Ontogenetic shifts in sound production and shared sonic mechanisms in two priacanthid fishes.

Sound production in teleost fishes relies on diverse anatomical adaptations, yet convergent mechanisms involving extrinsic sonic muscles acting on the swim bladder are widespread. This study investigates the acoustic and morphological features of two priacanthid species, Indo-Pacific glasseye Heteropriacanthus carolinus and moontail bullseye Priacanthus hamrur to explore interspecific similarities in sound production. Using recordings and anatomical analyses, we show that both species rely on a forced-response mechanism, where the contraction rate of fast sonic muscles determines the fundamental frequency. This is corroborated by the smaller diameter of sonic fibres compared to epaxial fibres in both species. Despite belonging to different genera, both species exhibit extrinsic sonic muscles originating from the first pleural rib and inserting on the anterior swim bladder. However, P. hamrur displays anterior bladder projections potentially involved in enhanced hearing, absent in H. carolinus. Acoustic signals were broadly similar between species, suggesting that the morphological shift in muscle insertion does not affect sound structure. Comparative analysis across geographically distant populations of H. carolinus (Indian and Pacific Oceans) revealed variation in acoustic features that was size-dependent rather than region-specific. Juveniles emitted continuous pulse trains with high fundamental frequencies, whereas adults produced more segmented calls with lower frequencies. These ontogenetic differences reflect developmental modulation of vocal output, not anatomical changes. Overall, our findings highlight the conserved nature of sonic mechanisms in Priacanthidae, the influence of body size on acoustic variation, and the potential role of swim bladder morphology in auditory enhancement rather than sound generation.