A fish is typically drawn as an elongated spindle, and in fact this is the most
common form, as it makes it easier to swim in open water. These hydrodynamic characteristics
permit rapid acceleration and not inconsiderable speeds (sometimes up to 20 km per
hour) in a medium (water) that offers a certain degree of resistance.
However, there are other forms, that are also all connected with the lifestyle
of the fish in question: bottom-dwellers have a flat stomach, while those that live
in water obstructed by plants and branches have compact, thin bodies that enable
them to squeeze through the obstacles. This is equally the case with the countless
fish in the coral reefs, which thread their way through the blocks of coral. Finally,
there are certain fish that are unclassifiable, so varied and strange are the forms
they flaunt, although they always correspond to a particular lifestyle.
Fish have several types of fins, each one playing a precise role. Their forms
and names are often used to classify them into different families.
Xipho (Xiphophorus helleri), bred with overdeveloped fins.
Of the unpaired fins (i.e. consisting of a single fin), the most noteworthy are
the dorsal and the anal fins. These serve to stabilize the fish when it is not going
very fast or is coming to a halt, and they are tucked in when the fish swims more
quickly. The caudal fin (incorrectly referred to as the tail) supplies propulsion,
in conjunction with the rear part of the body. In some species, particularly the
Characins and the catfish, there is a small extra fin between the dorsal and the
caudal fins, known as the adipose fin. although this is not really used. The paired
fins, attached symmetrically to each side of the body, are called pectoral and pelvic
fins. They are used for stabilizing, stopping, slowing down, or changing direction:
vertically, from the water surface to the bed, and vice versa, from side to side,
from left to right, from right to left. Fins consist of a membrane stretched on
spokes, and they can all be tucked in along the body, with the exception of the
caudal fin. The adipose fin is merely a fold of skin, without any spokes. When the
spokes are longer than the fins they are known as spiny fins, and they can represent
a danger to the aquarist, as in the case of the scorpion fish, for example.
Male fighting fish (Betta splendens).
SPECTACULAR FINS MAY BE NURTURE NOT NATURE
Some aquarium fish have fins that are very different in shape or size from those
that are found in nature. They are the result of patient breeding carried out by
aquarists over a period of years. The visual effect is guaranteed, but the fish's
behavior is sometimes altered, especially its velocity when moving around. Fish
with large fins in the form of sails have little more than a remote relationship
with their wild cousins, which have gone out of fashion and are no longer to be
seen in tanks. The purpose of these selections can sometimes be in doubt: they undeniably
result in highly attractive fish, but what advantage do they have over other stunning
The mucus, skin, and scales
Fishes' bodies are covered with a mucus that plays a double role: it reinforces
the hydrodynamics by "smoothing" the skin, and it affords protection against the
penetration of parasites or pathogenic elements. The latter point is extremely important,
and it explains why fish must not be moved by hand: this risks damaging the mucus
and facilitating the development of certain diseases.
Contrary to a widely held belief, the scales do not stick out of the body but
are an integral part of the skin, and they are visible through a fine layer of transparent
epidermis. When a scale is raised, damaged, or torn off, the skin itself is equally
affected and becomes vulnerable to the action of pathogens.
Every fish has a basic coloring that can be modified. Their shiny, metallic appearance,
derived from the crystals present in the cells of the skin, varies according to
the direction of the light striking them. A fish's color is a result of the different
pigments located in the epidermis. These can change, slowly, for reproduction and
camouflage, under the control of hormones, or more quickly, for flight or aggression,
controlled by nerves. The coloring of a fish can also vary when it is suffering
from disease or nutrient deficiency.
COLORING: PROTECTION, WARNING, AND SEDUCTION
The coloring of a fish varies according to its age and mood. Some fish living
in coral reefs reject individuals of their own species or a related species with
a coloring similar to their own (Pomacanthids, also known as angelfishes, for example)
because they consider newcomers as enemies wishing to appropriate their territory
and their food supply. This is why their offspring have a very different coloring
from that of adults, so as not to be considered intruders. In their desire to protect
themselves, some fish adopt a camouflage to merge in with their surroundings, or,
in contrast, reduce the intensity of their color to pass unnoticed. Thus, the vertical
black stripes on the scalare allow it to hide among submerged branches and plants
(see drawing above).
