The piranhas are bigmouths

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.

(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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