Hearing as an extreme sport: Underwater ears, infra to ultrasonic, and surface to the abyss

It has been argued that “hearing” evolved in aquatic animals. Auditory precursors exist in fossil Agnatha and cephalopod statolithic organs, but how/when/why did a dedicated acoustic receptor, the true primordial ear, first appear? Did hearing arise linearly or independently, in parallel, in and out of the water? Modern aquatic species have an extraordinary range of auditory systems, from simple pressure receptors to complex biosonar systems. What drives this breadth of “hearing”? Vertebrate ears reflect selective pressures. While vision, touch, taste, and olfaction are important, only hearing is ubiquitous. Even natural mutes, like goldfish and sea turtles, listen. Ears capture passive and active sound cues. Auditory structures, honed by habit and habitat, delimit species abilities to detect, analyze, and act on survival cues. Cochleae, from shrews to bats to wolves to whales, evolved from the essential papilla of stem reptiles, elongating, coiling, increasing in complexity that enhanced frequency discrimination, but with heads tuned to the physics of sound in their media. Air-water parallels evolved: ultrasonic echolocators and massive infrasonic specialists. The ear then is a window into the evolutionary push-pull driven by three tasks that shaped the several thousand elements packed into every auditory system: feed, breed, and survive another day.