The black bands of the scalare enable it to hide.
In some species, the male and female sport very different colorings, enabling
them to be distinguished - a gift of nature much appreciated by aquarists! This
is true of a large number of the Cichlids in the African lakes. At mating time,
the male can flaunt vivid colors, not only to seduce the female in the courting
ritual but also to impress his rivals and scare them off. This occurs with the meeki,
a Central American Cichlid - the underside of its head turns red at mating time.
The coloring of fish exists not merely to satisfy the eye;
it plays an equally important social role.
Certain fish, such as this murena, have very sharp teeth,
indicating that they are predators.
Whatever its form - conical, elongated, or stocky - the head houses some important
- first of all, there are the eyes, which have no eyelids and are highly mobile.
This mobility, coupled with their position on the side of the head, allows a
fish to command a broad field of vision - around 270°. In contrast, the clarity
of its vision is unexceptional: beyond a certain distance, it distinguishes
masses and forms rather than details. Fish are very sensitive to variations
in light - detecting low intensities of light, such as that of the moon - and
they can recognize colors.
- next comes the mouth, with a size and shape related to its feeding habits.
Carnivorous fish generally have a large mouth that can open wide and is endowed
with an array of pointed teeth, which are sometimes curved towards the back
to keep hold of their prey. Omnivorous and herbivorous fish have a smaller mouth,
with flat teeth ideally suited to grinding food.
Fish have a particularly wide field of vision
The position of the mouth can similarly reveal eating habits:
- a mouth in the upper position indicates a top-feeder;
- a mouth in the terminal position is the sign of a fish that hunts underwater;
- a mouth in the lower position indicates a bottom-feeder.
is aspirated through the fish's mouth, passes through the branchiae and is expelled
due to the movements of the operculum, which covers them. There is always some water
washing the branchiae of the fish.
Oxygen requirements are not directly proportional to the size of the fish, with
the smallest species being the greatest consumers of oxygen: ten fish weighing 1
g each consume more oxygen per gram of body weight than one fish of 10 g.
Fish that live on the bed or in dark environments (colored or turbid water) have
barbels around the mouth (Corfdoras, Botia, for example).
These appendages have a tactile and sensory role. By complementing or replacing
the eyes, they enable the fish to detect possible sources of nutrition.
The mouth of this marine fish (Forcipiger flavissimus, the
yellow longnose butterfly) allows it to capture its prey in the crevices of the
Two or four in number, these are located in front of the eyes. They play no part
in respiration but, extended inside the head by an olfactory sac, they perceive
and analyze smells.
This protects the branchiae and guarantees the circulation of water through the
regular movements of the valve, ensuring that the branchiae are always in contact
with the water from which they extract oxygen. The term "gills" sometimes incorrectly
used, refers to the opening produced by the movements of the operculum, which serves
as an exit for the water that has irrigated the branchiae.
The glass silurid detects its food partly as a result of
The blind tetra (Anoptichthys jordani) does not have any
eyes but detects its prey and enemies with its lateral line.
The lateral line
Running symmetrically along each side of the fish's body, the lateral line is
more or less visible, according to the species. It consists of a succession of pores
that communicate with a canal situated under the skin. This important organ does
not exist in any other vertebrates.
While the senses of taste and smell, highly developed in fish, allow them to
recognize a greater number of smells than humans, at very low concentrations, the
lateral line, with its special cells, detects and analyzes the vibrations of the
water and sends this information to the brain. In this way a fish can be aware of
the proximity of an enemy, of a prey... or of the approach of the aquarist. The
importance of the lateral line is apparent in the blind tetra (Anoptichthys jordani),
which never bumps into an obstacle even though it has no eyes.