Morphological innovations and evolutionary transitions in boxfish acoustic communication

Abstract Ostraciid boxfishes comprise two subfamilies: the Atlantic Lactophrysinae and the Pacific Ostraciinae. Although all species are reported to produce sounds, key acoustic and anatomical data remain missing, particularly for Atlantic taxa. This study compares the sound-producing mechanisms in both subfamilies and examines the anatomy of a representative from the sister family Aracanidae to infer the evolutionary history of acoustic communication. Atlantic species possess a novel structure, the sphaera sonica, composed of paired globular connective masses surrounded by fast-contracting sonic muscles with multidirectional fibres. Muscle contraction displaces these masses against the swim bladder fenestra, generating sound. In contrast, Pacific species lack this mass; their sonic muscles are divided into distinct extrinsic and intrinsic layers positioned around the fenestra. Aracanidae show no sonic system, but anterior body muscles might be homologous to those found in boxfishes. These observations suggest that the Lactophrysinae system represents a more ancestral configuration. The transition to the Ostraciinae condition involves reduction of the globular mass and increased muscular differentiation, probably enhancing control over bladder deformation and enabling more complex acoustic signalling. This study highlights how anatomical innovations in muscle arrangement have contributed to the diversification of acoustic mechanisms within the Ostraciidae.