Forebrain organization in elasmobranchs

It has long been known that many elasmobranch fishes have relatively large brains. The telencephalon, in particular, has increased in size in several groups, and as a percent of total brain weight, it is as large as in some mammals. Little is known, however, about the organization, connections, and functions of the telencephalon in elasmobranchs. Early experimental studies indicated that olfaction does not dominate the telencephalon and that other sensory modalities are represented, particularly in the pallium. We have investigated the intrinsic and extrinsic connections of the telencephalon in two elasmobranch species: the thornback guitarfish, Platyrhinoidis triseriata, and the spiny dogfish, Squalus acanthias. Tracers were injected into various parts of the forebrain and olfactory pathways were found to be extensive and were seen to involve the pallium. Injections into various parts of the pallium revealed a major input from the area basalis, which receives secondary and tertiary olfactory fibers. Nonolfactory input from the diencephalon appeared relatively minor and seemed to converge with olfactory information in the dorsal pallium and area superficialis basalis. Major descending projections were seen to originate in the dorsal pallium and terminate in the hypothalamus and - in the case of Platyrhinoidis - massively in the lateral mesencephalic nucleus. Descending pathways appeared mainly crossed in Platyrhinoidis, but not in Squalus. Our data indicate that the concept of the dorsal pallium as a nonolfactory area in elasmobranchs must be reconsidered, and we suggest that many telencephalic centers, including the dorsal pallium, are involved in olfactory orientation.