UNUSUAL BRAIN NUCLEUS IN THE DIENCEPHALON OF A PERCIFORM (NOTOTHENIID) FISH FROM ANTARCTICA

THIS cytoarchitectonic analysis on the rostral diencephalon of antarct ic perciform fishes confirms the previously established perciform dien cephalic body plan. However, it resulted in the detection of an additi onal, large and conspicuous brain nucleus (unusual nucleus, UN) in the antarctic nototheniid Pleuragramma antarcticum. This nucleus has not been seen before in any other perciform. The UN is probably homologous to a visually related nucleus (named nucleus rostrolateralis and late ral entopeduncular nuclear area, respectively), that has so far only b een found in two most primitive (osteoglossoform) and one most highly derived (atherinoform) species of teleost. Our present results indicat e that this brain region, which belongs to the ancient diencephalic or ganization of ray-finned fishes, is expressed in the brains of fishes due to their particular requirements regarding life styles rather than due to their phylogenetical relationships.