The Shark's Remarkable Senses

Sharks have a range of sensory systems on their heads and

bodies with which to explore the environment, pursue prey,

and avoid predators. In addition to touch, taste and a keen

sense of smell, sharks possess excellent vision at close range

and unique vibration and electromagnetic senses. Armed with

such equipment, it is not surprising that they have survived

so successfully for more than 200 million years.

The shark has frequently been referred to as a 'swimming nose' and there is considerable supportfor this claim. The nostrils, located on the underside of the snout just ahead of the mouth, open into spacious sacs lined with many folds of tissue which contain the cells responsible for detecting smells. When a shark takes water into its mouth to constantly aerate its gills, suction causes some water to flow in and out of each nasal sac. In addition, the forward motion of the shark brings water through the funnel-shaped nostrils into the sacs. A fleshy flap extending across the opening of each sac separates water flowing in from water flowing out. The odour-detecting system of a shark is therefore constantly bathed by a current of water whether the shark is swimming or resting. In one of the oddest of all sharks, the hammerhead, the nostrils, as well as the eyes, are located far apart at the ends of the 'hammer~ This fact, coupled with the shark's habit of swinging its head from side to side through a considerable arc as it swims. enables it to sample the smells in a much wider path through the water than would otherwise be possible.

Ralph E. Sheldon, working at the Marine Biological Laboratory in Woods Hole, Massachusetts, was the first to demonstrate scientifically that the sense of smell is important in guiding sharks to a meal. He found that smooth dogfish had no difficulty distinguishing a cloth- covered packet that contained crushed crabmeat from identical packets containing stones. When Sheldon plugged the sharks' nostrils with cotton wool so that a current of water could not enter the nasal sacs, they no longer homed in on the food packets, although they swam quite close to them. To eliminate the possibility that the cotton wool plugs merely made the sharks uncomfortable and discouraged them from eating, Sheldon plugged the nostril on one side only of several dogfish. After a brief period of adjustment all but one of the animals readily located the packet containing food. Several years ago, at the lerner Marine labratory, scientists tested the sense of smell of lemon sharks by implanting electrodes in various parts of their brains( see drawing below). They concluded that the sharks were capable of detecting dillutions of one part of tuna juice to 25 million parts of seawater. Albert Tester conducted laboratory tank experiments at Eniwetok in the 1960s and found that 'the response ofblacktip and gray sharks to a standard extract increased under conditions of progressive starvation.' He found that starved blacktips were so sensitive to smells that they responded to concentrations of grouper flesh in dilutions of only one part in 10 billion (109) parts of seawater.

REASSESSING THE MENTAL ABILITIES OF SHARKS

Brain wave records (right) show the response to various dilutions of tuna fish extract.    Brain of a lemon shark

 Sharks are often described as having that sharks compare well with many other 'primitive' brains -a claim that has not been vertebrates. The species with the largest borne out by recent studies. Brain size and brains in comparison to their body weights complexity vary greatly from species to are the dusky shark and the scalloped species, but some sharks obviously have hammerheads. In general, the active, fast- considerable mental ability, and are capable moving sharks have larger and more

of simple feats of learning. A comparison of complex brains than the slower, bottom- brain to body-weight ratio (see p 21) shows dwelling species.

 

 

A great white shark 'sniffs' the surface of the water as it homes in on the source of a smell. Sharks trace odors by following the scent and always turning upstream when it is detected.
The nostrils of the grey nurse shark Eugomphodus taurus are divided externally by a flap of skin. Water flows in through the openings on the outside of the nostrils, and out through the openings on the inner side of the flap. Once inside the shark's head the water passes through a funnel-shaped passage and into the nasal sacs, which are lined with sensitive cells. 

 

Sharks' eyes vary greatly in size and shape according to the species concerned. Not all have nictitating membranes as shown in the diagram (left). There has been some disagreement among researchers on whether the lens can be moved in and out to focus the eye on near and far objects.

 

  
In dim light the platelets of the tapetum lucidum reflect light back into the retina and improve the eye's sensitivity.
In bright light pigment cells obscure the platelets and prevent any light from being reflected back to the retina.

Sharks have excellent vision and experimental studies have now dispelled the wideiy-held notion that they have poor eyesight. As early as 1960, experiments were conducted in which large lemon sharks had discs placed over their eyes to blind them temporarily: The blinded sharks were then released into pens at the Lerner Marine Laboratory on Bimini in the Bahamas to compete for food with other sharks that had normal

vision. The adult lemon sharks with unimpaired vision had no trouble homing in on and consuming a 113- kg (250-lb) chunk of blue marlin, while those temporarily blinded were unable to locate such an attractive feast. Other studies at this laboratory confirmed that sharks were attracted to bright objects while non-reflective, dull colors were generally avoided.

