Comparative study of Japanese eel and yellowfin goby brains using a new brain size index

We propose a simple index of brain volume (D*L index), calculated using two-dimensional photographs of the brain, captured from the dorsal and lateral views. Here, D and L represent the dorsal and lateral brain areas in the images, respectively. Two distantly related species were assessed. The D*L indices of 11 Japanese eels, Anguilla japonica and 11 yellowfin gobies, Acanthogobius flavimanus were calculated. High correlation coefficients of D*L index with brain weights were obtained from both species, suggesting the validity and reliability of the index. The relative profiles of the olfactory bulb, telencephalon, optic tectum, and cerebellum were calculated using this index. Eels had a larger olfactory bulb and telencephalon, whereas gobies had a larger optic tectum. Principal component analysis showed two separate clusters for eels and gobies. Published images of two Anguilliformes species and four Gobiidae species were used to test the D*L index. Principal component analysis showed separate clusters for Anguilliformes and Gobiidae species. The D*L index is a suitable tool for data collection because it is simple, low-cost, and has reliability and validity.