Several species of catfish (Mochokidae) are capable of producing sounds and electricity, probably for communication. This has just been observed by researchers at the University of Liege. Having studied five species of these fish, the researchers have established that a single muscle which has evolved in different ways is specialized in both properties. Between two extreme cases, one where the fish produce sound and no electricity or vice versa, an intermediate group of species have kept both means of communication. This is an important study which aims to understand how certain fish have been able to discover electricity during the course of evolution.

It is public knowledge today that many fish emit sounds for communication purposes, defending their territories or as part of the mating ritual. They have also developed an efficient auditory system for communication. But sometimes, this function is not sufficient and can be rendered useless by the environment. Some species have developed other means of communication and have evolved in such a way that they can emit electric signals by means of electrocytes. Everyone has heard of electric eels, for example, but these are not the only creatures to have played at being the Benjamin Franklin of the aquatic environment.

Indeed, many species have yet to reveal their secrets with regard to this subject. Among them are certain catfish such as Synodontis. Eric Parmentier, lecturer and head of the Functional Morphology and Evolutive Morphology Unit at the University of Liege, requested Kelly Boyle, who at the time was a post-doctoral researcher in his unit, to carry out a study of these species. The result is a fascinating work on the morphological evolution of electricity-producing fish (1).

From sound to electricity

“We have been working on the emission of sounds by fish for more than ten years”, explains the researcher. “In the context of this study, we initially wanted to compare the production of sounds in certain species of  Synodontis. They presented anatomical differences and we wanted to establish how these variations could interfere in the production of sounds. Something intrigued me about one of the species. It showed all the anatomical elements necessary for the production of sounds, and yet we heard no sounds at all while the other fish were a lot more communicative. At the same time, we had read in a publication dating from the 1990s that certain catfish produced electricity, but without any further details. We decided to place electrodes in the aquarium and everything became more interesting from that point on”.  Although the fish seemed more disposed to producing sound, they actually emitted electrical discharges. Intrigued by the theory that certain catfish could possess both properties, Kelly Boyle observed a group of five species of Synodontis.

One of them emitted no sound but did emit a lot of electrical discharges. In contrast with this, another species only emitted sounds. Three of the species were capable of both properties. This was evidence of a varied evolutionary group for species of the same genus. “Being able to emit electricity is a very singular property and here we had the opportunity to discover a biological pathway that showed how this was possible”.

One muscle for two functions

Quite quickly, the young post-doctoral researcher noticed that the fish posessing both properties could not emit both of them at the same time. Either they produced a noise or they produced an electric signal but never simultaneously, as Eric Parmentier recalls. “We therefore imagined that the muscle which produced the sound could also be used for producing electricity and this proved to be the case. By blocking the activity of this muscle in some individuals, we noticed that they no longer emitted electricity”.

In Synodontis, the muscle responsible for the production of sound is the protractor muscle . This rapid muscle, which can contract and relax up to one hundred times a second is connected to a bony plate located at the front of the swim bladder. Just as in a balloon or a buoy, this bladder is an air-filled pocket  whose function is to stabilize the fish in the water.  When the muscle contracts the bladder stretches and when the musle relaxes the bladder returns to its initial shape. The rapid alternation between the cycles of contraction and relaxation produce the sounds... Not all fish produce sound in this way but it is the process that has been developed by the Synodontis studied here. 

(1) Kelly Boyle, Orphal Colleye, Eric Parmentier, Sound production to electric discharge : sonic muscle evolution in progress in Synodontis spp. catfishes (Mochokidae), Proceedings of the Royal Society, 2014.