most efficient for fast cruising. A broad truncate or rounded
caudal fin, as in rockcods (Epinephelus) or kob (Sciaenidae), is better for the sudden fast acceleration required of a
lurking predator.
Many fishes use their pectoral fins for propulsion. Labrids, scarids, pomacentrids, and the oceanic opahs (Lampris) cruise by flapping their pectorals like wings. In the
opahs, the pectoral fins are rather short and stiff, and the
muscles that operate these fins are enormous. The opahs
use their pectorals to "fly" through the water like penguins
flapping their wings. Although labrids and scarids depend
on their pectorals for cruising, they also use their caudal fin
when they are in a hurry. Stingrays and most other batoids
employ undulatory waves of their broad pectorals for propulsion and appear to fly effortlessly through the water.
Electric rays, guitarfishes and the sawfish use their tail and
caudal fin for swimming.
In the ocean sunfishes (Molidae), the stiff paddle-like
dorsal and anal fins are the primary means of propulsion. In
tetraodontoids (pufferfishes, porcupinefish and boxfish),
the short-based dorsal and anal fins as well as the pectorals
are used for slow swimming. But unlike the sunfishes, which
lack a tail and caudal fin, the tetraodontoids can use their
caudal fin when they want to swim faster. Fishes with longbased dorsal and anal fins (triggerfish, filefish, etc.) use
them for slow swimming by means of undulatory waves passed from the anterior end to the posterior end of the fin.
Accurate measurements of the speed of the fastest fishes
are difficult to make. The cruising speed of scombrids is in
the order of 2-10 km/h, but their burst swimming speed,
which they use when chasing prey or fleeing from predators,
is much faster. A wahoo (Acanthocybium) was timed at 77
km/h for a 5 second period of burst swimming, and a yellowfin tuna did 74 km/h for a 5 second period. The billfishes
(marlins, sailfish, swordfish, etc.) are also very fast swimmers,
and they are supposed to rival the tunas at their top speeds.
Coloration and bioluminescence
Fishes living in the well-lit epipelagic region of the sea are
countershaded (dark above and silvery below) to disguise
their shape and blend in with the featureless background illumination of open waters. Viewed from above, their dark
greenish blue backs match the colour of the deep ocean that
we, creatures of the air, are familiar with. Seen from below,
these fishes look like silvery streaks against a silvery mirror.
Below 300 - 400 m, the fishes are mostly uniform black; a
few are bright red, but appear black in their natural environment because the red component of sunlight is filtered
out in the first 10 m of water.
In the neritic zone, the pelagic fishes are generally silvery
BIOLOGY OF FISHES
and countershaded (e.g. the kingfishes, sardines and mackerels), but the more sedentary sublittoral fishes are often
coloured in a variety of hues and patterns. The many beautiful coral-reef fishes are among the most colourful of all
animals. The silvery iridescent colours of fishes are due to
special cells in the skin or scales. These cells, called
iridophores, contain crystals of guanine arranged in layers
which reflect several colours of light depending on the angle
of the incoming light and position of the observer.
The other (non-iridescent) colours of fishes are produced
by chromatophores. These cells contain various pigments,
and each chromatophore bears only one kind of pigment.
The chromatophore is typically irregular in shape, with
numerous branching processes extending outwards from the
centre. The pigment granules can be moved within the
chromatophore: when the pigment is dispersed throughout
the cell, the colour is intensified; when the granules are concentrated in the centre of the cell, the colour is diminished.
The skin of fishes may contain several different chromatophores (red, yellow, black, etc.) and it is the alternate contraction and expansion of the pigment in the various chromatophores that produces the changes in colour pattern.
Rapid colour changes are under nervous control. For
example, a dark galjoen swimming over dark rocks can
change immediately to a pale colour when it swims into a
light sandy area. The immediate contraction of the
melanophores is responsible for this quick change. Fishes
often have a night colour pattern that is quite different from
their daytime coloration, and a fish under stress may display
its night pattern during the day. A white-barred damselfish
that we collected at Inhaca looked vaguely familiar; when
kept in an acquarium, it changed to its daytime livery and
was then recognised as a common black-barred species.
Long-term colour changes, such as the transition from
juvenile to adult patterns, are effected by increases or decreases in the number of various chromatophores. This type
of colour change is under hormonal control, as is that of sexually dimorphic fishes, which change their colours when
they change their sex or become mature. Fishes that remain
a long time in one type of habitat are coloured to match that
habitat. In the Tsitsikama rivers, with their dark rocks and
dark water, the fishes are generally quite dark, and fishes
kept in a well-lit aquarium often become pale.
Some fishes are incredibly well camouflaged. Flatfishes
often match the pattern and colour of the substrate so well
that they are almost impossible to see unless they move.
The venomous stonefish looks just like a lump of coral rock
covered with algae, and is a real hazard to waders who
might tread on it. Klipvissies (Clinidae) are coloured to
match the seaweed in which they live.
Disruptive coloration is exhibited by many coral-reef
fishes. Bold black bars or stripes break up the general outline of the fish, and irregular patches of contrasting colour
or tones distract the eye from the overall shape of the fish.
