Encephalization and Brain Organization of Mobulid Rays (Myliobatiformes, Elasmobranchii) with Ecological Perspectives

Abstract

In the present study the brain mass to body mass ratio and external morphological features of the brain of Mobula japanica, Mobula thurstoni and Manta birostris (devilrays) are described. The Mobulids extended the upper boundary of the minimum convex polygon described earlier by other authors for batoids, which is plotted on a double logarithmic scale of brain to body mass, causing some change in the allometric coefficient. The encephalization quotient of Mobulas was higher than unity, therefore it can be concluded that the actual brain mass is greater than expected by the given body mass. M. japanica had the highest percentage (61%) of telencephalic mass from all batoids, while the brain mass of M. birostris was the highest of all fish studied so far. The gross morphology of the enlarged Mobulid telencephalon and cerebellum prominently resembled to that of Sphyrna mokarran (great hammerhead shark). A structural dimorphism of the highly foliated cerebellum was detected between genders of the M. japanica, albeit with a small sample size. No such gender-related dimorphism was detected in brain mass/body mass ratio. Other brain parts were similar to those of other elasmobranch species. The data are discussed in terms of their ecological and evolutionary significance