Verbal attacks

More seriously, one might wonder what motors this process of transformation which begets a dominant male from an immature individual and a female from a male. ‘We still do not know the exact nature of the factor which controls this sex change,’ points out Orphal Colleye.  ‘Is it chemical, visual or acoustic? It would certainly be very valuable to carry out research on the subject.’

Another question: how is the life of small groups of clownfish attached to an anemone governed? Answer: essentially through relationships of domination and submission within a hierarchical system based on the size of the individuals. In this context, it seems, and we will come back to this, that acoustic communication could play an important role.

In 2007, Professor Éric Parmentier, Director of the ULg’s Functional and Evolutionary Morphology Laboratory, and Orphal Colleye were the first to solve the mechanism through which clownfish emit aggressive sounds when defending their territory ⁽¹⁾ (Read the article Nemo really does speak). Up until then two procedures used by fish to generate sounds had been identified. The first consists of producing sounds known as stridulation, obtained by the friction between two hard parts of the body – for example the rubbing of the joint of the pectoral fin on the pectoral girdle or that of the pharyngeal teeth against each other.

The second technique brings into play the swimbladder, a diverticulum of the digestive tract forming a pocket of air under the vertebrae of the abdominal cavity. This air allows the fish to stabilise itself beneath the surface of the ocean whilst saving its energy; but it can also, in certain species, be employed to produce sounds. In effect certain fish make their swimbladder vibrate like a drum with the help of muscles known as ‘sonic’ which are either inserted directly into it or into a connected structure. Others expel the air contained in the bladder by the mouth (the case of the eel) or by the anus (herring, etc.). One could say that the eel burps and the herring farts.

What have the researchers at the Functional and Evolutionary Morphology Laboratory demonstrated as regards Clark’s anemonefish? That it is through the mouth that it produces aggressive sounds. Linking up a hydrophone, a high speed camera (500 images per second) and an X-ray machine (small lead balls had been inserted in several bony elements of the animals’ skulls) enabled them to shed light on the system’s intimate mechanism.

When the fish 'throws’ its head backwards, inertia brings about the opening of the mouth and, simultaneously, a pulling down of the tongue. Yet, as Eric Parmentier explains, the latter is connected to the lower jaw by two ligaments, and their being stretched beyond a certain point leads to a very rapid closing of the mouth (it takes less than 10 milliseconds), and causes the jaws to snap together. The subsequent snapping together of teeth is the source of a sound with an approximate duration of 20 milliseconds.

(1) Parmentier E., Colleye O., Fine L.M., Frédérich B., Vandewalle P., Herrel A. 2007. Sound production in the clownfish Amphiprion clarkii. Science 316 : 1006.