No one is quite sure how many different species of cichlid there are, but the great lakes in the African Rift Valley have some of the highest numbers of cichlid species in the world. How these fish have evolved into so many distinct species in the same waters has intrigued scientists for decades. Eric Parmentier from the University of Liège, Belgium, explains that two major forces had been thought to drive this dramatic example of evolution:modification of the jaw in response to different feeding environments and diets; and sexual selection where females choose males with specific colourings and patterns. However, a third evolutionary force could be at work:sound production. Parmentier explains that cichlids often make sounds during courtship and in combat, and that individual cichlid species could have evolved different sound production mechanisms to distinguish themselves from other species. But to discover whether or not sound production is a true evolutionary force, Parmentier and his colleagues had to find how cichlids make sounds (p. 3395).
Analysing the acoustics of the sounds produced by the Nile tilapia(Oreochromis niloticus), Parmentier and his student Nicolas Longrie quickly ruled out the possibility that the sounds were being generated by the fish grinding their teeth or as a result of the fish's swimming action. And when the team stimulated the fish to make the sound, they were able to change the sound's intensity by slightly deflating the fish's swim bladder. They realised that the swim bladder was involved in producing the sounds, but it wasn't clear how. Parmentier decided to take a closer look at the way the fish move to see if they could find out more.
Teaming up Quentin Mauguit, Parmentier and Longrie encouraged the fish to make the sounds naturally by placing a male and a female in each half of a gravel-bottomed tank divided by a transparent partition; the male could scoop out a nest without being disturbed by the female. Next Longrie introduced an intruder into the nesting male's side of the tank, and began recording and filming the nest builder's warning buzzes. Using high-speed cameras(250–500 frames s–1) to visualise movements that would not be clear to the naked eye, the team could see that the fish's pectoral fins rotated backwards as they made the sound. Could this fast fin movement contribute to a sound produced by the swim bladder?
Knowing that the bones of the pectoral and pelvic girdle are linked to the moving fins, Parmentier took a look inside the buzzing fish with X-rays. Collaborating with Sam Van Wassenbergh from the University of Antwerp, Longrie filmed the skeleton's movements with X-rays as the fish sounded off. Correlating the movements with different phases of the buzz profile, the team could see that during the first phase of a buzz the pelvic girdle and scapular bones moved backwards while one of the bones at the base of the anal fin moved forward, crunching the ribs together.
Suspecting that muscles in the fish's body were contracting to crunch the ribs against the swim bladder and generate the sound, Parmentier teamed up with Pierre Vandewalle and identified a muscle, which they named the vesica longitudinalis, that could pull the pelvic girdle and scapula backwards to compress the ribs and swim bladder to generate the sound.
Parmentier is quick to add that his rib-crunching hypothesis is currently just a theory. `The hardest thing is to find a way to definitively cut this muscle to prove the mechanism,' he says. But he is optimistic that he will eventually find a way to prove the theory and learn more about the role of sound production in cichlid evolution.
