Males of a tiny cyprinid species have specialised muscles and bones in the pectoral regions which are believed to help the males make croaking sounds.
Kevin Conway and Ralf Britz studied the miniature cyprinid Sundadanio axelrodi, and found that it exhibited extreme sexual dimorphism in the skeleton and musculature of the fifth rib, the pectoral girdle and a structure known as the Weberian apparatus.
Conway and Britz, who report their findings in the latest edition of the Journal of Fish Biology, believe that the modifications are responsible for the croaking sound that appears to be restricted to males of the species, as the structures are absent from females.
The structures are believed to be the first example of such dimorphism yet recorded in a species of teleost fish.
Read how to keep Sundadanio axelrodi in this article from Interesting Imports from 2004.
Conway and Britz wrote: "Male cyprinids are known to exhibit sexual differences in both colour and tuberculation pattern, but to date few have been reported to show skeletal dimorphisms.
"Sundadanio axelrodi not only exhibits the widespread dichromatism of males and females but also an extreme sexual dimorphism of the pectoral girdle, the axial skeleton and its associated musculature.
"Such features have not been reported, as far as is known, anywhere among teleosts and thus represent autapomorphies, or uniquely derived features of the genus Sundadanio."
The Weberian apparatus, a series of vertebrae that connect the swimbladder to the inner ear of Ostariophysan fishes, allow fish to detect pressure changes and sound and are often associated with sound production.
The authors wrote: "Based on sexual dimorphism present in S. axelrodi it is hypothesised that sound production is restricted to males of the species and may play a role during courtship and display.
"This hypothesis is supported somewhat by the observations of an aquarist who reported that croaking noises, audible outside the aquarium are only heard during antagonistic male-male display.
"The sound may be produced by two different mechanisms: by rubbing the enlarged fifth pectoral fin ray against the modified cleithrum (stridulation), as has been reported for some catfishes, or the bulbous muscle may function as a drumming muscle."
The tiny fish, which reaches around 2.2cm, was previously considered a member of the Rasbora genus, but has since been identified as a sister group to the miniature Paedocypris genus, which was first described last year.
For more information see the paper: Conway KW and R Britz (2007) - Sexual dimorphism of the Weberian apparatus and pectoral girdle in Sundadanio axelrodi (Ostariophysi: Cyprinidae). Journal of Fish Biology (2007) 71, 1562-1570.