Animals that dive have special physiological changes they have to make to ensure they can survive in areas of low oxygen concentrations along many others (Hill et al. 2016). Some of these changes include changes to metabolism due to the lack of oxygen, tissue cooling, delay in food processing, and so on. In addition to these changes, the sensory processes of diving animals change with respect to their depths in water.
Hearing
A significant aspect of sensory system that changes is hearing. For humans who are not adapted to diving for prolonged periods of time, we have a difficult time locating the direction and source of the sound (Lee 2015). When the "vibrations are translated to the inner ear", the "mechanical energy is translated to electrical pulses" in the brain where an animal is then able to process and react to the stimuli. However, for humans underwater our skulls vibrate to the sounds which leads to the difficulties in locating the source of the stimuli.
Therefore, marine mammals have developed a system which allows them to overcome this limitation and increase their chances of survival in the deep sea. An example of this are dolphins. The range at which dolphins can hear is from 20Hz to 160kHz which include ultrasonic sounds (Cross 2016). Dolphins utilize a special technique called echolocation in which they are able to vocalize, referred to as a "sonar to locate the target" (Kaveh & Farhoudi 2013). When the sound made by the dolphin, such as when the clicking "strikes an object, some of the energy of the sound-wave is reflected back towards the dolphin". When this is received by the same dolphin, another click sound is made and it is then able to analyze the time difference thus finding out the distance and location. The sound is produced in the nasal system of the dolphin (Au & Hastings 2008). Dolphins are able to hear through their ear openings (Cross 2016). They also use their foreheads for sound recognition and are able to feel sound vibrations in their jaw, which helps as sound travels differently underwater.
In continuation with this idea, marine animals have better hearing underwater because they are able to have sound enter through the lower jaw (BIAS 2013). This allows them to hear prey or predator signs underwater which aids in survival.
In other animals that live in the deep sea, such as the octopus or shark, they have systems to help detect vibrating sensations in the water instead of having dedicated ear organs like terrestrial mammals. An example of this is the lateral line, this is able to help detect the vibrations in the water to help locate predators and prey (Cross 2016). Another example is the statocyst in octopus, which help aid in hearing and balance. In sharks, they have 3 semicircular canals and 4 sensory membranes which help with both balance and sound. In fish, the swim bladder is used for buoyancy but also for sound production and hearing.