During the reproductive season, many fish species will use sound to communicate with one another, sending out calls to warn off rivals and serenade potential suitors. Although it may seem surprising to think of singing fish, it is actually quite common amongst fishes, as Eric Parmentier from the Université de Liège, Belgium, points out: ‘When studies began on fish sound production, scientists thought they were lucky when they found a fish that was able to make sounds, but in the future I think we will be lucky if we find a fish that is not able to make sound.’ Parmentier explains that, in fact, fishes have evolved many different ways to produce sound, from grinding their teeth to drumming their swim bladders, and he even jokes that this huge diversity keeps him in his research job. In his latest study, he turns his attention to the Gobiidae family, explaining that gobies are well known for their mating melodies but that the mechanism has remained elusive (p. 3189).
In the late winter of 2010 and 2011, with the help of several lab members, Parmentier travelled to Brittany, France, to capture the rock goby, Gobius paganellus. Back in the lab, the team selected aggressive males and introduced them to another fish. They filmed the encounters at 200 frames s−1 and captured any sounds using a hydrophone. Looking over the videos, Parmentier and his colleagues saw that the gobies seemed to be nodding their heads in time with their pulsed calls. ‘We were really happy’, recalls Parmentier, ‘because we thought the mechanism was at the level of the head’.
So how could this nodding produce sound? ‘We thought that the fish was able to take water into the mouth and [by nodding the head] change the pressure in the buccal cavity, helping the water to vibrate the bones’, explains Parmentier. To test their idea, the team cut part of the flap that covers the gills, which allowed water to leak out of the mouth cavity. They were disappointed to find that the gobies still produced noise. What's more, Parmentier had supplemented his Brittany catch with G. paganellus gobies caught in Venice with the help of Stefano Malavasi from the University Ca'Foscari Venice, Italy. Parmentier and Malavasi found that the Italian calls were more tonal than the French calls, but more importantly the Italian gobies didn't bob their heads up and down in time with the sound – yet more evidence that the head movements were not necessary for sound production.
Back at square one, the team decided to investigate the anatomical features in more detail. During the dissections, Parmentier noticed that the gobies were morphologically very similar to the Cottidae family of fish. Parmentier explains that cottids are thought to use their pectoral girdle (a bony structure supporting their pectoral fins) to make noise. It was too risky to cut the muscles associated with the pectoral girdle without completely damaging the fish, so the team carefully inserted tiny electrodes into the associated muscle to measure its activity during sound production. ‘We saw there was a perfect correlation between the sound production and the activity of the muscle’, recalls Parmentier. Speculating on the muscle's role in the gobies' songs, Parmentier says: ‘The muscle is going to push and pull the pectoral girdle, so I suppose that friction somewhere between the girdle and the ligaments, or bones, produces the sound.’ There is still more work to be done, and it remains a mystery how the French and Italian fish produce different types of calls, but we are certainly one muscle closer to understanding sound production in gobies.