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The piranhas are bigmouths

2/8/12

Fish are a lot chattier than had been thought. A team of researchers at the University of Liège has for several years been studying the sounds they emit in the water. Whether it is to find a sexual partner or to see off an enemy, certain fish know how to find the right words. A recent study carried out at the University of Liège’s Functional and Evolutive Morphology laboratory has just demonstrated that piranhas sometimes start to grunt to warn that they are about to attack.

PiranhaThe last we heard, Sandie Millot’s finger was healing nicely, thank you very much. She was recently bitten by a piranha she was holding in her hand. The bite caused a deep wound which required a visit to the Liège University Hospital Centre’s casualty department. The reputation of this Amazonian fish has been rightfully earned. It is a fierce carnivore, equipped with sharp and pointed teeth which are as cutting as a butcher’s knife. In normal conditions they find what they need to sustain themselves in their natural environment: fish, insects, animal corpses, etc. But last September, provoked by the increasing scarcity of their usual food, shoals of piranhas attacked bathers at a Brazilian resort. Several dozens of people had to receive hospital treatment for bites to their feet!

So what on earth was the University of Liège researcher doing, holding this voracious animal in her hand? ‘We are studying the sounds it emits, particularly in situations in which it is stressed,’ explains her boss, Professor Eric Parmentier, a lecturer and the Director of the University of Liège’s Functional and Evolutionary Morphology laboratory. ‘The fact of being taken out of the water and placed in the hand is perceived as an act of aggression by the fish.’ How about working with protective gloves? ‘They make too much noise on contact with the scales. There is thus a risk that we might not distinguish the sounds made by the animal.’

Sounds which in this case do not come from the mouth, but from an organ situated on the fish’s back: the swimbladder. It consists of a  internal gas-filled sac, situated between the digestive system and the vertebral column, and which contributes to the control of the animal buoyancy . It is this that allowx the fish to remain at mid-levels under the surface of the water, without having to make the slightest effort to compensate for a bone density which tends to make it sink. Fish which do not have such an organ, such as the shark for example, have to move permanently to avoid drifting to the bottom. ‘We for a long time thought that the swimbladder was the key organ in sound production in fish,’ explains Eric Parmentier. ‘But it is more complicated than that, as we have just demonstrated in the study (1) published in The Journal of Experimental Biology. The swimbladder is surrounded by a muscle – the sonic muscle – which has an incredible capacity for contraction: up to 150 times per second! In reality the swimbladder just responds to this muscle contraction.’

To demonstrate this, the Liège researchers opened up the flanks of anaesthetised piranhas. They then attached a tiny reflective disc to the swimbladder and pointed a laser beam in the direction of this disc, so that the slightest movement made by the swimbladder would be recorded by the laser beam. ‘It was in this way that we could see that the movements of the swimbladder are entirely dependent on the contraction of the sonic muscle,’ concludes Eric Parmentier. In other words, contrary to what has been supposed by the scientific literature for decades, the swimbladder is not capable of resonance: a high-speed sustained muscle contraction  is required to produce sound.

The piranha warns before attacking

The second part of the study consisted of studying in what environmental conditions the piranha emits sounds. The equipment required for such a demonstration? A few piranhas (they can be obtained on the market), an aquarium, a hydrophone (in other words a waterproof microphone) and lots of...patience! The aim here was to link types of fish behaviour to characteristic sounds. After having studied at length the sounds and the images recorded, the Liège researchers identified three sounds manifestly linked to aggressive manoeuvres. The first of these manoeuvres is termed ‘frontal display.’ When two piranhas face up to each other they emit a kind of bark of intimidation. If that is not sufficient, the piranha begins to circle around its adversary and it produces a noise which can be likened to the beating of a drum. The final stage of the aggressive display consists of snapping its jaws. Contrary to the first two sounds, this one does not come from the swimbladder but from the mouth. And if all of these warning shots are not enough, then we have a real battle on our hands!

A new study under way at the Functional and Evolutive Morphology laboratory aims at studying the production of sounds according to the size of the piranhas. It was during one of these fish handling sessions that Sandie Millot had her finger gashed. ‘We also want to study the ultrastructure of the sonic muscles,’ explains Eric Parmentier.

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.

Piranha2

(1) Millot S, Vandewalle P, Parmentier E (2011). Sound production in red-bellied piranhas (Pygocentrus nattereri, Kner): an acoustical, behavioural and morphofunctional study. Journal of Experimental Biology 214 : 3613-3618. November 2011.


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