Diversification of brain morphology in antarctic notothenioid fishes: basic descriptions and ecological considerations.

The Notothenioidei, a perciform suborder of 120 species, dominates the ichthyofauna of the Southern Ocean around Antarctica. Unlike most teleost groups, notothenioids have undergone a corresponding ecological and phyletic diversification and therefore provide an excellent opportunity to study the divergence of the nervous system in an unusual environment. Our goal is to evaluate notothenioid brain variation in light of this diversification. To provide a baseline morphology, we examine the gross morphology and histology of the brain of Trematomus bernacchii, a generalized member of the family Nototheniidae. We then examine the variation in brain gross anatomy (32 species) and histology (10 species) of other notothenioids. Our sample represents about 27% of the species in this group and includes species from each of the six families, as well as species representing diverse ecologies. For comparison we reference the well-studied brains of two species of temperate perciformes (Perca flavescens and Lepomis humilis). Our results show that, in general, notothenioid brains are more similar to the brains of temperate perciforms than to the unusual brains of cave-dwelling and deep-sea fishes. Interspecific variation in gross brain morphology is comparable to that in Old World cyprinids and is illustrated for 17 species. Variation is especially noteworthy in the ecologically and geographically diverse family Nototheniidae. Measurements indicate that sensory regions (olfactory bulbs, eminentia granularis, and crista cerebellaris) exhibit the most pronounced variation in relative surface area. Association areas, including the corpus cerebelli and the telencephalon, exhibit moderate variation in size, shape, and lobation patterns. Regulatory areas of the brain, including the saccus vasculosus and the subependyma of the third ventricle, are also variable. These regions are best developed in species living in the subfreezing water close to the continent. In some species the expanded ependymal lining forms ventricular sacs, not previously described in any other vertebrate. Three species, including two nototheniids (Eleginops maclovinus and Pleuragramma antarcticum) and the only artedidraconid in our sample, have distinctive brains. The unique brain morphology of Pleuragramma is probably related to a sensory (lateral line) specialization for feeding. Within the Nototheniidae, a phyletic effect on cerebellar morphology is evident in the Coriiceps group and in the Pleuragramminae. Neither phyletic position nor ecological factors (water temperature, position in the water column, dietary habits) alone fully explain the pattern of notothenioid brain diversification.