Squeaker catfishes which produce audible sounds by wiggling their pectoral fins generate noise in the same sort of way that sound is generated when brake shoes press against car wheels, say scientists.
The sound production process, which is known as stridulation, is common among Synodontis and other members of the Mochokidae family of catfish and is achieved via rapid jerky movements of the stout pectoral fin spines.
New research by scientists from the University of Liege in Belgium has shown that the sound is produced when ridges on the top of the spine rub which results in a croaking sound analagous to the noise made when a brake shoe presses against a car wheel.
Synodontis, which are known as squeakers due to their use of stridulation, often make noise during territorial disputes and frequently croak when captured or removed from the water.
According to high-speed video footage produced in the study, each Synodontis call consists of a series of pulses which occur during a sweep of the spine.
The individual sweep of the mochokids' spine is made up of a series of jerky movements achieved through contractions of key muscles around the spine which pull and press it against the spinal fossa.
Ridges in the spines rub against the spinal process with each jerky movement resulting in a squeaking noise which can be clearly heard by other fishes or predators.
For more information see the paper: Parmentier E, Fabri G, Kaatz I, Decloux N, Planes S, Vandewalle P (2010) - Functional study of the pectoral spine stridulation mechanism in different mochokid catfishes. J Exp Biol. 2010 Apr ; 213(Pt 7): 1107-14.