EXTERNAL ANATOMY

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.

Hippocampus kuda.

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.

Pterois volitans.

The fins

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 natural specimens?

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.

Coloring

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.

Euxiphipops navarchus.

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.

The head

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.

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.

Breathing

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

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 coral.

The nostrils

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

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 its barbels.

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.