the mouth open anyway). The branchial pump method is
used by sedentary species to force water through the gills.
By alternately expanding and contracting the mouth and gill
cavities, water is sucked into the mouth and pushed through
the gills. The operation of this branchial pump can be seen
in fish in an aquarium as they expand and contract their
opercula.
Since water contains only about three per cent of the oxygen in an equivalent volume of air, it is not surprising that
some fishes have developed means for aerial respiration.
Air breathing is best developed in freshwater fishes like the
lungfishes, which have developed lungs as well as gills.
There are a few marine fishes that use aerial respiration to
supplement the oxygen gained from water respiration. In
freshwater eels (Anguilla), about 10 per cent of the oxygen
uptake is normally through the skin, but if the eel is in air
some 66 per cent of its oxygen is taken in through the skin
and the rest is brought in through the gills. The mudskipper
(Periophthalmus) is also able to live for long periods in air,
and it acquires oxygen in the same way as the eel. Some
species of mullet (Mugilidae) frequently jump out of the
water. In warm stagnant water, which contains little dissolved oxygen, mullet jump more frequently than they do in
well-oxygenated habitats. The mullet apparently jump out
of the water to take in air, which is then kept in bubbles in a
chamber above the gills. These air bubbles provide an additional source of oxygen in stagnant water.
Osmoregulation
The physiological process by which the salt concentration
of blood and other tissues is maintained within the limits
necessary to sustain life is called osmoregulation. In marine
bony fishes, the salt concentration of the blood and tissues is
less than that of sea water, thus they lose water by diffusion
through the gills and skin. To make up for this loss of water,
bony fishes drink sea water and absorb the water (along with
salt ions which they do not need) through the gut. The unwanted ions are excreted via the gills, urine and faeces.
The salt content of cartilaginous fishes (Chondrichthyes)
is only slightly higher than that of teleosts, but due mainly to
the high concentration of urea in the blood, there is no tendency to lose water through the gills or skin. Cartilaginous
fishes (and the coelacanth, which also uses urea to osmoregulate) are somehow able to tolerate concentrations of
urea in their blood that would be fatal to most other animals. Influx of ions is countered by excretion of ions via the
gills, urine and rectal gland. The rectal gland appears to be
unique to the coelacanth and chondrichthyes; it accounts
for most of the Na+CI- excreted by these fishes.
Some elasmobranchs have adapted to life in fresh water.
In these species, the concentration of urea and salts is much
reduced compared to their levels in a marine environment.
The freshwater stingrays of South America (Potamotrygon
BIOLOGY OF FISHES
spp.) cannot retain urea in their blood, and they will die if
placed in sea water. The Zambezi shark (Carcharhinus
leucus) , however, often occurs in rivers; juveniles are
known from Malawi and the Kruger National Park!
Euryhaline fishes, like the Zambezi shark, are able to tolerate wide fluctuations in salinity. The estuarine fish fauna
comprises euryhaline species like moonies (Monodactylus) ,
mullet (Mugilidae) and kob (Argyrosomus). Stenohaline
species canot tolerate changes in salinity and are thus confined either to fresh water or to sea water. Most estuarine
fishes are euryhaline marine species, but a notable exception is the freshwater cichlid Oreochromis mossambicus,
which is able to tolerate estuarine salinities.
In hagfishes, the salt concentration of the blood is similar
to that of sea water, and hagfishes cannot tolerate even halfstrength sea water. Some lampreys live in marine environments, although they must return to fresh water to breed.
Lampreys regulate their ionic concentrations in a manner
similar to that of teleosts.
Temperature
With a few notable exceptions, fishes are "cold-blooded"
(poikilothermous) animals, which means that their body
temperature is dependent on the temperature of the surrounding water. Except for certain oceanic fishes that often
move through the thermocline, fishes cannot tolerate rapid
changes of more than a few degrees Celsius. When an east
wind blows along our southern coast, it causes upwelling of
cold bottom water; this influx of cold water in the nearshore
environment often kills fishes or makes them very lethargic.
Some large, fast-swimming fishes (tunas, marlins and
lamnid sharks) are able to maintain their body temperatures above the ambient water temperature. See the section
on "Swimming" (p. 12) for a discussion on these warmbodied fishes.
Some Arctic and Antarctic fishes have an organic "antifreeze" compound in their blood to prevent its freezing in
the -1 ° to - 2°C temperatures of their environment. Deepsea fishes can obviously live in low temperatures, for the
water of the bathypelagic region is between 2° and 4 0c. At
the other extreme is the African freshwater cichlid, which
can live in hot springs of 40°C.
Swim bladder, buoyancy and sound production
Most fishes have a gas-filled swimbladder in the upper
part oftheir abdominal cavity. In some primitive fishes (e.g.
lungfishes and the freshwater bowfins of North America)
the swimbladder is connected to the oesophagus and functions as an accessory respiratory organ. Although some
marine fishes retain the duct from the swimbladder to the
15
used by sedentary species to force water through the gills.
