The turquoise waters of the coral reefs are packed with species that still harbour many secrets. This is the case of Onuxodon, small fishes that live inside the shells of pearl oysters. An unusual and discreet shelter that makes any behavioural studies rather difficult. And yet, recent research conducted at the University of Liège has established that they emit sounds from dusk till dawn. A nocturnal activity that complicates observations, but has led scientists to assume that this is a call for a mate during reproduction cycles. One convenient yet surprising particularity seems to be the role of the amplifier, played by the oyster shells.

There’s no need to dive into the oceans’ murky depths to reveal the mysteries of the underwater world. Take Eric Parmentier and Loïc Kéver, from the Functional and Evolutionary Morphology Laboratory at the University of Liège, for instance; they stopped at the coral reefs of French Polynesia. These tropical waters are shared by a good many species, with some of the shyer ones remaining very discreet. This is the case of Onuxodon which belong to a group of fish more commonly known as pearlfish. "They were given this name because certain remains of dead fish were found on the oyster shell valves, completely surrounded by mother-of-pearl", Eric Parmentier tells us. "But not all species from this group live in oysters. Other species from the Carapidae family are also capable of penetrating and living inside echinoderm hosts such as sea cucumbers or starfish. Other ones have a free lifestyle and don’t depend on a host."

Besides this particular behaviour, we know that certain species of Carapidae leave their refuge at night to feed, while others display parasitic behaviour and eat their host’s genitals. In the case of Onuxodon, and many others, what they eat remains a mystery. They seem capable of eating small worms (annelids) and small crustaceans. If the prey is found inside the oyster, the behaviour would be of mutual benefit (the fish cleans the host, the bivalve protects the fish). However, we can’t exclude the possibility that the fish feeds off the host’s tissues, thus making it a parasite. Or, if the fish hunts its prey outside the host, this can be seen as commensalism (the oyster protects the fish but doesn't get anything in return). A study in progress should provide more information, by comparing the carbon and nitrogen isotopes of the fishes and the oysters. However, in this particular study, the two scientists focused on another characteristic of this delicate fish: its ability to produce sound.    

Extensive underwater research

Eric Parmentier had the chance to study the morphology of Onuxodon and conclude that these fish must be capable of producing sounds. This idea was further reinforced by the recording of species found in sea cucumbers. In addition to several missions dedicated to other research in French Polynesia, he went looking for these fish in their natural environment, only to return empty-handed for several years. "One day", Loïc Kéver remembers, "the head of diving at a centre Eric Parmentier knew well, left on a mission in the atolls of the Tuamotu Archipelago.  During a dive in the Makemo atoll, he collected several pearl oysters. When they were opened, he discovered Onuxodon and sent them to us. We studied and identified them, and we went on a mission in November and December 2011."

(1) Loïc Kéver, Orphal Colleye, Marco Lugli, David Lecchini, Franck Lerouvreur, Anthony Herrel, Eric Parmentier, Sound production in Onuxodon fowleri (Carapidae) and its amplification by the host shell, The Company of Biologists, 2014