13
caudal fin, as in rockcods (Epinephelus) or kob (Sciaenidae), is better for the sudden fast acceleration required of a
lurking predator.
Many fishes use their pectoral fins for propulsion. Labrids, scarids, pomacentrids, and the oceanic opahs (Lampris) cruise by flapping their pectorals like wings. In the
opahs, the pectoral fins are rather short and stiff, and the
muscles that operate these fins are enormous. The opahs
use their pectorals to "fly" through the water like penguins
flapping their wings. Although labrids and scarids depend
on their pectorals for cruising, they also use their caudal fin
when they are in a hurry. Stingrays and most other batoids
employ undulatory waves of their broad pectorals for propulsion and appear to fly effortlessly through the water.
Electric rays, guitarfishes and the sawfish use their tail and
caudal fin for swimming.
In the ocean sunfishes (Molidae), the stiff paddle-like
dorsal and anal fins are the primary means of propulsion. In
tetraodontoids (pufferfishes, porcupinefish and boxfish),
the short-based dorsal and anal fins as well as the pectorals
are used for slow swimming. But unlike the sunfishes, which
lack a tail and caudal fin, the tetraodontoids can use their
caudal fin when they want to swim faster. Fishes with longbased dorsal and anal fins (triggerfish, filefish, etc.) use
them for slow swimming by means of undulatory waves passed from the anterior end to the posterior end of the fin.
Accurate measurements of the speed of the fastest fishes
are difficult to make. The cruising speed of scombrids is in
the order of 2-10 km/h, but their burst swimming speed,
which they use when chasing prey or fleeing from predators,
is much faster. A wahoo (Acanthocybium) was timed at 77
km/h for a 5 second period of burst swimming, and a yellowfin tuna did 74 km/h for a 5 second period. The billfishes
(marlins, sailfish, swordfish, etc.) are also very fast swimmers,
and they are supposed to rival the tunas at their top speeds.
Coloration and bioluminescence
Fishes living in the well-lit epipelagic region of the sea are
countershaded (dark above and silvery below) to disguise
their shape and blend in with the featureless background illumination of open waters. Viewed from above, their dark
greenish blue backs match the colour of the deep ocean that
we, creatures of the air, are familiar with. Seen from below,
these fishes look like silvery streaks against a silvery mirror.
Below 300 - 400 m, the fishes are mostly uniform black; a
few are bright red, but appear black in their natural environment because the red component of sunlight is filtered
out in the first 10 m of water.
In the neritic zone, the pelagic fishes are generally silvery
BIOLOGY OF FISHES
and countershaded (e.g. the kingfishes, sardines and mackerels), but the more sedentary sublittoral fishes are often
coloured in a variety of hues and patterns. The many beautiful coral-reef fishes are among the most colourful of all
animals. The silvery iridescent colours of fishes are due to
special cells in the skin or scales. These cells, called
iridophores, contain crystals of guanine arranged in layers
which reflect several colours of light depending on the angle
of the incoming light and position of the observer.
The other (non-iridescent) colours of fishes are produced
by chromatophores. These cells contain various pigments,
and each chromatophore bears only one kind of pigment.
The chromatophore is typically irregular in shape, with
numerous branching processes extending outwards from the
centre. The pigment granules can be moved within the
chromatophore: when the pigment is dispersed throughout
the cell, the colour is intensified; when the granules are concentrated in the centre of the cell, the colour is diminished.
The skin of fishes may contain several different chromatophores (red, yellow, black, etc.) and it is the alternate contraction and expansion of the pigment in the various chromatophores that produces the changes in colour pattern.
Rapid colour changes are under nervous control. For
example, a dark galjoen swimming over dark rocks can
change immediately to a pale colour when it swims into a
light sandy area. The immediate contraction of the
melanophores is responsible for this quick change. Fishes
often have a night colour pattern that is quite different from
their daytime coloration, and a fish under stress may display
its night pattern during the day. A white-barred damselfish
that we collected at Inhaca looked vaguely familiar; when
kept in an acquarium, it changed to its daytime livery and
was then recognised as a common black-barred species.
Long-term colour changes, such as the transition from
juvenile to adult patterns, are effected by increases or decreases in the number of various chromatophores. This type
of colour change is under hormonal control, as is that of sexually dimorphic fishes, which change their colours when
they change their sex or become mature. Fishes that remain
a long time in one type of habitat are coloured to match that
habitat. In the Tsitsikama rivers, with their dark rocks and
dark water, the fishes are generally quite dark, and fishes
kept in a well-lit aquarium often become pale.
Some fishes are incredibly well camouflaged. Flatfishes
often match the pattern and colour of the substrate so well
that they are almost impossible to see unless they move.
The venomous stonefish looks just like a lump of coral rock
covered with algae, and is a real hazard to waders who
might tread on it. Klipvissies (Clinidae) are coloured to
match the seaweed in which they live.
Disruptive coloration is exhibited by many coral-reef
fishes. Bold black bars or stripes break up the general outline of the fish, and irregular patches of contrasting colour
or tones distract the eye from the overall shape of the fish.
13