Central Mechanosensory Lateral Line Centers and Pathways Among the Elasmobranchs

Abstract

The central neuronal pathways and connections required to process mechanosensory lateral line information probably arose, concomitantly with the peripheral receptors, in the first vertebrates. The cartilaginous fishes (Chondrichthyes) share the mechanosensory component of the lateral line system with all other anamniotes, although there are vast differences in the central nervous systems of those vertebrates that possess lateral line sensory capabilities. Even among the elasmobranchs, there are different levels of brain organization and profound variations within each level (Northcutt 1978). It is important to identify mechanosensory lateral line centers and their fiber relations with other octavolateralis centers among those elasmobranchs that possess different patterns of brain organization, in order to establish homologies and to better understand the evolution of mechanoreception in chondrichthyans. However, our knowledge of the central mechanosensory lateral line pathways is limited to only a few elasmobranchs, e.g., the batoids (thornback guitarfish Platyrhinoidis triseriata, clearnose skate Raja eglanteria, and the little skate Raja erinacea), the squalomorph sharks (spiny dogfish Squalus acanthias), and the galeomorph sharks (carpet shark Cephaloscyllium isabella). Central lateral line pathways have been studied mostly in skates that are considered advanced elasmobranchs in terms of brain organization (Northcutt 1978), but some information is available in the spiny dogfish and carpet shark which, in contrast to skates, are more primitive in terms of brain organization. The purpose of this chapter is to delineate some of the central mechanosensory lateral line centers and their connections at each level of the neuraxis and to establish homologies in these few species of cartilaginous fishes.