14.8: Communication and Navigation

The majority of species must be able to communicate at least on a rudimentary level—for example, to provide an indication of sex and attract a mate. Most predators cannot rely on finding their prey by chance but must actively seek them out. Many marine species must navigate from place to place in migrations or to find suitable habitat, food, and mates. Humans and most other terrestrial animals use their five senses—vision, smell, hearing, touch, and taste—to perform these functions, with vision being the predominant sense. Light is absorbed and scattered in seawater, and most of the ocean is dark. In addition, when light is present, its path is distorted by refraction. Therefore, marine species rely much less on vision than most terrestrial species do.

In the marine environment, hearing, chemical sensing that is equivalent to taste or smell, and touch have assumed much greater importance. Marine organisms have also developed other senses, including the ability to sense electrical fields and probably the Earth’s magnetic field. Our knowledge of these unusual and different senses and even of how marine organisms use their greatly enhanced hearing, chemical sensing, and touch is still extremely limited.

Although vision is much less useful in the oceans than on land, it is still widely used by many marine species, and many adaptations of vision are found in species that live in the ocean environment. The most important adaptations are greatly enlarged eyes or multiple-lens eyes that can see in very low light levels and distinguish subtle changes in light intensity (Fig. 14-18a,b). Most marine species do not see sharply focused images, and many, particularly some marine invertebrates, have bizarre-looking eyes (e.g., Figs. 14-18a, 14-35a). Some species have light-sensing organs that cannot be readily identified as eyes. For example, the giant clam (Fig. 14-35d) and many other benthic invertebrates are able to sense very small variations in light intensity, which enables them to withdraw into their shells, tubes, or other protective environments when predators approach.

Chemical sensing is very important in the marine environment, but very little is known about it. For example, anadromous and catadromous species are thought to use a well-developed chemical sensing capability to detect and navigate to their home streams. They apparently can sense small differences in the composition of chemicals present in very low concentrations in these home streams. Sharks also appear to have an extremely keen sense for chemicals. They are attracted to blood even when they are kilometers away from its source and the concentration of blood chemicals is exceedingly small. Adventurous scuba divers deliberately attract sharks by dumping blood and shredded fish into the water.

Sound travels through seawater with relatively little loss of intensity. Consequently, sound sensing, or hearing, is well developed in many marine species, and sound is used extensively for communication. Many species, particularly cetaceans such as whales and dolphins, use sound not only for communication, but also to locate objects, including prey. Most cetaceans can generate sound pulses that bounce off objects in their path. They focus and process the resulting echoes, and thus are able to “see” the objects. This echolocation ability is extremely sophisticated and capable of high sensitivity and precision in at least some species. For example, the dolphin echolocates by emitting low-frequency clicks to scan objects at distances up to hundreds of meters. As it nears the object, the dolphin uses higher-frequency clicks to get a more detailed “picture.” With their echolocation abilities, dolphins can identify specific objects that differ only slightly in shape, size, thickness, or material composition. Dolphins and other cetaceans can produce and sense an extraordinary range of sound frequencies far beyond human hearing at both high and low frequencies.

We do not know exactly how cetaceans produce sound pulses or how they focus and sense returning echoes. However, some species are believed to create the sounds by moving air within hollow bony structures in the head. The bulbous brow of most cetaceans (Fig. 12-25) contains oil- or fat-filled structures believed to be responsible for focusing sound pulses when they are generated. Returning echoes appear to be captured by the fat-filled lower jaw, which conducts the sound directly to the inner ear, allowing for precise navigation and hunting in total darkness.

In addition to cetaceans, many other marine species produce sounds thought to be used primarily for communication. For example, certain fishes are known to generate sound by using their swim bladders, and they may sense other fishes’ sounds or echoes in the same way. Scuba divers, if they listen carefully, can hear a continuous cacophony of noises in the oceans that sounds like the static on a radio. Some species use sound for more sinister purposes. The pistol (or snapping) shrimp (Fig. 14-35e) has one very large, overdeveloped claw, which, when snapped closed, creates a loud noise that can be heard across a room if the shrimp is in an aquarium. The concussion of the shrimp’s snapping sounds stuns its invertebrate or small fish prey, sometimes into unconsciousness. The stunned prey is then easy game for the shrimp. Other species, notably killer whales, also may generate and use intense sound pulses to stun or even to kill their prey.

Fishes can sense small changes in pressure through sensory organs located in a row along each side of their bodies that form what is called the “lateral line.” Lateral-line sensors are apparently used by schooling fish to sense motions of their neighbors. Together with visual clues, these sensors may explain how schooling fish can turn in unison. Sharks also may be able to sense pressure variations, since they are attracted from long distances by the thrashing of a sick or dying fish. The low-frequency pressure variations caused by such a fish are transmitted through the water as very low-frequency sound waves, and the shark’s ability to sense such pressure fluctuations may simply be an extension of its “hearing.”

Evidence is steadily accumulating that many marine species are able to sense magnetic fields, including the Earth’s magnetic field. This ability may be vital for migrating species such as sea turtles. In addition, some marine species are able to generate and sense electrical fields.