BIOLOGY OF FISHES
When a boldly patterned fish is swimming over the complicated, multicoloured background of a coral reef, the fish is
difficult to recognise and hard to follow.
Another type of disruptive coloration is the eye stripe, a
black band or bar on the head that passes across the eye and
makes it less obvious. Some butterflyfishes not only have a
black eye stripe, but they also have a false eye spot or conspicuous ocellus at the rear end of the dorsal fin. This false
eye spot is supposed to deflect the attack of a predator to a
less vulnerable part of the fish.
Another function that colour patterns may serve is that of
species (or sex) recognition. Sexual dichromatism seems to
be most highly developed in the hermaphroditic species
(e.g. parrotfishes, wrasses and some angelfishes). This conspicuous difference in males and females is obviously important in the reproductive behaviour of these species.
Some fishes exhibit sexual dichromatism only when they are
about to spawn.
Species recognition is important not only for mating, but
also for territorial fishes and for those fishes that want to
"advertise" their presence to others. The cleaner wrasse,
Labroides dimidiatus, picks parasites from other fishes and
is thus generally welcomed by reef fishes which benefit from
its grooming. The cleaner wrasse has a conspicuous pattern
of bright blue with a broad horizontal black band from the
snout to the caudal fin. The cleaner occupies a particular
area (territory) of the reef, above which it swims in a distinctive, jerky up-and-down fashion. Fishes recognise the
cleaner and come to its cleaning station to be picked over
for their parasites. The cleaner wrasse is protected from predation by being recognised as a cleaner, and another unrelated species has capitalised on its good reputation. The
false cleaner (Aspidontus taeniatus) is a blenny that mimics
the cleaner wrasse; it has the same blue and black-banded
colour pattern, and it swims in the same jerky fashion. The
false cleaner is, however, not welcomed by other fishes; if
allowed to approach another fish, it will bite off a piece of
fin or skin from the body of the unsuspecting host. As long
as the mimic is relatively rare (compared to the model), it is
afforded some safety from predation and can successfully
practise its game of deception.
Bioluminescence, the ability to produce light, is better developed in fishes than in any other group of animals. At
least 45 families of marine fishes have luminous species, but
for some reason (biochemical?) no freshwater fishes are
luminous. Most of the luminous fishes are found in the deep
sea at depths of 300 - 1 000 m, but there are some luminous
shallow-water species, too (e.g. the pineapple fish, Monocentrus, and the apogonid Siphamia).
Most luminous fishes produce substances that react
chemically to produce light without heat. This reaction
takes place in light organs (photophores) and is under nervous control. Some species rely on symbiotic luminous bacteria for their light. These bacterial light organs glow con14
tinuously, and the light can be turned off only by covering or
occluding the light organ in some way. In the flashlight fish
(Anomalops), the large bacterial light organ under the eye
is "turned off" by rotating it down into a black-pigmented
pocket under the eye.
The structure of the photopores may be complex, with a
lens to focus the light and a reflector to direct the beam
through the lens. Some fishes (e.g. Malacosteus) have large
headlight photophores that emit red light. Most deep-sea
fishes are unable to see red light, but Malacosteus has the retinal pigment necessary to perceive red light and is thus able
to illuminate its prey with light that the prey cannot detect.
It seems that fishes have anticipated the development of the
infrared night vision devices that were invented for use by
military forces.
Bioluminescence in fishes serves a variety of functions.
Light organs may be used as lures on the end of barbels or
fishing rods. Some fish have bright headlight organs, which
could be used to search for prey. Many midwater species
have numerous ventrally directed light organs, which may
be used to match the faint downwelling light from the surface and thus obscure their silhouette. And probably all
light organs serve the additional function of species recognition signals.
Respiration
The gills of fishes are equivalent to the lungs of other vertebrates. The bright red colour of the gills of a live fish indicates the abundant blood supply and the thin walls of the gill
lamellae, which are the actual sites of gas exchange. In the
lamellae, the blood is separated from the water flowing
through the gills by a very thin membrane; oxygen diffuses
from the water into the blood, and carbon dioxide moves
from the blood into the water across this thin membrane.
Another adaptation to facilitate oxygen uptake by the gills
is the countercurrent system of blood flow in the lamellae,
where the flow of blood in the lamellae is opposite to the
water flow across the lamellae.
Fast-swimming fishes like scombrids have a high
metabolic rate and increased oxygen consumption, compared with more sluggish fishes like catsharks or anglerfishes. The gills of the more active fishes are "designed" for
greater efficiency in the uptake of oxygen: the lamellar
walls are thinner and the number of lamellae is greatly increased, providing a much greater surface area for diffusion.
The water current for gill ventilation is provided either by
the "branchial pump" or by simple "ram ventilation". In the
latter method, the fish simply swims with its mouth open,
and the water flow through the mouth and gills is accomplished by the movement of the fish through the water.
