The auditory system of cartilaginous fishes

Cartilaginous fishes (Chondrichthyes), including sharks, skates, rays, elephant fishes and chimaeras, have been in existence for over 400 million years and represent early stages of the evolution of extant jawed vertebrates. The sensory systems of cartilaginous fishes, including their hearing apparatus, have adapted to a diverse range of ecosystems, from the deep ocean to freshwater rivers, revealing high levels of morphological diversity. Since sound travels such long distances underwater, this environmental cue may represent an important signal in the behavior and survival of this group of fishes but there are still many gaps in our understanding of their hearing system. Based on current knowledge, cartilaginous fishes are most sensitive to low frequency sounds (< 1500 Hz) and there is abundant support that their inner ears use only particle displacement detection rather than the detection of sound pressure, but further studies are needed to corroborate the observations of long distance detection of sound sources that might be based on the detection of pressure oscillations. This review investigates the diversity and functional significance of the inner ear of chondrichthyans from a range of habitats and explores what is known about their hearing capabilities. How underwater sounds are processed by the central nervous system, the impacts of sound on acoustic ecology and behavior, and the potential effects of anthropogenic sound are also examined. Some suggestions for future work are presented to fill the large gaps in our knowledge of the hearing abilities of this important group of vertebrates.