Thursday, March 5, 2015

Aquarium: Fish (Anatomy and Biology)

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.


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.


The coloring of a fish varies according to its age and mood. Some fish living in coral reefs
reject  individuals of their own species ora 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). 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.

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.


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.


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.


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.

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.


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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