undertake vertical migrations of several hundred metres every
evening in order to feed on the abundant plankton in the
epipelagic region. By feeding at night in the surface waters,
they are safe from birds and other predators that hunt their
prey visually. At dawn, the myctophids return to depths of
100 -1000 m.
The elf (Pomatomus saltatrix) migrates from Natal to the
southern Cape coast during the late summer months to
feed, and returns to Natal waters to breed from September
to December. This species also occurs along the Atlantic
coast of the U.S. and on the south coast of Australia where
it is well known as a migratory fish.
The freshwater eels (Anguilla) that live in the rivers of
our south and east coasts must return to the open ocean to
spawn. The transparent larvae (leptocephali) drift south in
the Mozambique current from somewhere near Madagascar. When they enter freshwater, they turn into the wormlike little elvers that ascend the rivers, even wriggling up
dam walls. Females go farther inland than males do, and
the eels stay and grow in freshwater for about 14 years.
When they are ready to return to the sea for breeding, they
stop eating, their eyes enlarge, and their dark skin becomes
silvery. They apparently migrate to some place near
Madagascar to spawn, but once they leave the rivers they
are never seen again. The European eel (Anguilla anguilla
Linnaeus, 1758) traverses more than 3 000 km of the Atlantic Ocean to breed in the Sargasso Sea south of Bermuda. It
takes two or three years for the leptocephali to reach the
European rivers where they will feed and grow for some
years until it is time for them to set off on their own breeding
migration.
Fishes that return to the sea to spawn, like the freshwater
eels, are called catadromous. Our freshwater mullet
(Myxus capensis) is a catadromous species that spends most
of its life in the fresh waters of our rivers, but returns to the
sea to spawn. Unlike the elvers of the freshwater eels, the
young of Myxus cannot get past dam walls, and this species
is therefore becoming increasingly restricted in its available
habitat.
Anadromous fishes, which return from the sea to spawn
in freshwater, do not occur in southern Africa, but they are
well known in the northern hemisphere. Some species of
Pacific salmon return to the same stream from which they
emigrated years earlier. Biologists have used this attraction
to their birthplace to breed large salmon, by taking the ova
gut, the swimbladder functions mainly to provide buoyancy.
In the spiny-ray fishes, the swimbladder has no connection with the gut, and any change in the amount of gas in the
swimbladder is a slow process of secretion or resorption. If
the fish is pulled rapidly to the surface from a depth of20 m
or more, the gas in the swimbladder cannot be adjusted to
the sudden decrease in external pressure. The result is the
gross expansion of the gas and therefore of the swimbladder, such that it may rupture or push the everted stomach
into the mouth. Anglers know that if they hook a large
rockcod in deep water and can bring it halfway up to the surface, the battle is won, as the expanding gas in the
swimbladder makes it very difficult for the fish to swim back
down to the bottom.
To get around the problem of rapid gas expansion with
sudden changes in depth, fishes have either replaced the
gas-filled swimbladder with a partly or wholly fat-filled
swimbladder, or they have lost their swim bladder completely. In many species of lanternfishes (Myctophidae),
the swim bladder gas is progressively replaced with lipids
(fatty oils that are less dense than sea water) as the fish ages.
The swimbladder of the coelacanth (Latimeria) is completely filled with lipid (mainly low-density wax esters), and
the body muscles are also rich in wax esters. The oilfish
(Ruvettus) is notorious for the purgative properties of its
flesh, which is due to the abundance of wax esters in the
body muscles. Latimeria and Ruvettus have thus evolved a
similar solution to the problem of buoyancy in their up-anddown world. These two quite unrelated species occupy the
same habitat at the Comores, and they apparently make
considerable depth changes in their search for the same
prey.
Sharks (and other chondrichthyes) lack a swimbladder,
but they also produce large amounts of lipid to provide
buoyancy. The lipid is stored in the liver as oil (mainly
squalene, a very low-density oil that incidentally also makes
an excellent lubricant for fine machinery). The livers of
some sharks are enormous - up to 25 per cent of the total
weight!
The swimbladder is used by some fishes to produce or
amplify sounds. By contracting muscles attached to the gasfilled swim bladder , vibrations are produced that can be
heard as growls, grunts, hoots and drumming noises. The
muscles are either wholly attached to the swimbladder (intrinsic muscles) or attached by one end to the swimbladder
and by the other to the skull, vertebral column or ribs (extrinsic muscles). Toadfishes (Batrachoididae) have a welldeveloped swimbladder with large intrinsic muscles. The
North American oyster toadfish (Opsanus tau) is known for
its repertoire of loud hoots, grunts, yelps and boat-whistle
sounds. These noises are used in courtship and in defence of
territory. The cave bass (Dinoperca petersi) has a large,
thick-walled swimbladder that is almost completely enveloped by three pairs of intrinsic muscles. Divers familiar
16
with this species report that it is often seen at the entrance to
its cave and makes a loud drumming noise when approached.
Migrations
Migrations are regular mass movements of animals from
one place to another, usually for purposes of breeding or
feeding. Many mesopelagic fishes (e.g. myctophids) undertake vertical migrations of several hundred metres every
evening in order to feed on the abundant plankton in the
epipelagic region. By feeding at night in the surface waters,
they are safe from birds and other predators that hunt their
prey visually. At dawn, the myctophids return to depths of
100 -1000 m.
The elf (Pomatomus saltatrix) migrates from Natal to the
southern Cape coast during the late summer months to
feed, and returns to Natal waters to breed from September
to December. This species also occurs along the Atlantic
coast of the U.S. and on the south coast of Australia where
it is well known as a migratory fish.
The freshwater eels (Anguilla) that live in the rivers of
our south and east coasts must return to the open ocean to
spawn. The transparent larvae (leptocephali) drift south in
the Mozambique current from somewhere near Madagascar. When they enter freshwater, they turn into the wormlike little elvers that ascend the rivers, even wriggling up
dam walls. Females go farther inland than males do, and
the eels stay and grow in freshwater for about 14 years.
When they are ready to return to the sea for breeding, they
stop eating, their eyes enlarge, and their dark skin becomes
silvery. They apparently migrate to some place near
Madagascar to spawn, but once they leave the rivers they
are never seen again. The European eel (Anguilla anguilla
Linnaeus, 1758) traverses more than 3 000 km of the Atlantic Ocean to breed in the Sargasso Sea south of Bermuda. It
takes two or three years for the leptocephali to reach the
European rivers where they will feed and grow for some
years until it is time for them to set off on their own breeding
migration.
Fishes that return to the sea to spawn, like the freshwater
eels, are called catadromous. Our freshwater mullet
(Myxus capensis) is a catadromous species that spends most
of its life in the fresh waters of our rivers, but returns to the
sea to spawn. Unlike the elvers of the freshwater eels, the
young of Myxus cannot get past dam walls, and this species
is therefore becoming increasingly restricted in its available
habitat.
Anadromous fishes, which return from the sea to spawn
in freshwater, do not occur in southern Africa, but they are
well known in the northern hemisphere. Some species of
Pacific salmon return to the same stream from which they
emigrated years earlier. Biologists have used this attraction
to their birthplace to breed large salmon, by taking the ova