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An oyster for an amplifier
3/3/15

Nocturnal activity and its limits for the observer

While nearly all the sounds recorded were emitted at night, leading the researchers to believe that their function relates to the reproduction cycle, the specific time at which the sound is produced presents a major inconvenience. "We have no idea whatsoever what happens when they produce these sounds", the young researcher regrets. "We can’t define any associated behaviour since we can’t observe them. We were able to catch the emission of a sound during the day, from the inside of an oyster.  However, we don’t know whether they also produce sounds in open water, for instance." Another grey area is linked to a subsequent discovery concerning the morphology of these fish. The morphology of the rocker bone varies according to gender. It is smaller in females, has a different shape, and the sonic muscle isn't attached to the same points of the bony structure as in males. "The differences are very subtle but lead us to believe that the mechanism doesn’t function in exactly the same way. And yet, we didn’t record a variety of different sounds. We did notice variations in the pulses, but it was always the same type of sound". Observations that lead to two hypotheses. Either the morphological differences between the two sexes don’t have any particular effect on the characteristics of the sounds emitted, or only one of the two genders emitted sounds during the recording. In which case, the researchers will have to conduct further missions to collect other types of sounds.

"Now we have to carry out further investigations concerning behaviour and sound production. But to achieve this, we have to be able to observe them." The operation may appear simple. Just put infrared cameras in position at night time, and there you have it! An easy enough system to set up in a laboratory, but far more difficult to do on an isolated atoll in the middle of the Pacific Ocean. And bringing back live fish isn’t any easier either. Only eight centimetres long and no more than one centimetre wide, Onuoxodon is far from being the most robust underwater species. It would be rather complicated to inflict such a long journey on so a fragile fish.

Makemo atoll

From the limits of the aquarium to widespread propagation in the lagoon 

The studies carried out in an aquarium did have certain advantages. In particular, it allowed the researchers to attribute the sounds emitted to the species studied without a doubt. But, above all, they demonstrated their limits. They don’t reflect the true conditions of the fishes’ environment, or the way they evolve in it. "And the aquarium poses real problems concerning resonance, since there is a constant and distorted background noise", Loïc Kéver points out. "This is what encouraged Eric Parmentier to return to Makemo the year after our mission, and subsequently make a recording in the lagoon".

In particular, the operation allowed the researchers to confirm that Onuxodon emitted sounds from dusk till dawn, but not only then. The biologists also observed that the peaks recorded were followed by a second set of weaker, yet similar, peaks. By taking into account the depth at which the oysters were to be found (some twenty metres), and the speed of the sound’s propagation in the water, i.e. 1500 metres per second, they were able to deduce that the amount of time between the two recorded peaks was the time it took for the sound to reach the surface from the place where it was emitted, and return back again. "The first sound recorded therefore came directly from the fish. But the sound spreads in all directions. It doesn’t just go towards the hydrophone. When it reaches the surface, the majority of it is reflected, a bit like light off a mirror, then returns in the other direction, and is then recorded a second time. What was surprising, for so small a fish, was that the sound emitted was sufficiently powerful to cover such a distance, i.e., some forty metres."

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