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Fish can smell each other very well
The olfactory and gustatory senses of fish are very sensitive and they are vital.

Communication under water

Life under water makes high demands on the sense organs of aquarium inhabitants. The latest research has shown that e.g. the olfactory sense and the gustatory sense of fish are highly sensitive. This is a good thing, as the fish need them not only to navigate but also to communicate under water.

 "Directly behind the nostrils, fish have their olfactory bulb into which there is a constant flow of water from their surroundings," as Dr. Gabriele Gerlach, professor for biodiversity and animal evolution at the University of Oldenburg (Germany) explains. "The olfactory organ is lined with more than a million nerve ends per square centimetre. Due to this fish can even perceive scents which are only present in low homeopathic concentrations in the water."

Their fine sense of smell is useful for the swimming fish in many ways: "For one thing, scents inform the fish about their surroundings. In addition to that, they communicate by means of releasing urine, that is by releasing scents into the water. For example males can tell if there is a female ready to mate nearby. If this is the case, they will purposefully squirt urine in the direction of the female to attract attention to themselves as potential partners. By means of smell fish can also tell who is of the same family. It enables the young to stay with their families. Sexually mature fish do not mate with their siblings.

However, fish not only have an excellent olfactory sense. They can also taste. "Their gustatory sense is not restricted to the mouth," Dr. Gerlach explains. "The catfish for example has taste buds on the clearly visible barbels as well as distributed all over the skin." Other fish perceive taste with the help of extensions to their fins. Similar to the olfactory organ, the gustatory organ can also perceive minimal concentrations of molecules. For example fish can taste their food already at quite a distance and then purposefully move in the right direction.

Source: IVH (German Industrial Association for Pet Supplies)