The sea is actually a very noisy place. Sound is a convenient mediator of information in water, since it travels far, irrespective of daylight and visibility. This has been exploited by all sorts of animals living in the sea. However, the acoustical properties of water put special demands on sea animals trying to use sounds. This has lead to a rich variety of adaptations in both sound generators, receptors and use in the sea. Among the noisiest sea creatures is a Crustacean, the snapping shrimp Alpheus japonicus, producing a pistol shot like, broadband (<20-200 kHz) sound with their claws. They do so in connection with prey capture, resulting in the immobilizing of small fish. It is not the sound itself, but a powerful jet of water ejected through a hole in the claw, that blocks out the fish's linear system. The sound is produced when two smooth surfaces, pressed tightly in the open claw, are suddenly forced apart when the claw is closed. Less noisy are fish, mostly limiting themselves to silent, low frequency grunts used in social contexts. Sometimes the fish bladder is used as a resonance chamber, amplifying the sound. Hearing is mostly limited to the low frequency range, even if recent research shows that e.g. herrings Clupea harengus may hear up to over 10 kHz. Birds are not believed to display much underwater social behaviour, nor sounds. But loons (Gaviidae), penguins (Spheniscidae), cormorants (Phalacrocoracidae), diving ducks such as the eider duck Somateria mollissima, and auks (Alcidae) all make noise when swimming, mainly cavitation noise. All marine mammals, when they returned to a life in the sea, had to re-adapt to the acoustical properties of water. In order to be effective, both the terrestrial sound generation and reception systems had be remodelled. This has occurred to a various extent in present day marine mammals. Water dwelling Carnivores, such as the polar bear Thalarctos maritimus, may be good swimmers and divers, but have not developed special underwater adaptations in neither sound generation nor hearing. The otter Lutra lutra and the mink Mustela vison chase and catch fish under water, but then apparently rely on vision and touch (vibrissae). They too have normal terrestrial ears. Propeller-like noise from swimming minks, which is believed to be cavitation noise from the front paws, has been reported. Pinnipeds are much more adapted to life in water, although they have maintained an important link to terrestrial life in connection with reproduction. Therefore they have a sound repertoire for use both in air and in water. Their calls are low frequency, often composed of trains of pulses, but FM sounds also occur. These calls may be heard at great distances. 0ne species, the leopard seal Hydrurga leptonyx, is reported to produce ultrasonic sounds in connection with fish chasing. Often the throat region is seen to be moving and/or inflated in connection with the seals' underwater sounds, indicating that the larynx may be involved in their production, but most often no air is expelled into the water. Or cavity resonance may be involved, e.g. in the bell-like sound of walrus, which is believed to be produced the pharyngeal pouches. Seal hearing is acute and partly adapted to the water physics. The Cetaceans have cut every link to terrestrial life, with no need for compromises in their acoustical adaptations. Baleen whales use very low frequency and sometimes very intense sounds, in order to reach far with their communication signals. Their sound generation mechanism is largely unknown, but may involve resonance in the lungs and/or trachea. Their hearing shows clear morphological adaptations to underwater demands, but so far their characteristics are rather unknown. Odontocetes have gone the farthest in their acoustical adaptation, and have developed the most sophisticated sonar in the animal kingdom. In at least the Delphinids the sonar clicks are concentrated into a narrow beam pointing forward, approximately along the long axis of the rostrum. This is done by means of the melon, a fatty tissue structure in front of the blowhole. Most Delphinids also produce FM whistles in the 5-25 kHz range. The most well-known of these is the so called signature whistle, which is believed to be like an acoustical fingerprint. The sonar clicks as well as the whistles are produced by means of a new sound generation system in the nasal cavities, powered by air pressure created in the bony nares. The main part of the sonar click is purely a tissue phenomenon, whereas the whistles are produced in the air within the nares. The air used during sound production is collected in diverticula just below the blowhole, and is then pulled back to the bony nares to be used again. In its extreme, the pulsed sounds may be powerful enough to debilitate prey. This has been suggested for the sperm whale, where the large head, with the spermaceti organ, may be a huge sound amplifier. The hearing in most Delphinids is extremely acute, ranging from below 1 kHz to 150 kHz. The sounds are entering via the lower jaw, and is guided to the middle ear by mean of a special fatty tissue channel. The hearing is not affected by water depth, indicating that the air in the middle ear is not involved in the sound transmission.