Fish communicate between themselves in different ways. Ever since Aristotle we know that fish produce sounds. Contrary to what is observed in other vertebrates, the mechanisms used are characterised by their variety. There are two main types.The first consists of a form of sound production called stridulation. This is the result of friction between two of the body’s harder parts. For example, rubbing a pectoral fin’s joint against the pectoral girdle, or the rubbing of pharyngeal teeth against each other.

The second category of sound producing mechanism involves the swimbladder, in other words the diverticulum of the digestive tube which forms a pocket of air under the abdominal cavity's vertebra. This bladder allows the fish, which is heavier than water due to the density of its skeleton, to keep its position beneath the water’s surface without expending energy excessively. But it is also used by various species to generate sound. A well known example is that of the eel, which burps during the breeding season in order to attract females. The fish contracts the muscles found within the abdominal cavity and the air, transported into the oesophagus by a small pneumatic canal, is propelled towards the outside through the mouth. The herring, however, expels air through the anus. A team led by Wilson at the University of British Columbia has shown that the latter is indeed a communicative act and not an exaggerated tendency to flatulence!

The swimbladder also contributes to another mechanism which uses the muscles which entirely surround it. Their contractions produce different levels of pressure within the organ and these are the source of sound production. Nevertheless, the bladder has little or no resonance capacity. It has been established that the main frequency of sound emission corresponds to the frequency of muscle contraction, between 300 and 500 Hertz for the Oyster toadfish (Opsanus tau)! For its part the University of Liège’s Functional and Evolutive Morphology Lab has discovered that Pearlfish (Carapidae) produce sounds through a complex mechanism involving slow contractions (a frequency of 1 to 5 Hertz) of the muscles connected to the swimbladder.

We cannot yet classify this in any major way as it is only found in one group of fish, and it could be considered as a variant of the first mechanism.