By alternately expanding and contracting the mouth and gill
cavities, water is sucked into the mouth and pushed through
the gills. The operation of this branchial pump can be seen
in fish in an aquarium as they expand and contract their
opercula.
Since water contains only about three per cent of the oxygen in an equivalent volume of air, it is not surprising that
some fishes have developed means for aerial respiration.
Air breathing is best developed in freshwater fishes like the
lungfishes, which have developed lungs as well as gills.
There are a few marine fishes that use aerial respiration to
supplement the oxygen gained from water respiration. In
freshwater eels (Anguilla), about 10 per cent of the oxygen
uptake is normally through the skin, but if the eel is in air
some 66 per cent of its oxygen is taken in through the skin
and the rest is brought in through the gills. The mudskipper
(Periophthalmus) is also able to live for long periods in air,
and it acquires oxygen in the same way as the eel. Some
species of mullet (Mugilidae) frequently jump out of the
water. In warm stagnant water, which contains little dissolved oxygen, mullet jump more frequently than they do in
well-oxygenated habitats. The mullet apparently jump out
of the water to take in air, which is then kept in bubbles in a
chamber above the gills. These air bubbles provide an additional source of oxygen in stagnant water.
Osmoregulation
The physiological process by which the salt concentration
of blood and other tissues is maintained within the limits
necessary to sustain life is called osmoregulation. In marine
bony fishes, the salt concentration of the blood and tissues is
less than that of sea water, thus they lose water by diffusion
through the gills and skin. To make up for this loss of water,
bony fishes drink sea water and absorb the water (along with
salt ions which they do not need) through the gut. The unwanted ions are excreted via the gills, urine and faeces.
The salt content of cartilaginous fishes (Chondrichthyes)
is only slightly higher than that of teleosts, but due mainly to
the high concentration of urea in the blood, there is no tendency to lose water through the gills or skin. Cartilaginous
fishes (and the coelacanth, which also uses urea to osmoregulate) are somehow able to tolerate concentrations of
urea in their blood that would be fatal to most other animals. Influx of ions is countered by excretion of ions via the
gills, urine and rectal gland. The rectal gland appears to be
unique to the coelacanth and chondrichthyes; it accounts
for most of the Na+CI- excreted by these fishes.
Some elasmobranchs have adapted to life in fresh water.
In these species, the concentration of urea and salts is much
reduced compared to their levels in a marine environment.
The freshwater stingrays of South America (Potamotrygon
BIOLOGY OF FISHES
spp.) cannot retain urea in their blood, and they will die if
placed in sea water. The Zambezi shark (Carcharhinus
leucus) , however, often occurs in rivers; juveniles are
known from Malawi and the Kruger National Park!
Euryhaline fishes, like the Zambezi shark, are able to tolerate wide fluctuations in salinity. The estuarine fish fauna
comprises euryhaline species like moonies (Monodactylus) ,
mullet (Mugilidae) and kob (Argyrosomus). Stenohaline
species canot tolerate changes in salinity and are thus confined either to fresh water or to sea water. Most estuarine
fishes are euryhaline marine species, but a notable exception is the freshwater cichlid Oreochromis mossambicus,
which is able to tolerate estuarine salinities.
In hagfishes, the salt concentration of the blood is similar
to that of sea water, and hagfishes cannot tolerate even halfstrength sea water. Some lampreys live in marine environments, although they must return to fresh water to breed.
Lampreys regulate their ionic concentrations in a manner
similar to that of teleosts.
Temperature
With a few notable exceptions, fishes are "cold-blooded"
(poikilothermous) animals, which means that their body
temperature is dependent on the temperature of the surrounding water. Except for certain oceanic fishes that often
move through the thermocline, fishes cannot tolerate rapid
changes of more than a few degrees Celsius. When an east
wind blows along our southern coast, it causes upwelling of
cold bottom water; this influx of cold water in the nearshore
environment often kills fishes or makes them very lethargic.
Some large, fast-swimming fishes (tunas, marlins and
lamnid sharks) are able to maintain their body temperatures above the ambient water temperature. See the section
on "Swimming" (p. 12) for a discussion on these warmbodied fishes.
Some Arctic and Antarctic fishes have an organic "antifreeze" compound in their blood to prevent its freezing in
the -1 ° to - 2°C temperatures of their environment. Deepsea fishes can obviously live in low temperatures, for the
water of the bathypelagic region is between 2° and 4 0c. At
the other extreme is the African freshwater cichlid, which
can live in hot springs of 40°C.
Swim bladder, buoyancy and sound production
Most fishes have a gas-filled swimbladder in the upper
part oftheir abdominal cavity. In some primitive fishes (e.g.
lungfishes and the freshwater bowfins of North America)
the swimbladder is connected to the oesophagus and functions as an accessory respiratory organ. Although some
marine fishes retain the duct from the swimbladder to the
15