This method of ram ventilation is used by fast-swimming
fish and by most plankton feeders (which swim about with
When a boldly patterned fish is swimming over the complicated, multicoloured background of a coral reef, the fish is
difficult to recognise and hard to follow.
Another type of disruptive coloration is the eye stripe, a
black band or bar on the head that passes across the eye and
makes it less obvious. Some butterflyfishes not only have a
black eye stripe, but they also have a false eye spot or conspicuous ocellus at the rear end of the dorsal fin. This false
eye spot is supposed to deflect the attack of a predator to a
less vulnerable part of the fish.
Another function that colour patterns may serve is that of
species (or sex) recognition. Sexual dichromatism seems to
be most highly developed in the hermaphroditic species
(e.g. parrotfishes, wrasses and some angelfishes). This conspicuous difference in males and females is obviously important in the reproductive behaviour of these species.
Some fishes exhibit sexual dichromatism only when they are
about to spawn.
Species recognition is important not only for mating, but
also for territorial fishes and for those fishes that want to
"advertise" their presence to others. The cleaner wrasse,
Labroides dimidiatus, picks parasites from other fishes and
is thus generally welcomed by reef fishes which benefit from
its grooming. The cleaner wrasse has a conspicuous pattern
of bright blue with a broad horizontal black band from the
snout to the caudal fin. The cleaner occupies a particular
area (territory) of the reef, above which it swims in a distinctive, jerky up-and-down fashion. Fishes recognise the
cleaner and come to its cleaning station to be picked over
for their parasites. The cleaner wrasse is protected from predation by being recognised as a cleaner, and another unrelated species has capitalised on its good reputation. The
false cleaner (Aspidontus taeniatus) is a blenny that mimics
the cleaner wrasse; it has the same blue and black-banded
colour pattern, and it swims in the same jerky fashion. The
false cleaner is, however, not welcomed by other fishes; if
allowed to approach another fish, it will bite off a piece of
fin or skin from the body of the unsuspecting host. As long
as the mimic is relatively rare (compared to the model), it is
afforded some safety from predation and can successfully
practise its game of deception.
Bioluminescence, the ability to produce light, is better developed in fishes than in any other group of animals. At
least 45 families of marine fishes have luminous species, but
for some reason (biochemical?) no freshwater fishes are
luminous. Most of the luminous fishes are found in the deep
sea at depths of 300 - 1 000 m, but there are some luminous
shallow-water species, too (e.g. the pineapple fish, Monocentrus, and the apogonid Siphamia).
Most luminous fishes produce substances that react
chemically to produce light without heat. This reaction
takes place in light organs (photophores) and is under nervous control. Some species rely on symbiotic luminous bacteria for their light. These bacterial light organs glow con14
tinuously, and the light can be turned off only by covering or
occluding the light organ in some way. In the flashlight fish
(Anomalops), the large bacterial light organ under the eye
is "turned off" by rotating it down into a black-pigmented
pocket under the eye.
The structure of the photopores may be complex, with a
lens to focus the light and a reflector to direct the beam
through the lens. Some fishes (e.g. Malacosteus) have large
headlight photophores that emit red light. Most deep-sea
fishes are unable to see red light, but Malacosteus has the retinal pigment necessary to perceive red light and is thus able
to illuminate its prey with light that the prey cannot detect.
It seems that fishes have anticipated the development of the
infrared night vision devices that were invented for use by
military forces.
Bioluminescence in fishes serves a variety of functions.
Light organs may be used as lures on the end of barbels or
fishing rods. Some fish have bright headlight organs, which
could be used to search for prey. Many midwater species
have numerous ventrally directed light organs, which may
be used to match the faint downwelling light from the surface and thus obscure their silhouette. And probably all
light organs serve the additional function of species recognition signals.
Respiration
The gills of fishes are equivalent to the lungs of other vertebrates. The bright red colour of the gills of a live fish indicates the abundant blood supply and the thin walls of the gill
lamellae, which are the actual sites of gas exchange. In the
lamellae, the blood is separated from the water flowing
through the gills by a very thin membrane; oxygen diffuses
from the water into the blood, and carbon dioxide moves
from the blood into the water across this thin membrane.
Another adaptation to facilitate oxygen uptake by the gills
is the countercurrent system of blood flow in the lamellae,
where the flow of blood in the lamellae is opposite to the
water flow across the lamellae.
Fast-swimming fishes like scombrids have a high
metabolic rate and increased oxygen consumption, compared with more sluggish fishes like catsharks or anglerfishes. The gills of the more active fishes are "designed" for
greater efficiency in the uptake of oxygen: the lamellar
walls are thinner and the number of lamellae is greatly increased, providing a much greater surface area for diffusion.
The water current for gill ventilation is provided either by
the "branchial pump" or by simple "ram ventilation". In the
latter method, the fish simply swims with its mouth open,
and the water flow through the mouth and gills is accomplished by the movement of the fish through the water.
This method of ram ventilation is used by fast-swimming
fish and by most plankton feeders (which swim about with