On the Coincidence of Loss of Electroreception and Reorganization of Brain Stem Nuclei in Vertebrates

Abstract

Only anamniotic vertebrates, such as lampreys, sharks, bony fish, and amphibians possess electroreceptive organs. Some bony and cartilaginous fish species have developed electric organs and some bony fish communicate with electric signals. This electrocommunication parallels in many ways the auditory system. Some of these fish have developed a large brain sometimes approaching the brain-body weight ratio of mammals (Bell and Szabo, 1986). These enlarged brain areas, in part related to electrocommunication, may form as much as 50% of the brain volume but may not be recognizable at all in related species. Clearly the sometimes laminated areas of the brain devoted to the processing of electrosensory information display a very large variability in size not matched by any other sensory modality. With respect to the relative increase in size and the laminar organization these changes rival in a way those underlying the evolution of the mammalian cerebral cortex. Understanding of the mechanism(s) through which this variability is achieved in the specialized electrosensory system may help to understand more general problems of vertebrate brain evolution such as the phylogeny of the mammalian cortex.