In fish

There are times when ft seems that fish have a sixth sense, especially when a net is put into the aquarium! In fact, they analyze the situation with their lateral line and their sense of smell and in this way they deduce whether there is a possibility of danger. This premonition of a danger or a particular event seems to be heightened in those fish that are normally the most friendly. It is therefore perhaps better to talk of hypersensitivity in these species.

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 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".

• The mouth of this marine fish (Forcipiger flavissimus, the yellow longnose butterfly) allows it to capture its prey in the crevices of thecoral.

Theglass silurid detects its food partly as a result of its barbels. •

Aquarium Guide

• The blind tetra (Anoptichthys jordani) does not have any eyes but detects its prey and enemies with its lateral line.

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

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.

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