In the vast and diverse world of marine life, communication methods have long intrigued scientists. Recent groundbreaking research conducted by an international team led by Professor Éric Parmentier from the University of Liège has shed light on a previously overlooked but crucial mode of interaction among boxfishes—acoustic communication. This study not only reveals the intricate sound-producing mechanisms of these uniquely armored fish but also provides profound insights into their evolutionary history, underscoring sound as a pivotal driver in their adaptation and diversification.
Boxfishes, belonging to the family Ostraciidae, are easily distinguished by their unmistakable angular, box-like shape—a form created by the fusion of bony plates into a rigid carapace. This armor serves as an effective defense mechanism against predators. However, this skeletal innovation restricts body flexibility, contradicting the locomotor strategy of most fishes that rely on body undulation for propulsion and maneuvering. Instead, boxfishes employ an unconventional swimming mode, utilizing exclusively their pectoral, dorsal, and anal fins to navigate aquatic environments with a precise, drone-like gliding motion. This unique locomotion not only defines their physical behavior but also intersects intriguingly with their sound communication adaptations.
Historically, studies on fish evolution have emphasized visual features, such as coloration patterns, for species recognition and sexual selection. This focus, prevalent in research on groups like cichlids and clownfishes, has led to acoustic signals being marginalized. Professor Parmentier highlights that many fishes, including boxfishes, rely heavily on sound in social interactions ranging from courtship to territorial defense. The novel investigation into boxfish acoustic communication sought to elucidate the anatomical and functional mechanisms underpinning these vocalizations, filling significant gaps in marine bioacoustics.
Boxfishes are taxonomically divided into two main subfamilies: Ostraciinae, native exclusively to the Indo-Pacific region, and Lactophrysinae, found in the Atlantic Ocean. All species within Ostraciidae are capable of producing sounds, yet their acoustic systems vary remarkably between these geographical clades. The research team uncovered a novel sonic apparatus in Atlantic boxfishes, named the sphaera sonica. This apparatus comprises two spherical masses of connective tissue enveloped by fast-contracting sonic muscles. These muscles rhythmically contract and relax, causing the spheres to oscillate within the swim bladder, generating distinctive sound waves akin to pool wave generators. This discovery contrasts sharply with Indo-Pacific species that lack such globular structures, instead possessing strategically arranged sonic muscles that directly interact with their swim bladders in a more specialized design.
The evolutionary implications of these findings are profound. The presence of the sphaera sonica in Atlantic species suggests they are relictual forms, representing an ancestral state within the Ostraciidae lineage. Meanwhile, species from the Indo-Pacific have progressed toward more refined sonic mechanisms, possibly indicating an adaptive radiation driven by socio-environmental pressures demanding enhanced communication efficacy. This divergence exemplifies how anatomical and functional modifications in sound production can influence evolutionary trajectories among fishes.
Further anatomical comparisons between the Ostraciidae and their close relatives, the Aracanidae, revealed a fascinating divergence linked to their armor rigidity. Unlike the heavily armored Ostraciidae, Aracanidae possess a more flexible external carapace and lack a discernible sound-producing mechanism. Investigation into muscle homologues demonstrated that sonic muscles in Ostraciidae likely evolved from muscles originally used for undulatory swimming in Aracanidae ancestors. The ossified carapace in Ostraciidae restricted these muscles from fulfilling their locomotor function, prompting evolutionary repurposing into sonic muscles for producing communication sounds. This transition highlights a remarkable case of functional exaptation, where structures are co-opted for new uses in response to changing selective pressures.
The evolutionary narrative of boxfish sonic systems encapsulates a remarkable shift: from mute, undulatory swimmers to vocally active species employing increasingly sophisticated sound production mechanisms. This transformation underscores the dynamic interplay between morphological innovation and behavioral adaptation. It challenges traditional views that primarily consider visual traits and posits acoustic communication as a key ecological and evolutionary driver within aquatic ecosystems.
Additionally, the study’s implications extend beyond biological curiosity into applied sciences. The unique swimming mode of boxfishes, relying on fin propulsion rather than body flexibility, has inspired advancements in underwater robotics and maneuverable submersible design. Understanding the biomechanics of boxfish locomotion can inform engineering solutions, enhancing the capacity of robotic systems to navigate complex marine terrains with precision and flexibility reminiscent of these natural swimmers.
Acoustic communication in boxfishes plays critical roles in reproduction, territoriality, and social cohesion. Distinct sound patterns, described as “hums” and “clicks,” vary between species and are integral to their behavioral repertoires. Oscillogram analyses reveal that hums in species such as Ostracion solorensis and Ostracion meleagris consist of bursts of peaks, while clicks involve either single or multiple pulses depending on the species. These acoustic signatures enable species-specific recognition and possibly mate selection, contributing to reproductive isolation and speciation processes.
The discovery of specialized sound mechanisms and their evolutionary origins invites a reevaluation of fish communication paradigms. Rather than relegating acoustic signals to secondary status, this research advocates for the inclusion of sound as a central factor in fish behavioral ecology and evolution. It proposes that the evolutionary history of fishes might be more comprehensively understood by integrating acoustic data with anatomical and genetic studies, opening new avenues for research in marine biology.
Taken together, this integrative approach unveils the hidden world of fish vocalization, exemplified by the boxfishes’ unique adaptation. It highlights how morphological constraints imposed by evolutionary innovations such as bony armor can drive the emergence of novel communication strategies. The study thereby enriches the broader narrative of evolutionary biology, showcasing the complexity and versatility of life in the oceans.
This research, published in the Biological Journal of the Linnean Society, epitomizes the synthesis of functional morphology, evolutionary biology, and bioacoustics, offering a transformative perspective on how fishes communicate and evolve. It stands as a testament to the value of interdisciplinary studies in uncovering the intricacies of the natural world, fostering a deeper appreciation of the evolutionary forces that shape biodiversity beneath the waves.
Subject of Research: Evolutionary biology and functional morphology of acoustic communication in boxfishes (family Ostraciidae)
Article Title: Morphological innovations and evolutionary transitions in boxfish acoustic communication
News Publication Date: 2-Oct-2025
Web References: http://dx.doi.org/10.1093/biolinnean/blaf079
Image Credits: University of Liège / E. Parmentier
Keywords: Boxfish, Ostraciidae, Acoustic communication, Evolution, Functional morphology, Sonic muscles, Swim bladder, Sphaera sonica, Fish bioacoustics, Locomotion, Evolutionary innovation, Marine biology
