The first oysters collected weren’t inhabited. The scientists turned their research to passes, places where the ocean’s waters communicate with those of the lagoon. The fish return here to set up home when they reach adulthood. "That’s where we found them. Approximately 30% of oysters were infested. We collected them so we could put them in an aquarium. But the change of pressure was too accelerated and too great for the Onuxodon. They were all dead by the time we reached the surface. We had to find a gentler way of bringing them up." The oysters and their discreet landlords could finally be placed in an aquarium.

The hypothesis of a reproductive function

Waterproof microphones, known as hydrophones, were placed in the aquariums at the same time as the cameras. But no sounds were recorded in the first few days. "We attempted several manipulations, for instance, encouraging several fish to go into the same oyster, to observe their reactions." But to no avail. The Onuxodon remained silent. Up until the day when the researchers left the microphone on at nightfall. "That’s when we detected the sounds. There were even a lot of them, at certain moments. The fact that these sounds were emitted at dusk is probably related to a reproductive function or, in any case, a call to attract a mate that was outside the oyster." When sound production has this function in fish, it is not unusual for it to occur predominantly at night. Because according to the reproductive cycle, if mating takes place in the early hours, so will the egg-laying. Thus, the eggs will reach the open sea when it is dark. They will be protected from the danger of predators who use sight to hunt. This hypothesis concerning reproduction is all the more likely since several species close to Onuxodon, which are already better known, produce sounds for this purpose.

A morphology tailored to produce sound

The biological mechanism at the origin of this sound producion is quite original and bears witness to the ability of tissue to adapt when subject to certain constraints. In Onuxodon, the front of the swimbladder has been "transformed" into a mineralised structure (rocker bone). It sits at the front of this bladder a bit like a cork on a bottle of champagne. A pair of muscles link the eye’s upper orbit to the rocker bone. The contraction of the sonic muscles pull the rocker bone forwards. When the muscle relaxes, the bony part quickly draws back into the swimbladder initiating  a sound.