When fish cry underwater, something happens
The senses of the fish
Underwater there is violent grunt, croak and growl. Because sounds from water rarely reach a human ear, we often assume that the underwater world is mute. Not even close.
Fish make sounds to communicate with other fish. They grind their teeth, let out bubbling water or create sounds by specifically releasing air from the swim bladder.
The gurnard, for example, generates its eponymous growl in this way. Some schooling fish communicate by "shouts", the anglerfish can even squeak like a mouse.
Fish have their ears everywhere
You can't see them, but fish have ears: small fluid-filled tubes behind the eyes that function like the inner ear of terrestrial vertebrates. Hitting sound waves set small ear stones made of lime floating in the liquid vibrating.
This movement in turn excites fine sensory cells that transmit their information to the brain. In some fish species, the inner ear is connected to the swim bladder, so that vibrations from the bladder are transmitted to the ear.
But that's not all. Fish have another highly specialized organ for sensing pressure waves from their surroundings. A "normal" ear is not enough underwater to locate the exact direction of the sound.
A person on land knows from which direction a noise is coming because the wave reaches the ear facing the sound source a little faster than the ear facing away.
However, sound waves propagate four times faster under water than in the air. The difference in time with which sound reaches the ears is no longer sufficient to locate the source. A more sensitive organ is needed.
Sensitivity and ear in one
The main sensory organ of the fish is the lateral line system: a highly sensitive sense of remote touch with which the animals can perceive vibrations, currents and sounds in the water - and their place of origin.
On the side of the fish's body under the skin there is a tube filled with mucus, which is connected to the outside world through fine pores. Externally, the side line can be seen as a fine vertical stripe.
As in the fish's ear, the fluid-filled tube contains fine hair cells that are excited by the impact of pressure waves. The swimming fish pushes a column of water in front of it. If this hits an obstacle such as a stone, a prey fish or an enemy, the pressure wave is thrown back and hits the sideline organ of the fish.
The strength and direction of the wave inform the fish about the distance, shape and size of the obstacle. With the sideline, a fish can locate the origin of the pressure wave much more precisely than with the ear.
The dense and spacious packing of the hair sensory cells allows a finer temporal resolution. The sideline organ also helps ensure that fish do not collide in the school.
Most fish are naturally nearsighted. You can only see objects up to a meter away in focus. In essence, a fisheye works like a human, but the lens is spherical and rigid. Unlike other vertebrates, it cannot change shape to focus on objects at different distances.
In order to focus their attention on objects that are further away, fish simply pull the entire lens further back into the eye with the help of a special muscle. With a more or less sharp result.
Since the visibility underwater is less than on land, it is not so important for fish to be able to adjust their eyes to very different distances. Some deep-sea fish have huge eyes to make better use of the little remaining light.
Taste the environment
In water, smells become taste. In fish, the sense of smell and taste are one and the same. Water is a soup made from thousands of dissolved chemical substances. It makes sense to use them for guidance.
The sense of taste is not just limited to the mouth. In the catfish, for example, sensory buds are distributed all over the skin. He can perceive even the smallest concentrations of fragrance molecules.
In fish there are four small nostrils near the eyes, which are closed with an inlet and outlet valve. Behind it is an odor chamber, which is lined with a cellular carpet made up of more than a million nerve endings per square centimeter. The information from the nerve cells is processed in the olfactory lobe of the brain.
In salmon, this area takes up more than half the volume of the brain. Understandable, because the migratory fish orientate themselves on their long journey with the help of the smell. The trout's sense of smell and taste, for example, is more than a million times finer than that of humans. An eel could even smell a single lump of sugar sunk in Lake Constance.
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