Fish and amphibian auditory neuroanatomy

Abstract

Morphological variation in the primary projections of the inner ear and acousticolateralis variation in agnathans, chondrichthians, bony fishes, and amphibians is reviewed and used to examine several hypotheses concerning auditory evolution in anamniotes. The acousticolateralis hypothesis regarding the origin of the inner ear is rejected on the basis that all vertebrates, including the earliest fossil agnathans, possess both lateralis and labyrinth systems. Embryonic evidence regarding development of the labyrinth, lateralis, and other special sense organs suggests they are serial homologues sharing a common neural crest-placodal origin, and that all special senses are equally old phylogenetically. The labyrinth hypothesis of the origin of audition can not be rejected or accepted, as there is insufficient evidence regarding the phylogenetic distribution of audition in anamniotes. The hypothesis that audition arose by the invasion and replacement of lateralis fibers by acoustic afferents in the acousticolateralis medullar centers is rejected, based on new experimental evidence that these systems in fishes maintain primarily separate central pathways. Finally, the acousticolateralis area of Latimeria is discussed and compared with that of amphibians.