Aquarium fish Anatomy & Biology
There are more than 30,000 species of fish, more or less evenly distributed
between fresh water and sea water, and of these some 1,500 are of interest to
the aquarist. Fish embody a great anatomical and biological diversity and
richness, and this can be clearly seen in aquariums.
Whatever
type of aquarium you choose, a minimal knowledge of the anatomy and biology of
the species you are raising is an essential prerequisite. The information
below, presented in layman's language, allows you to keep your fish in good
health, in the best possible conditions, to feed them appropriately so that
they can grow, and to facilitate their reproduction — in short, to understand
them better in order to take better care of them.
External Anatomy of a fish
The
body
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.
The
fins of a fish
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. 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.
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.
The
mucus, skin, and scales of a fish
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.
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
natural specimens?
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.
Coloring
of a fish
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 OF A FISH
The coloring of a fish varies
according to its age and mood. Some fish living in coral reefs
Thus, the vertical black stripes on
the scalare allow it to hide among submerged branches and plants (see drawing
above). 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
head of a fish
Whatever
its form - conical, elongated, or stocky - the head houses some important
organs: - 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. 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.
How
fish breathe
Water
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.
The
barbels of a fish
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
nostrils of a fish
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.
The
operculum of a fish
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
lateral line of a fish
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 (see box, p. 47). 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.
Internal
Anatomy of a fish
The sum
of the internal organs accounts for roughly 50 to 60% of the body weight in a
classically shaped fish.
The
brain of a fish
This is
fairly simple in fish, when compared to other more evolved animals. The parts
corresponding to sight and smell are particularly well developed, demonstrating
the importance of these two senses.
The
fish skeleton
Obviously,
this supports the fish's body, but it is less sturdy than that of a land
animal, as a fish, partially freed from gravity, is "carried" by the
water. Nevertheless, the relative fragility of the skeleton is a handicap and
it is not uncommon to find fry that emerge from their egg "twisted".
The
fish respiratory and circulatory system
This
system is highly distinctive. The blood loaded with carbon dioxide is pumped by
the heart to the branchiae, where it is oxygenated. Nature has provided fish
with eight branchiae (four on each side), each made up of two leaves. The total
surface area of these essential organs, when spread out, would be nearly equal
to that of the fish's body.
The
vivid red color of the branchiae is due to their abundant irrigation of blood;
a darker color is a symptom of a respiratory problem. The branchiae are fragile
organs, susceptible to damage from suspended sediment or parasites, resulting
in a reduced intake of oxygen, with all its unfortunate consequences. After
traveling through the branchiae, the vivid red blood, rich in oxygen, irrigates
the body, and the oxygen goes on to break down the foodstuffs I the organs.
The
fish in the Belontiid family, which live in water in which the oxygen is
sometimes rarefied, have a special organ for trapping atmospheric oxygen.
The
fish digestive system
This
has no special characteristics, apart from the fact that the stomach can
stretch to hold large prey, especially in carnivorous fish. This is where
digestion starts, and it then continues in the intestine. With large prey the
process can last several days, but in an aquarium, with artificial food, it
will not take more than a few hours.
The
fish excretory system
This
allows undigested matter to be evacuated through the anus in the form of
excrement, or feces. The urine is formed
in the
kidneys, situated under the spinal column; it is evacuated through the urinary
pore. It is worth mentioning that fish also excrete nitrogenous
substances via the branchiae. All excreted substances contain nitrogen and are
toxic for animals, but in a well-balanced aquarium they are eventually
converted into nitrates and thus because no harm.
DO
FISH RECOGNIZE THEIR OWNER?
Countless aquarists have noticed that
some of their fishes react more enthusiastically to their presence than to that
of strangers. If they do "recognize" the person who looks after them,
how do they do it? They are capable of distinguishing the special
characteristics of the vibrations caused by the footsteps of such and such
person, which are transmitted to the water of the aquarium. Not only has that,
their sense of sight, although not perfect, helped them in this task of
"recognition."
Which
of the two recognizes the other first? •
The
swim bladder of a fish
Fish
have a swim bladder, also known as an air bladder. This is an organ connected
to the digestive system, which fills up with gas and helps fish to regulate
their flotation when moving between two different types of water. They empty it
to dive and fill it up when they need to come nearer the surface.
Bottom-dwelling fish generally have asmaller swim bladder, or none at all, as
they rarely swim in open water.
The
reproductive organs of a fish
Males
have two testicles that are linked to the vas deferens. While females have ovaries
extended by the oviducts. In both cases the sexual products - the spermatozoa and
ova - are expelled via the genital orifice. As the fertilization of the eggs is
external and takes place in the water, there are no organs for coupling and
fertilization, except in the case of livebearers.
