Allometric Brain Development and Neuroecological Adaptations in Juvenile and Adult Raja polystigma

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INTRODUCTION: Chondrichthyans represent some of the earliest diverging lineages of jawed vertebrates, making them key models for studying the evolution of vertebrate brains. Despite their evolutionary significance, Mediterranean species remain understudied. This research focuses on the speckled skate (Raja polystigma), an endemic Mediterranean benthic species with distinct life history traits, such as bathymetric segregation and postnatal shifts in diet. These traits provide a unique opportunity to explore how ecological factors influence postnatal brain development and neuroecological adaptation in cartilaginous fishes. METHODS: We examined the allometric relationship between brain mass and body mass in postnatal individuals of R. polystigma and assessed the relative growth of major brain regions, including the olfactory bulbs, telencephalon, diencephalon, optic tectum, cerebellum, and medulla oblongata. Data were analyzed using log-transformed linear regressions to determine differential growth rates and patterns of regional specialization during development. RESULTS: Our analysis revealed that brain growth scales with negative allometry relative to body mass, indicating a slowdown in brain growth as individuals mature. Region-specific trends showed that the olfactory bulbs, cerebellum, and medulla oblongata grow at a faster rate than the rest of the brain, suggesting enhanced development of sensory and motor capacities. Conversely, the optic tectum exhibited slower growth, implying a reduced visual reliance in adults. The telencephalon and diencephalon scaled isometrically with brain mass, suggesting stable roles in cognitive and integrative functions throughout postnatal development. CONCLUSION: These findings highlight how ecological and behavioral shifts during development shape brain organization in R. polystigma. Enhanced growth of non-visual sensory regions and motor centers may reflect adaptations to a benthic lifestyle and bathymetric niche. This study contributes to our understanding of neuroecological evolution in Mediterranean chondrichthyans and underscores the value of R. polystigma as a model for investigating brain development in relation to ecological specialization.

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