# Brain Morphology and Quantitative Assessment of Sensory Brain Areas in Southern Bluefin Tuna, Thunnus maccoyii (Scombridae, Teleostei)

## TL;DR

This study uses MRI to analyze the brain structure of southern bluefin tuna, revealing that the cerebellum is the largest brain region and vision plays a significant role.

## Contribution

The study provides the first MRI-based quantitative analysis of brain regions in southern bluefin tuna.

## Key findings

- The optic tectum has a larger absolute volume than the olfactory bulb, eminentia granularis, and cristae cerebelli.
- The corpus cerebelli occupies 35% of the total brain volume and shows shark-like features.
- MRI provides more accurate brain volume measurements compared to idealized ellipsoid methods.

## Abstract

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.

The absolute and relative volume of major sensory and integrative brain regions of the southern bluefin tuna Thunnus maccoyii are examined using magnetic resonance imaging (MRI). A dominant cerebellum projects into the midbrain ventricular space, while the optic tectum represents a smaller proportion of total brain volume than previously estimated using idealized ellipsoid methods.