A comparative morphometric analysis of the optic nerve in two cetacean species, the striped dolphin (Stenella coeruleoalba) and fin whale (Balaenoptera physalus)

A comparative study was made on one Mysticete (the fin whale, Balaenoptera physalus) and one Odontocete species (the striped dolphin, Stenella coerule oalba) by measuring several morphological characteristics seen in cross sec tions of the optic nerve. We found that the two cetacean nerves share a num ber of specializations that distinguish them from the optic nerve of terres trial mammals. Fiber density is approximately two-fold lower than in land m ammals. A corresponding increase in the cross-sectional area occupied by as trocytes is observed. A population of "giant" (up to 15 mum in diameter) op tic axons is present in both the B. physalus and the S. coeruleoalba nerve. It is argued that these features probably reflect common adaptations to th e constraints imposed by tile aquatic environment. "Giant" optic axons migh t ensure short-latency detection of preys and other targets during navigati on while the increased astroglial content might be related to the maintenan ce of neuronal function during periods of anaerobic metabolism under water.