Each in their own place

In research work published in November 2012 in the journal PLoS One⁽²⁾, Orphal Colleye tackled the question of communication in clownfishes by looking into the way the interactions between the individuals living under the same ‘roof’ are governed. Carried out by an underwater camera (25 images per second) maintained in a watertight tank and linked to a hydrophone, his observations were carried out both in the field and on groups of fish taken from their natural environment in Japan, together with their anemone, and subsequently placed in an aquarium.

What did the ULg biologist discover? The presence, alongside aggressive sounds, of submissive sounds whose production mechanism nonetheless remains unknown – the animal’s jaws do not move – and whose emission goes hand in hand with a characteristic form of behaviour the specialists have termed ‘headshaking.’ ‘The subordinate fish positions itself to the side of the dominant fish and quivers its head and body, which brings about a vibratory wave starting at the head and running down the fish body,’ points out Orphal Colleye.

In his opinion the acoustic signals of aggression and submission could play an important role in the lifestyle of small clownfish groups sharing the same sea anemone. Not only do the interactions of aggression and submission prove to be extremely frequent in these groups, but it also appears that both aggressive and submissive sounds, in terms of sound frequency and duration, are related to the size of the sound emitter. The larger the fish, the longer is the sound duration and the lower is the frequency. In other words, it reflects the hierarchy in force within the group.

In principle, thus, everyone knows their place. And it is likely, according to Orphal Colleye’s analysis, that the aggressive sound signals emitted by a hierarchically superior individual towards a subordinate and the submissive sounds produced in return by the latter have the goal of avoiding confrontations which could escalate into the bodily harm of the ‘belligerents’. It all takes place as if the dominant individual was handing out orders (‘Stay in your place, I’m in charge here’) to the subordinate, which confirms its allegiance to the former (‘No problem, you’re the boss’). ‘Certain researchers have had the idea of placing individuals of the same size in the same sea anemone. This similarity of size had the effect of causing conflicts which led to one of the protagonists fleeing, or even led to death,’ Orphal Colleye informs us.

In expressions of submissive behaviour, the visual aspect is evidently important. Not only because, as we have already mentioned, the sound message is always accompanied by headshaking, but all the more so in that it can happen that this display is carried out without the emission of the slightest sound. ‘We can thus imagine that the sound signal represents a form of strengthening the submission message transmitted by headshaking’, notes the researcher.

(2) Colleye O., Parmentier E. 2012. Overview on the diversity of sounds produced by clownfishes (Pomacentridae): importance of acoustic signals in their peculiar way of life. Plos ONE 7 : 11.