**4. Acoustic habitat**

The marine species use sound or acoustic signals for numerous biologically critical functions, and thus they can safely be said to possess acoustic vision as they perceive the world around them through sound. These biologically critical functions include communication (for group cohesion and coordination), navigation and exploration (for sensing the environment around), echolocation (for foraging and detection of prey), survival (avoiding predators) and many more. They may generate (vocalization) and receive (listen) sound, based on the soundscape of their habitat. Thus, acoustic habitat is critical for their well-being and survival. The vocalization and hearing is species specific and thus needs deeper understanding. The vocalization is relatively possible to monitor for varied range of species. However, the hearing of species is near impossible to monitor as the psychoacoustic study cannot be undertaken in the natural environment as the sound stimulus and its impact cannot be studied, without taking the animal into captivity. The auditory systems of smaller species that can be taken in captivity have been studied to some extent, but these animals in captivity may not respond in the same manner as in their natural habitat. Thus, such studies will have their own biases and may not reflect the true animal behavior. The vocalization in the large species like the big whales has been studied in their natural environment in a limited sense as their habitats are spread across the vast expanses of the oceans and also inaccessible in many cases. Out of the 126 sub-species of the whales (including dolphins and porpoises), only 25 have been studied for their acoustic characteristics, and most of them have only been studied for their vocalization as these are extremely large in size so cannot be taken in captivity in a lab environment [Chapters 2, 8].

Typically, it is assumed that the vocalization and the hearing have to be overlapping, with the hearing frequency band being much larger than the vocalization range. Further, it is interesting to note that some species may require hearing

#### *Changing Ecosystems and Their Services*

sensitivity far beyond their own vocalization to be able to detect any predatory threat through their vocalization. For example, a harbor seal may need to be able to detect the vocalization of a killer whale, though it may not vocalize in the same band. Thus, significant study is required to understand the acoustic habitat in the marine environment, both in terms of vocalization and auditory system (perception of sound) for varied marine species. The sound scape in their natural habitat will play a critical role in this study as these animals are known to adapt to their natural settings [Chapters 2, 8].

Marine animals have adapted to their acoustic habitat by developing specialized vocalization and hearing organs. The sound generated by fish is largely low frequency up to 1 kHz that use multiple mechanisms for the same. These include [Chapters 2, 8] the following:


The cetaceans have two major suborders namely Odontoceti (toothed whales) and the Mysticeti (Baleen whales) with a distinct and complex mechanism to generate and receive sound. The Odontocetes generate a variety of sounds using a complex system of air sacs and specialized soft tissues that vibrate as air moves through the nasal passage. The Mysticetes use the larynx (without the vocal cords) for sound generation. Marine vertebrates generate sound by closing their enlarged claws to create a bubble that cavitates. Snapping shrimps are known to generate sound with very high intensity using the cavitation process. Crabs are known to generate sound by drumming on the substrate with both their claws. Marine invertebrates use stridulation and rapid muscle contraction for sound generation like the spiny lobster [Chapters 2, 8].

The auditory system for acoustic perception of sound varies based on the fact that the particular marine species is exclusive water dweller or mixed. Cetaceans (exclusive water dwellers) and pinnipeds (seals, sea lions and walruses are mixed dwellers) show significant differences as the cetaceans have no external pinnae, and their ear canals are nonfunctional and narrow that are clogged with debris and dense wax. The narrow ear canal is not attached to the tympanic membrane (ear drum), thus not connected to the middle ear. In toothed whales, the lower jaw is surrounded by specialized fats which along with a thin bony area called the pan bone is known to play a critical role in channelizing the sound to the middle ear. The middle and inner ears of cetaceans are encashed in bones that are located in a cavity outside the skull. The complexity of the inner ear determines the sophistication of the auditory process [Chapters 2, 8].

In pinnipeds, the external ear flaps, or the pinnae are reduced or absent. Muscles and cartilage valve along the external ear canal function to close the ear canal to water. In general, the middle and inner ears in pinnipeds, polar bears and otters are similar to those of terrestrial mammals, and the mechanism for perception of sound is also similar. Depending on their lifestyles, some species hear best in air, whereas others hear better underwater.

The fishes have developed a unique mechanosensory (lateral line) system that senses vibration and water flow. The fish body is considered to be acoustically transparent as the density is approximately the same. The fish's body moves in concert with the traveling sound wave, and the sound gets picked up by bones in the inner ear called otoliths that are denser. The displacement/bend of the otoliths deforms

#### *Acoustic Habitat Degradation Due to Shipping in the Indian Ocean Region DOI: http://dx.doi.org/10.5772/intechopen.90108*

the cilia on the hair cells located in the inner ear that is picked up by the brain as sound. Otoliths are the species-specific sensory organs, made of calcium carbonate, whose shape and size determine the acoustic characteristics of the sensed signal. The proximity of the swim bladder and the inner ear significantly determines the sensitivity to sound by the fish species. The density of the gas inside the swim bladder being lower than the fish's body and that of the seawater allows the swim bladder to deform due to sound pressure waves [Chapters 2, 8].

The acoustic characteristics of the vocalization by the marine animals that contribute to the soundscape in the underwater domain are highly species specific based on their intensity, frequency and time duration. The purpose for vocalization could vary from one-way communication signals to two-way echolocation signals for active sensing. The size of the animal also has a bearing on the acoustic characteristics, as bigger animals tend to generate low-frequency signals, whereas smaller animals tend to produce high frequency, sensitive to the mechanism of sound generation; size being comparable to the wavelength of the signal.

The large size Mysticetes produces sounds for communication over long ranges and senses the environment at low-frequency band ranging from 10 to 2000 Hz. These large animals migrate over large areas and need to communicate over large ranges, thus use low frequency that attenuate far less. These signals are categorized as tonal calls, frequency-modulated sweeps, pulsed tonals for echolocation and broadband grunts. They use echolocation to sense the environment around them rather than for foraging. The Odontocetes use mid-to-high frequency sound in the frequency band of 1–200 kHz. These signals are categorized as broadband clicks with species-specific peak energy between 5 and 150 kHz, burst pulse click trains for echolocation used for foraging and other active sensing requirements and tonal and FM whistles for communication ranging from 1 to 25 kHz. Pinnipeds that are semi-aquatic breed produce a limited array of barks and clicks in the frequency range of 1–4 kHz [Chapters 2, 8].

The non-toothed cetaceans have been found to be incapable of echolocation. The Odontocetes have very sophisticated sonar processing abilities with directed beams in space to locate, track and intercept prey. The fatty melon in the forehead acts as an acoustic lens to focus the acoustic beam. The freshwater dolphins like the Ganga river dolphins and harbor porpoises have been known to have very specialized clicks in the frequency range of above 100 kHz for foraging. These animals have long beaks that form narrow beams to be able to direct high energy in the front for locating small fish for food. The sperm whales generate sonar pulses with intensity of the order of 223 dB underwater, which is equivalent to 160 dB in air, louder than a jet during take-off [Chapters 2, 8].

The marine animals have evolved their vocalization and auditory system to be able to exploit the acoustic potential of the undersea domain, in spite of the severe limitation of the propagation conditions and low SNR. The natural sound from the animals is also complemented by the noise due to wind and others due to human intervention [Chapters 2, 8].
