Senses
Life in the water poses different challenges than the terrestrial environment for finding food, avoiding predators, attracting mates, and orienting in space. Therefore, the senses of marine animals work slightly differently from the ways we as land dwellers might be used to. Rays and sharks might lack cone cells, meaning they do not have color perception; while bony fish on the other hand, especially those in protected habitats near the surface, have excellent color vision. Accordingly, they often sport very vivid colors which they use for communication and the attraction of mates. Conspicuous though they might be, when a predator notices them they can hide quickly in the reef. In the open waters this flamboyance would be dangerous as well as futile, as the range of vision is often limited there. Sound, on the other hand, propagates a lot better in water than it does in the air. Sounds can be perceived in the seas over distances of many kilometers. Therefore, many marine animals rely more on their sense of hearing than on their vision. Acoustic information is used for communication as well as for orientation. This is complemented by very a sensitive olfactory sense. Salmon, for instance, can find their way back to the streams where they hatched over vast distances, guided by smell. In octopodes, the sense of smell is located in the tentacles and combined with tactile sense, allowing them to discriminate between a crab and a crab-shaped object presented to them in an experiment. But in some fish, even more sophisticated senses have developed; the lateral line enables them to detect differences in pressure in the water; magnetoreception gives them magnetic compass orientation; and electroreception provides them with information about their surroundings even in the darkest and most turbid waters.
Sources: Tyack, Peter L. and Edward H. Miller, “Vocal Anatomy, Acoustic Communication and Echolocation”, Marine Mammal Biology: An Evolutionary Approach, ed. b Rus Hoelzel, Durham (2002): 142-184.
Attenborough, David, Life on Earth, (1979): 122-130.
van Giesen, Lena et al., “Molecular Basis of Chemotactile Sensation in Octopus”, Cell, vol. 183, issue. 3, 2020.