The sea is not always as silent as you might think. A great deal of fish chatter goes on beneath the waves on coral reefs. And when you choose to make your home in a clam or pearl shell, acoustic communication becomes even more imperative. Loic Kéver and colleagues explain that four species of Carapidae take up lodgings in the shells of molluscs and one member of the family, Onuxodon fowleri, makes its home in the shells of black-lip pearl oysters. Explaining that most Carapidae are capable of producing sound by vibrating their swimbladders, Kéver and his colleagues were curious to find out whether Onuxodon could also produce sound and if so, could the sounds be heard beyond the fish's shell homes (p. 4283).
Kéver recalls that finding the elusive animals was not easy. ‘Eric Parmentier had tried to find them in many islands,’ says Kéver. However, Franck Lerouvreur eventually succeeded in locating the fish at the remote Makemo Island atoll in French Polynesia, where 70% of the oyster shells boasted pearlfish lodgers. And Kéver recalls that the diving conditions on the atoll were idyllic. ‘There is only one small scuba club, which means that we were always alone, except for a few locals‘, Kéver says with a smile. However, there were some drawbacks to the remote location, ‘There was limited infrastructure and electricity,’ Kéver explains. And he adds that bringing the tiny fish to the surface was also risky because of the pressure difference: it took almost an hour to bring the oysters and their fishy lodgers to the surface at the end of each dive session.
Back on land, Kéver, Parmentier, Lerouvreur, Orphal Colleye and David Lecchini transferred the oysters with their fish into tanks to record the fish's sounds from outside of their oyster homes after dark. Analysing the structure of the calls, the team found that the each sound could last as long as 3 s and comprised trains of up to 40 broadband pulses that were dominated by three frequencies (212 Hz, 520 Hz and 787 Hz). Also, when Marco Lugli tested the shells' acoustics, they found two frequency bands (250 Hz and 500 Hz) that were amplified within the shells – possibly for communication with other pearlfish residents – while frequencies around 1000 Hz were amplified inside and out of the shells. ‘Amplification probably improves the efficiency of communication by increasing the propagation distance of the sounds’, says Kéver.
Having identified the main features of the fish's distinctive calls, the team brought some of the animals back to Europe to learn more about their unique sound-production system. After Kéver dissected the fish to begin learning about their anatomy, Anthony Herrel used high-resolution CT scans to reveal their unique sound-production system. Kéver describes the structures, saying, ‘The rostral [front] end of the swimbladder forms a mineralised structure – called the rocker bone – on which insert the primary sonic muscles’, adding that four of the vertebrae near to the swim bladder are also modified. Speculating that the rocker bone provides a solid anchor for the sonic muscles that vibrate the swimbladder, Kéver says, ‘it is quite exceptional to see that soft tissue can be hardened when subject to certain constraints’. He also points out that there are differences between the male's and female's rocker bones. ‘[They] should allow the emission of two different signals and thus the recognition of the sex of the emitter’ says Kéver, but points out that this is yet to be confirmed.