Brain Morphology and Quantitative Assessment of Sensory Brain Areas in Southern Bluefin Tuna, <i>Thunnus maccoyii</i> (Scombridae, Teleostei)

A quantitative comparison of the absolute and relative volumes of different brain areas is useful for predicting the sensory capabilities and behavior of large pelagic teleosts, which are difficult to study in the field or in vivo. However, the size of pelagic teleost brain regions has only been approximated using the idealized ellipsoid method, which is susceptible to over- or underestimation, as it assumes the shape of brain regions to be an idealized ellipsoid or half-ellipsoid. This study examines the gross morphology and volumes of different sensory brain areas of southern bluefin tuna Thunnus maccoyii using magnetic resonance imaging (MRI). The results show that the optic tectum (568 ± 11 mm³) has a larger absolute volume compared to the olfactory bulb (50 ± 5 mm³), eminentia granularis (62 ± 9 mm³), and cristae cerebelli (47 ± 3 mm³), suggesting the significance of vision for T. maccoyii. The full segmentation of a T. maccoyii brain allowed the quantification of the integration areas, which reveals that the corpus cerebelli (1299 mm³) occupies the largest proportion (35%) of total brain volume, whereas the optic tectum only occupies 15% of total brain volume. The corpus cerebelli also exhibits a rostro-caudal elongation with multiple horizontal sulci, which resemble the corpus cerebelli of some species of sharks. The results reveal that the brain of T. maccoyii is dominated by the locomotive area of the corpus cerebelli and highlight the benefits of using MRI when performing quantitative analyses on the brain volumes of large pelagic teleosts.