Relationships between the body, brain, and major brain regions have traditionally been used to infer cognitive abilities across all vertebrates, providing vital information about life history traits, behavior and even “intelligence”. Indeed, we have shown that broad variability in the size and complexity of major brain areas, including the olfactory bulbs, telencephalon, diencephalon, optic tectum, tegmentum, cerebellum, and medulla oblongata, are highly correlated with habitat and/or specific behavior patterns in both bony and cartilaginous fishes, even in phylogenetically unrelated species that share certain lifestyle characteristics² . Although these studies are not a functional analysis, we suggest brain morphology may serve as a tool to make predictions about the behavioral ecology, sensory specializations, and predatory habits in cartilaginous fishes.

More recently, new neuronal scaling rules based on a method of accurately assessing the number of neurons in the brain in mammals or isotropic fractionation¹ , suggest that brain mass may be a poor predictor of cognitive ability and enhanced associative function. Here, we present the first application of this technique in fishes, using the Port Jackson shark, Heterodontus portusjacksoni, and the barramundi, Lates calcarifer, as model species. The total number of neurons (as compared to non-neuronal glia) was measured in the brain and its component parts using the isotropic fractionation method. We suggest the isotropic fractionation method, coupled with flow cytometry, serves as an effective tool to quantify neuronal scaling in early vertebrates. Further, we present data on total brain cell counts in our representative fish species in the context of currently available mammalian data, and discuss the potential for conservation of neuronal scaling across vertebrates. These data will pave the way for future work to assess whether the number of neurons within the major brain regions show a linear relationship or reveals differential rates of addition in relation to predicting higher cognitive abilities and/or more complex behavioral repertoires in fishes, with implications for how “intelligence” has evolved across vertebrates.