Caracterização citoarquitetônica do córtex auditivo primário e secundário de Golfinho-Clímene (Stenella clymene): a quimera acústica do Atlântico

Abstract

Hearing is a fundamental sense for odontocetes, essential for communication and environmental perception, and is reflected in neuroanatomical adaptations according to each species' habits. Despite its importance, the neurological study of cetaceans faces challenges due to limited access to samples and the rarity of species, leading to gaps and controversies in the literature, with some species lacking any neuromorphological description. The clymene dolphin, Stenella clymene, is among the undescribed species, and its characterization is made even more urgent by its unique phylogenetic origin: it is the only known hybrid dolphin species in nature. In this context, the present study addresses the cytoarchitecture of the auditory cortex of Stenella clymene, an odontocete endemic to the Atlantic Ocean, for which neuroanatomical data are unprecedented. This includes a detailed analysis of the secondary auditory area (A2), which is poorly characterized across the entire cetacean taxon. Additionally, the study aimed to compare these findings with its phylogenetic ancestries to better understand its hybrid nature. Furthermore, a comprehensive protocol for the perfusion and collection of small cetacean encephalons was developed and detailed, seeking to optimize sample quality and enhance comparability between studies, as complete documents of this nature are scarce in scientific publications. Samples were obtained from a specimen rescued alive on the coast of northeastern Brazil, which unfortunately succumbed during rehabilitation. Histological analysis identified five cortical layers in both primary (A1) and secondary (A2) auditory areas, with the notable absence of a typical defined layer IV, although morphological distinctions may suggest otherwise. Variations in thickness and morphological characteristics were observed between A1 and A2, corroborating hierarchical segregation, alongside relevant inter-hemispheric and functional divergences. This study also described findings of cytoarchitectural structures not previously detailed in other cetaceans. While similarities with Stenella coeruleoalba were noted, differences with other species indicate significant variability. The results expand the knowledge of cetacean cortical organization, providing an unprecedented characterization of Stenella clymene and a detailed account of A2. Thus, these findings provide a solid foundation for future investigations into the auditory function and the ecological and behavioral adaptations of these species, while also enabling more comparative studies through the increased methodological standardization facilitated by the developed protocol.