Forebrain Organization in Elasmobranchs

Abstract

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.

Identification of Striatal and Pallidal Regions in the Subpallium of Anamniotes

“…Chondrichthyes or cartilaginous fishes are the most ancient living group of jawed vertebrates and represent a small clade, but they exhibit striking variation in brain size and complexity [Hofmann and Northcutt, 2012]. They are divided into two sister subclasses, the primitive holocephalians (ratfishes and chimaeras) and the elasmobranchs, which are in turn divided into sharks and batoids (skates and rays).…”

Glycine‐immunoreactive neurons in the brain of a shark (Scyliorhinus canicula L.)

“…This evidence suggests that the subpallial Gly‐ir population might modulate secondary olfactory centers. In the thornback ray, the basal superficial area projects to the dorsomedial pallium and is involved in higher order olfactory circuits (Hofmann and Northcutt, ). No similar glycinergic population has been observed in the telencephalon of other fishes.…”

The neuroecology of the water-to-land transition and the evolution of the vertebrate brain

“…Sharks, for example, had been characterized as 'swimming noses', with olfactory information dominating the forebrain, by the pioneer comparative neuroanatomists [178], as reviewed in [179]. A revisionist period followed, showing that multiple sensory modalities also had access to the aquatic forebrain [77], but of late, the pendulum has swung back to acknowledge that yes, olfaction appears to dominate, though not as completely as the first claims pronounced [180][181][182][183]. For vision, particularly considering mammals, it is instructive to notice just how much of the cortical surface is captured by primary visual cortex and extrastriate visual representations in even nocturnal animals like rats and mice.…”

“…R. Soc. B 377: 20200523 by olfaction to integrative multisensory, dependent on niche, is emphasized for chondrichthyans (sharks and rays) [180,236]. A general absence of conserved structure in the pallial aspect of the forebrain of teleosts is claimed, the most species-numerous of the radiations [6], though not without controversy [181,182].…”