Morphologic features of the cerebellum of the Atlantic stingray, and their possible evolutionary significance

The morphology of the cerebellar corpus in cartilaginous fishes varies from a smooth, relatively simple structure to a complex, multilobed structure. Atlantic stingrays possess a trilobed corpus that includes an anterior lobe, divided into rostral and caudal lobules, and a posterior lobe. The corpus in this stingray is assymetrical. This asymmetry was examined in the stingray population of Galveston Bay. In 49% of the animals the axis of the caudal lobule was right of the midline, in 27% it was across the midline, and in 24% it was to the left. This variation is not related to size, sex, or an asymmetry in the cranial volume, but might reflect a variation in the cerebellar developmental program. To gain insight into the factors that have driven cerebellar hypertrophy in cartilaginous fishes, the neural connections of the lobules of the cerebellum of the Atlantic stingray were examined using biotinylated dextrans. It was found that, for the most part, the stingray cerebellum receives inputs from the same diencephalic and brainstem nuclei as in cartilaginous fishes with a bilobed cerebellum. However, in stingrays there is greater segregation of inputs. The anterior lobe rostral lobule receives inputs from the accessory optic nuclei, the caudal lobule receives inputs from trigeminal and octavolateral nuclei, and the posterior lobe receives inputs from the spinal cord. The rostral lobule and posterior lobe also receive input from midbrain nuclei that do not appear to be present in cartilaginous fishes with a morphologically simple corpus. Therefore, it is proposed that the complex hypertrophy of the corpus in stingrays might result from a combination of functional specialization of the lobes, and the acquisition of new inputs.