In order to verify that the protractor muscle was also behind the electric discharges, and especially in order to understand the process, it was necessary to observe the ultrastructure of samples under the electron microscope. “A typical muscle is composed of cells that are mostly covered in myofibrils, which cause the contractions. The more myofibrils there are, the greater the strength of the muscle will be. We observed that in fish that produced only sound, the myofibrils took up less place than on a typical muscle but were present in greater proportion in the cell overall”. On the other hand, the fish that is incapable of producing sound but can produce electricity had very few myofibrils.


It is therefore the protractor muscle that also produces electricity. “We also constantly produce electricity with for example each beat of our hearts”, explains Eric Parmentier. We do so at a lower voltage; the electricity is produced by the transfer of ions between the cells. For example, it is because muscles create electricity that the hammer-headed shark is able to detect prey, even when they are immobile, hiding in the sand. Its large snout is packed with electrosensitive cells. And the fish that is hidden in the sea bed continues to produce electricity with the beating of its heart despite the fact that it is not moving. The shark can then locate it”.

Several different evolutions within the same species

The study is unique in that it shows that the same muscle equipped with different ultrastructures has been able to acquire different properties. “In other fish”, explains Eric Parmentier, “such as eels or elephant-fish, we have known for a long time that electrocytes were myogenic (of muscular origin). However, these species have never revealed a similar evolutionary process to that of Synodontis, because they are all exclusively electric. There are no longer any intermediate stages existing within their groups”. 

This new knowledge comletes other research, carried out on piranha fish in this instance (2).  This study of specimens of different ages made it possible to establish that the muscles capable of producing sound were located in the back muscles. In the youngest specimens, the cells of the sound-producing muscles contain a large nucleus and few myofibrils which develop with age. At a point during growth of the muscles, the development of myofibrils is blocked and they specialize in the emission of sounds. “This observation only concerns piranhas”, admits the researcher. “But it is possible to imagine that this property extends to other species and put forward the theory that, while they are growing, Sydontis develop a sound-emitting muscle. By blocking deeper this developmental process, they create a muscle capable of emitting electricity”. This is only a theory, because such observations have not been carried out on catfish. It remains a plausible theory nonetheless.

(2) Millot S, Parmentier E (2014) Development of the ultrastructure of sonic muscles: a kind of neoteny? BMC Evolutionary Biology 14: 24