Hearing and sensing under water
We know that fishes have excellent memories and are able to recognise familiar objects or other species. Red Sea clownfish for example can recognise their mate after 30 days separation. And a tiger shark will recognise the person that feeds it regulary with some nice pieces of tuna. Fishes also have excellent sensory systems, although one of the problems of course is that we cannot ask fishes to report their sensory eperiences like we do in humans. So, for example we dont really know what a goldfish or a shark listens to. Will the famous tiger shark Emma of the Bahamas (https://www.facebook.com/EmmaTheShark/ react when visitors call her name? But we know much more about the sensory system and basic perception processes in fishes. Lets start with hearing and the lateral line.
The lateral line system allows a fish to detect movement and vibrations in the surrounding water. Its crucial for its spatial awareness and the ability to navigate in space. All fishes depend on this system for orientation, predatory behavior, and social schooling. Lateral line receptors form somatotopic maps within the brain informing the fish of amplitude and direction of flow a current. It all starts with a hair cell in a gelatine substance. These hair cells are lined up in a long lateral line running from head to tail. The lateral line is open to the environment via a series of openings (called lateral line pores) in a long canal. The basic sensory unit is called a neuromast. This is a small bundle of sense hairs and their sensory cells with a flexible little pole or cup (cupula) op top filled witn a jelly like fluid. (See insert above*). The cupula bends just like a joystick with movement of the water in the canal of the lateral line or on the surface of the skin. Mechanical motion of water caused by objects or sounds in the environment is then transmitted via the neuromast to the brain. But disturbance in the water caused by the fishes own movements counteract the activity caused by movement of external signals. Thus suppressing interference with the biologically relevant signals.In some fish species, the receptive organs of the lateral line have been modified to function as electroreceptors, the organs used to detect electrical impulses.
Fishes also have an auditory perception organ in the inner ear called otolith (bony particles) which differs from that in mammals. They also have hair cells in the inner ear (the vestibular system) similar to the neuromast. The differential movement between the hair cells and the heavier otolith is interpreted by the brain as sound. This is a rather crude form of auditory perception. For example in a shark its function is to detect sounds that have a direct survival function; like splashing and the sounds of an injured prey which create different sound frequencies. Bony fishes like carps and goldfish have a better hearing, because their swim bladder which is near the otolith amplifies sounds like a hearing aid.
Marine mammals The lateral line system of fishes is not so different from the system that mammals use for auditory perception and orientation. One major difference however is that in mammals the hair cells in their gelatin fluid are located in the inner ear. Where they respond either to vibrations of sound that move haircells in the cochlea, or to movements of the head that move haircells in the semicircular canals. Marine mammals such as dolphins and whales do not have the lateral line of fishes but rely on the same but more refined auditory system as land mammals. Probably they have adapted in a late phase of evolution to their watery environment by developing refined pressure detection of surrounding water. Instead of outer ears and eardrums they have small openings in the head transferring sound waves through a narrow canal to their middle and inner ears. Their refined auditory perception also allows them to understand complex sounds and communicate by producing sounds for mutual interactions. The same holds for a marine mammal like the Florida manatee. But in addition to a sound detection system in the inner ear, manatees also have numerous body hairs or vibrissae. Facial vibrissae are used for direct contact and tactile exploration, like the whiskers of cats or rodents, but with their body vibrissae the manatee can also detect vibrations and movement of water in their environment, just like fish do with their lateral line.
Avians like songbirds have an auditory system that is not so different from the auditory perception in mammals. Even more fascinating is that thery have analogous brain regions for auditory perception and vocal production as in humans***.
Noise and marine life. Noise caused by motor boat traffic along coral reefs could also have an impact on marine life. This was conluded in a recent study on the behavior of damselfish. Both playback of motorboat noise and direct disturbance by motorboats elevated metabolic rate in Ambon damselfish (Pomacentrus amboinensis), which when stressed by motorboat noise responded less often and less rapidly to simulated predatory strikes. The dusky dottyback (Pseudochromis fuscus) its natural predator even consumed more than twice as many damselfish when motorboats were passing in a field setting **
Listening underwater. Thoughts on Sound and Cetacean hearing. F.W. Reysenbach de Haan. In: Wales, Dolphins and Porpoises. K.N. Norris. UCLA press 1966.
Reep, R.L et al. (2011). Manatee vibrissae: evidence for a “lateral line” function. Annals of the New York Academy of Sciences Volume 1225 pages 101–109, April 2011
***J.J. Bolhuis, et al. (2011): From songs to synapses: Molecular mechanisms of birdsong memory BioEssayspp. 377 - 385