Evolutionary insights into the sound-producing mechanisms of boxfishes

Ostraciid boxfish are categorized into two subfamilies: the Pacific Ostraciinae and the Atlantic Lactophrysinae. While all species are known to be capable of sound production, the sound-producing mechanism in Atlantic species remaining unidentified. This study aimed to compare the sound-producing mechanisms of boxfish in these two subfamilies and to examine the anatomy of a species from the sister family, Aracanidae, to understand the evolution of acoustic communication in boxfish. Atlantic species feature a novel sound-producing apparatus we termed the sphaera sonica. This structure consists of two globular masses of connective tissue surrounded by fast sonic muscles with fibres oriented in various directions. Contraction of these muscles push the masses into the swim bladder at the swim bladder fenestra, which initiates sound production. In contrast, Pacific boxfish species, lack the globular mass and their muscle fibres are distinctly separated and organized into two distinct muscles that cover the swim bladder fenestra. Comparative anatomical analysis reveals that no sonic mechanism exists in Aracanidae, though muscles located at the anterior part of the body may be homologous to the sonic muscles found in Ostraciidae. Within Ostraciidae, the mechanism is considered more primitive in Atlantic species. The evolutionary transition from the Lactophrysinae system to the Ostraciinae involves the reduction of the globular mass and the development of a more sophisticated organization of sonic muscles. This adaptation likely provides improved sonic muscles control and facilitates the production of more complex signals in Ostraciinae.