A shark's eye is a somewhat flattened version of the same eye that II vertebrates have., with an iris, le and retina, and three fluid-fille chambers contained within a tough envelope of cartilage, the sclera. In the shark, the aperture of t~ iris (pupil), varies in shape and again, contrary to popular belief, may open and close quite rapidly: In the nurse shark Ginglymostoma cirratum, for example, the pupil can dilate to its maximum size 24 to 39 seconds after entering the dark, and can constrict to its minimum size 5 to 13 seconds after emerging into the light again. In the brown shark Carcharhinus plumbeus maximum dilation and constriction take place in 40 and 25 seconds respectively: In every case the shark's pupil constricted in bright light more rapidly than it dilated in dim light or darkness.

The retina of the shark's eye contains many light-sensitive rod and cone cells, with there being many more rods than cones. The cone cells determine the extent of the animal's visual acuity and colour vision, and the large number of rod cells improve a shark's visual sensitivity; and therefore its ability to tell the difference between an object, especially a moving one, and its background in very dim light. In recent years experiments have shown that sharks have much greater visual ability than researchers first thought.

The sensitivity of the shark's eye in dim light is greatly improvedby a remarkable structure, the tapetum lucidum, a mirror-like reflecting layer that lies under the retina. The tapetum is made up of tiny platelets, silvered with guanine crystals, that can reflect incoming light back through the retina to restimulate the light- sensitive rods and cones. The tapetum therefore enables sharks that feed at night -as most species that live in the open seas do -to make the most of the small amount of light that is available.

The tapetum, however, has an even more remarkable abilit): For those sharks that also feed during the day; bright light must be prevented from reaching the delicate cells of the retina. This is achieved not only by reducing the size of the opening in the pupil, but also by a curtain of pigment that temporarily screens each of the tiny platelets of the tapetum. Each platelet is covered by cells that contain pigment, and black granules temporarily fill the cells in bright light, thus preventing light from being reflected back into the retina. Conversely; as the shark's eye becomes accustomed to the dark, these black granules

The bulbous ampullae of Lorenzini are shown in this cross-section connected to canals that lead to the skin surface, and to nerves. The diagram of the shark's head shows the locations of skin pores leading to both the ampullae of Lorenzini and the lateralis system. Skin pores show up clearly in this view of the head ofa grey nurse shark. They lead to the ampullae of Lorenzini - sensory organs which enable sharks to detect very weak electric fields, and therefore any prey that may be buried under sand on the sea floor.

Ampullae of Lorenzini

Lateral line canals

 
A researcher uses an electrode at the end of a pole to test the electrical sense of this shark. The extraordinary sensitivity of this sense has only been fully appreciated in recent years.

withdraw to the base of the cells to expose the reflective surface of the platelets again. The lens of a shark's eye is nearly spherical and rigid, unlike the ellipse-shaped, elastic lens that mammals have. Some sharks can focus on an object by moving the lens towards or away from the retina, much as a camera can be focused. The movement of the lens is controlled by a muscle -the protractor lentis.

It has now been conclusively- demonstrated that sharks can locate their prey in the open sea by detecting the minute electric fields they generate. One of the first to suspect that sharks may be sensitive to tiny electric currents was Sven Dijgraaf, working at the University of Utrecht more than 45 years ago. He noticed that the blindfolded, small-spotted catsharks Scyliorhinus canicula he was using in behavioural experiments would turn rapidly away from a rusty steel wire located a few centimetres from their heads. However, when a glass rod was held ~t the same distance it did not produce the same response. Dijgraaf assumed that his sharks were responding to electric currents generated at the surface of the rusty wire. But what receptors did the sharks employ for doing this? A clue was provided in the early 1960s by R.W Murray who demonstrated that the ampullae of Lorenzini on the snouts of sharks responded to weak electric fields. When Dijgraaf and one of his bright graduate students, A.J. Kalmijn, severed the nerves to these ampullae the sharks were no longer able to detect electric currents.   It remained for Kalmijn to show that a shark may employ these unique electroreceptors to locate pre~ In a classic series of experiments he demonstrated that a small-spotted catshark can locate a live flounder buried beneath the sand in a laboratory aquarium. It did this by detecting minute electric pulses generated by the resting flounder. The shark was even able to find the flounder when it was placed in an agar chamber that permitted seawater and electric pulses to pass through. When the agar chamber was covered with a plastic film, electric pulses from the flounder were blocked, and the shark could not locate it. Finally; when a pair of electrodes were buried in the sand to simulate the flounder, Kalmijn's sharks attacked the electrodes.

