To be successful in the open oceans, very sharp senses are needed, but not necessarily the same ones land creatures use.

Vision
Even if the sea is clear, and often it isn’t, vision in water is limited to distances of less than 30m. It is most important when predators are closing in on their prey. Some marine creatures have very large eyes and, generally speaking, these are the ones that live in or dive to the depths. Their large eyes mean creatures can take advantage of what little light there is. A good example is the swordfish that dives after jellyfish. Bony fish seem to rely on vision to a greater extent than fish with cartilage.

Fish generally don’t need eyelids, but some sharks have a membrane that functions in much the same way - to protect the eye and to cut out any brightness.

Touch
In place of vision, fish have a thin canal of sensory cells called a lateral line (see lateral line). This allows the fish to detect vibrations in the water enabling it to determine where objects and other animals are. It picks up changes in water pressure and the sound of fast swimming prey.

Seals are mammals and they don’t have lateral lines. Scientists studying common or harbour seals now believe that their whiskers are particularly sensitive to water movement. Seals in the study group could detect a herring from a distance of 180m, even with their eyes covered, but they didn’t do anything like as well if their whiskers were shielded.

Cartilaginous fish such as sharks have sense organs called ampullae of Lorenzini that pick up electrical signals. It’s believed that they use them to locate prey and for navigation (see magnetic fields).

Hearing
Sound travels very quickly through water, about four times as fast as it does through air. Fish pick up the sound waves using simple inner ears that are found behind their eyes. The ears contain similar sensors to those found in the lateral line. Some fish amplify the sound using their swim bladders.

Smell
Fish are highly sensitive to smell. They have olfactory sacs and nostrils, usually found on both sides of the head. All fish leave behind them proteins and waste materials and predators can detect the trail of their prey, as long as they are down-current. Sharks are particularly good at sniffing out a meal and are famous for being able to identify the most minute concentrations of blood in water (less than one part to a million). Fish use smell to find food, identify mates and avoid predators. Some fish even use smell to find their way, for example, salmon seem to be able to remember the smell and taste of the water in their spawning grounds. Marine mammals hardly use smell at all.

Taste
A fish has taste buds that are found not only in its mouth, but also on its skin and fins. The taste buds respond to chemicals in the water. Bottom dwelling fish have barbels that stick out like whiskers from around the mouth. These are used to find food on the sea floor.

Echolocation
Marine mammals also need to be able to find things at sea and dolphins do so using echolocation. It works like the sonar system developed for use in submarines. The dolphin makes a series of clicks in a nasal sac on the top of its head. The clicks are focused into a beam as they pass through a lens of fat (called a melon). When the beam of sound hits an object or another animal, it bounces back towards the dolphin and is picked up in fat-filled holes in its lower jaw. From there the sounds travel to the middle ear, then to the inner ear and through the auditory nerve to the brain.

We know that dolphins have very large brains, and it’s thought that a lot of that brain power is needed to process the information they receive from echolocation. Whales also use this means of detection.

Voice
Whales and dolphins also communicate by sound. The male humpback sings during the breeding season so the song might be used to announce his presence to a mate or to compete with other males. It may not be music as we’d recognise it, consisting of a series of moans, squeaks and whistles but it’s quite appropriate to call these noises song, because they are arranged in phrases and themes that are repeated in a regular pattern.

The low frequency sound can be picked up as far as 160km away. All the males in a population sing the same song and they can keep going for days on end. A single whale can sing for up to 20 hours at a time. The songs gradually change according to location and time of year. Other whales, marine mammals and some fish communicate by sound.