Contrary to the saying ‘dumb as a carp,’ fish actually communicate with sounds!

The proverb that says “dumb as a carp” is quite wrong… Fish are far from being voiceless and emit a whole bunch of sounds. Of course, it is not easy for our human ears to pick up them, and even less so if we are not aware of them! Fortunately, thanks to bioacoustics and the use of particular tools, we are now able to study what the fish say… And they are rather talkative! A team of researchers from the Functional and Evolutionary Morphology Laboratory at the University of Liège (Dr. Marine Banse and Prof. Eric Parmentier), particularly looked into the case of fish. Holocentridae, better known as Soldier Fish Thanks to their study, published in the journal. Plos Onewe better understand the different sounds emitted by this species and the behaviors with which they are associated.

Water, a rather noisy environment

The idea that water is a calm place, without any noise… Forget it immediately! Aquatic life, on the contrary, is very noisy: it talks down there!

Because yes, aquatic environments are full of noise, nothing to do with space, filled with emptiness, where sounds do not propagate. In fact, sound waves travel better and faster in water than in air. On the other hand, they encounter other constraints, due for example to water movements, temperature differences or even salinity. Water is a moving medium in which the propagation of sound is therefore not homogeneous.

Despite everything, this does not prevent aquatic life from expressing itself. We are talking about biophony, and this concerns both whale songs and the noises that sea urchins make while grazing on algae. Moreover, in the Mediterranean, it is these noises which represent the vast majority of the soundscape of rocky reefs.

The fish are not left out and also communicate noisily. It is currently estimated that there are nearly a thousand species of fish capable of voluntarily producing sounds. This number is probably underestimated, because many species are known to possess sound-producing mechanisms. We just haven't been able to record these yet.

Exactly, how do scientists “listen” underwater? They use what are called hydrophones. These are a kind of underwater microphone that allows you to pick up the sounds of marine life.
These instruments are part of what is called passive acoustic monitoring. This non-intrusive technique consists of capturing sounds without disturbing ecosystems. We only record existing sounds, there are no wave emissions, as with sonars for example.

Using these techniques, we can map the presence of different species, particularly fish. There is no need for human presence and scientists are not limited by night hours or weather conditions. With certain limits despite everything, particularly if there is heavy rain which can make it difficult to hear low amplitude noises.

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What do the fish say?

Fish are capable of making several types of sounds. These can take the form of grunts, clicks, squeaks or even hisses.

Fish produce sounds in different ways. Some are able to vibrate their swim bladder, an organ that provides flotation and stability, using fast muscles. Others will make chirping sounds by rubbing their teeth or bones against each other. In short, it’s a joyous cacophonous mix that takes place underwater. Moreover, it is considered that a noisy habitat is a healthy habitat. The greater the acoustic diversity, the greater the biodiversity.

All of these sounds are useful to fish. They allow them to communicate with each other. They can be used to defend their territory, but also for courtship during the mating season. Certain sounds can also be used to warn of imminent danger.

Often, sounds are specific to specific behaviors. Thus, the Dascyllus (damselfish fish), can emit 6 very distinct sounds associated with different behaviors such as mating, chasing or even fighting.
Other species will use sounds to reinforce visual communication; the same type of sound then being used in several behavioral contexts.

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Communication in soldier fish

Soldier fish are tropical fish that are rather discreet during the day.
Credits: Shutterstock/Sakis Lazarides

The new study published in the journal Plos One was particularly interested in the family of soldier fish, the holocentridae.

These are fish living mainly in coral reefs in tropical regions. They are found more than a hundred meters deep! They are rather discreet during the day, which they spend hidden in caves, their large eyes giving them effective night vision. The latter is therefore useful for going hunting after dark.

Previous studies have shown that these fish produce several sounds linked to a wide variety of behaviors. They could be sounds related to territory defense, alarm cries or alerts in the event of predators.
This resulted in the description of 5 types of sounds: beats, growls, staccatos, rumbles and bangs.

However, these descriptions were rarely accompanied by physical descriptions. It is therefore not certain that the authors systematically used the same terms to describe similar sounds. On the contrary, different terms could have been used to describe the same type of sound and vice versa.

In their new study, the researchers therefore wanted to compensate for this lack of precision and these uncertainties by revisiting and clarifying the main characteristics of the 5 types of sounds previously identified. They also wanted to clarify in which behavioral contexts the fish emitted them.

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Vocal communication which mainly reinforces visual communication

Scientists recorded the sounds produced by soldier fish in the wild and showed that the holocentrids emitted sounds during 6 different behaviors. These were accompanied by events of acceleration in swimming, pursuit of individuals of the same or different species, competition, social signaling and body tremors.

In total, the researchers recorded 1,382 sound events for all behaviors in 6 species of soldier fish.
Generally speaking, aggressive behaviors (chasing and competition) are longer and made up of more sounds. Indeed, communication can be based on a kind of “sentence” composed of several “words” (sounds).

The acoustic analysis showed that the sounds emitted by soldier fish are not linked to specific behaviors. The latter are in fact associated with a multitude of different behaviors, without any precise association. In other words, there is no sound specific to behavior.
Indeed, all types of sounds were produced in the different behaviors. This therefore suggests that these sounds would not indicate specific behavior in soldierfish, but rather serve to improve visual communication, at least during the day.

This combination of sound and visual behavior would result in a lower level of aggression compared to just making sounds. This would therefore help to reduce conflicts.
In addition, it would help fish to communicate more effectively, whether within the same species or between individuals of different species.

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