DO FISH
SLEEP?
When an aquarium is abruptly switched
on in the morning, it is noticeable that its occupants do not immediately
resume their normal activity. Some of them are on the bottom of the tank, some
in the plants, and others remain almost immobile in the water. It is difficult
to speak of sleep in the generally accepted sense of the word, but it is
certain that fish have periods of restfulness, of varying degrees. This can be
verified at night, with the aid of a small flashlight: the fish are practically
stationary (apart from nocturnal species), but their eyes are not closed as
they have no eyelids.
DO FISH FEEL
PHYSICAL PAIN?
The sensation of pain is sent to the
brain via the sensory nerves. As fish are endowed with the latter, it can be
assumed that they feel pain when they are hit or wounded, and perhaps even when
they are sick.
FISH
BIOLOGY
How
do fish swim?
It is
essentially the rear part of the body, particularly the caudal fin, which
serves to propel the fish, while the other fins play a stabilizing and steering
role. Of course, the more hydrodynamic a fish's form, the more it is capable of
setting off abruptly and swimming quickly, indispensable for catching prey or
fleeing an enemy. Aquarists are sometimes advised not to let quick and lively
fish (like Barbs) cohabit with slower and more placid species (like loaches),
as the latter may be frustrated in their attempts to eat the food provided by
their owner.
General
Fish Behavior in Aquarium
Fishes'
behavior in an aquarium reflects their lifestyle in a natural habitat, albeit
modified by the fact that they are living in a more cramped environment, coming
into contact with other species more quickly and easily. Fish from the same
species can behave differently from one aquarium to another, according to th
capacity and the other occupants.
• Fish Territorial
behavior
When
fish are in their original biotope, their territorial behavior is reproduced in
captivity, and is sometimes even intensified. A territory is a living space –
either permanent or temporary (as in the reproduction period) - with an
extension proportional to the size of the fish. Its occupant rebuffs
individuals from the same species, from related species, or even from totally
different ones. The surface area must be sufficient for the fish to find
refuge, foodstuffs, and fish of the opposite sex with which to reproduce. With
some fish, particularly marine species, it is important to plan a territory in
the aquarium that will provide shelters and hideaways.
• Fish Group
behavior
Strength
is to be found in unity, and living in a group permits a better defense against
enemies. Indeed, from a distance a group or school of fish takes on the
surprising and intimidating an enemy.
Group
life also facilitates reproduction, as an individual has a greater chance of
finding a fish of the opposite sex. A group's unity and organization are
governed by a series of signals which are invisible to human eyes: the use of
the lateral line, for example, prevents fish from colliding with each other.
• Fish Dominance
behavior
The
biggest members of a species dominate the smallest ones: when the latter get bigger
they are ejected from the territory. Dominance behavior has practical and social
implications, as the dominator will have priority in food and the choice of a fish
of the opposite sex. At the bottom of the social ladder, the most dominated
fish is permanently subject to aggression and harassment and has to hide most
of the time, with its growth being prejudiced as a result. This is the case
with some species of African Cichlids.
• Prey-predator relationships of fish
Some
fish feed on other smaller ones in a natural habitat, giving rise to
incompatibilities in an aquarium: take care, for example, not to let South
American Cichlids cohabit with Characins.
• Aggression
Sometimes
an aquarium is a stage for aggression between different species. This
aggression is always justified, as it is related to the defense of territory or
offspring. It is a problem of space – these phenomena are rarely seen in big
aquariums. However, a new fish introduced into a tank will often be considered
as an intruder, or prey, and will be harassed.
Fish
Growth and longevity
Unlike
human beings, fish continue to grow throughout their life, quickly at first,
and then more slowly with age. The size of fish in aquariums is mostly smaller
than that found in the wild, undoubtedly as a result of the restricted living
space at their disposal. This can easily be put to the test: an individual
whose size has seemingly stabilized starts to grow if it is put into a bigger
tank.
As
regards longevity, this varies according to the species: a year, more or less,
for the small species, and two to five years for the majority of fish. Some
patriarchs live to the ripe old age of ten or more - these are large fish,
particularly marine species. It is very difficult to postulate an optimal life
span for a given species in captivity, as environmental conditions introduce
too many variable factors.
In
nature, the biggest fish often feed on smaller ones – obviously something to
avoid in an aquarium.
HOW
TO REDUCE ACTS OF AGGRESSION
IN
AN AQUARIUM?
Only put together those
fish which are known to be compatible, particularly in the case of marine fish,
and give them as much space as possible. Be sure to provide a number of nooks
and crannies, appropriate for the dimensions of the residents. Another solution
is to mix species with different lifestyles – for example, free swimmers (like
Barbs and Danios) and bottom dwellers (like loaches) - that will not compete
with each other.
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