And do the piranhas also make sounds in order to mate? ‘Certainly,’ estimates Eric Parmentier, ‘but that has not been studied yet.’ Mating calls are well known amongst certain fish. Written works dating back to the seventeenth century already described certain fishermen with their ears glued to the hull of their boat to listen to the ‘singing’ of the meagres which had come to reproduce in the mouth of the Rhône river. That being said, the more research advances the more it reveals the great diversity of this underwater communication. But it is impossible to draw general conclusions from this study of a single species. Certain fish seem not to have mating calls, such as the clownfish for example. ‘Maybe it doesn’t need one,’ observes E. Parmentier, ‘given its social organisation: a single female lives in a sea anemone, surrounded by a cohort of males whose descending size gives the order of precedence in terms of inseminating the ‘queen mother.’ The sounds made by the males serve to defend their position rather than courtship.’ On the other hand the clownfish well and truly has a war cry, yelled to all and sundry when the territory is under threat (read the article Nemo really does speak). Other fish have a very wide panoply of sound. The Dascyllus flavicaudus, a damsel fish, produces at least six different sounds, of which several are used for the reproductive function: one to attract the female, another to synchronise the emission of gametes in the water (which increases the chances of insemination), etc.

Fish also have ears

And then there is an entire section of underwater communication which demands more studies: receiving the message. How do fish hear? The Functional and Evolutionary Morphology laboratory has just equipped itself with an anechoic chamber to carry out research in this area. It consists of a small room which is completely soundproofed and isolated from vibrations in which is installed an aquarium fitted with a loud speaker, linked to a computer and an amplifier which emits sounds and different frequencies and amplitudes.  The researchers graft an electrode between the skin and the skull of the fish in order to record the electrical activity of its brain. It is thus possible to draw up an audiogram of the fish and to discover from what sound levels its brain reacts. The equipment, of which it is a rare example in Europe, will at first respond to fundamental research questions. ‘One of our researchers,’ explains E. Parmentier, ‘wants to understand how fish which are swept away by the sea kilometres from their native coral reef, whilst they are still in an egg and larvae state, are capable of finding their way back later, when they are adults. It is perhaps the noise of the reef which enables them to get their directional bearings.’ To get to the bottom of the matter the researcher, Laetitia Berten, went to record measurements in a French research station in Tahiti. From her boat she immersed her microphone in the water whilst progressively moving further away from the reef, as far as a couple of kilometres, to record the sounds of the reef. The  second part of the study will consist of drawing up an audiogram of certain fish typical of the milieu to verify if they are capable of hearing the sounds of the reef, and above all up to what distances?

The ULg researchers would also like to study the impact on fish of the noise pollution caused by human activity. Are they capable of hearing the noises of ships, drilling platforms, offshore wind turbines, etc. And if it is the case, what is the impact of this pollution on their health and behaviour? Another research project involves medicine. It is known that certain antibiotics have a tendency to destroy acoustic cilial cells, which are necessary for the reception and transmission of auditory messages to the brain. Studying the effect of these antibiotics on the auditory system of fish is valuable as, unlike mammals, they have the capacity to regenerate their cilial cells. A treatment for deafness might well be found in the oceans’ depths.