During the summer of 1976, Kalmijn and K.J. Rose continued their work in the open sea offCape Cod, Massachusetts. They succeeded in getting the nocturnal,

The lateralis system alerts sharks to low-frequency vibrations, such as distant disturbances in the water made by struggling fish, or the erratic movements of a human swimmer. This sensitivity to vibration is shared by most fishes and some aquatic amphibians. There are still many unanswered questions about the role that this system plays in the everyday lives of sharks, in spite of the fact that it has recently been the subject of considerable research.

Tests have shown that the lateralis system is extremely sensitive. Blinded sharks are even able to detect the wall of a large tank without touching it, apparently by sensing water waves reflected from the surface.

 

bottom-feeding dusky smoothhound Mustelus canis to 'home in' on live electrodes which were giving off weak electric (DC) pulses. The sharks ignored 'dead' electrodes and other stimuli. After observing this identical behaviour in hundreds of other dusky smoothhounds in their natural environment, Kalmijn and Rose concluded that the sharks can detect their prey at close range using only their electric sense.

So responsive are sharks to a very weak electric field that Kalmijn believes they may also be sensitive to the earth's magnetic field and can use this ability to navigate. The dusky smoothhound regularly migrates southward from the waters off Cape Cod, Massachusetts, for the winter months, and is apparently endowed with a good sense of direction. The ampullae of Lorenzini may therefore provide the sharks with a very accurate internal electromagnetic compass.

During his experiments Kalmijn found that 'within the frequency range of direct current up to about eight hertz (cycles per second), sharks respond to fields of voltage gradients as low as a hundred-millionth of a volt per centimetre. That would be equivalent to the field of a flashlight battery connected to electrodes spaced 1000 miles [1600 km] apart in the ocean'! This remarkable electrical sensitivity is greater than that possessed by any other animal investigated so far.

Sharks have an acute vibration sense called the lateralis system which is located in their heads and bodies. It consists of several small canals that open at intervals to the surface through tubes to pores in the skin. Each canal is filled with seawater and contains clusters of sensory cells, called neuromasts, on its inner surface. From each neuromast several hairlike projections, enclosed in a gelatinous dome, stick out into the interior of the fluid-filled canal. Vibrations that reach the shark's head and body are transmitted to the neuromasts and cause the hairlike projections to move very slightly; thereby triggering a nerve impulse to the brain.

More than half a century ago George H. Parker of Harvard University found that a dogfish that could not see or hear was still able to detect disturbances in the water as long as its lateralis system was undamaged. When he severed the nerves of the lateralis system, the shark ceased to respond.

In addition to the lateralis system, sharks also detect vibrations with their ears. The ears of a shark -one per side -are enclosed in its cartilaginous brain case, and are made up of the same basic components that are found in all vertebrate ears -three semicircular canals, the utriculus, the sacculus, and their related structures.

 A major function of a shark's ears, and indeed any fish's ears, is to provide the creature with information about its position and speed. In a watery world there are few visual clues to help with this task. The three semicircular canals are at right angles to one another and therefore detect movement in anyone of the three spatial planes. Information of acceleration and deceleration is provided by the otoliths when they move, and liquid surging in the canals tells the shark's brainabout the extent of a turn.

Recent evidence suggests that sharks can pick up low frequency sounds from great distances. Work by A.N. Popper and R.R. Fay in the 1970s showed that the shark's ear did this by detecting the movement of water particles, which carry vibrations, rather than from changes in sound pressure. The three fiuid-filled semicircular canals in the shark's ear detect changes in the speed and direction of theshark's movement. Other organs in the sacculus and macula neglecta contain otoliths- tiny granules of calcium carbonate -which move when the shark changes position, and send signals to the brain to tell it about the animal's attitude in the water. The macula neglecta is especially sensitive to vertical movements, and it can also pick up vibrations from the roof of the animal's skull. Information from both the ears and the lateralis system may be used to locate the source of a sound quickly and accurately; It appears that the macula neglecta may be particularly important for doing this, a theory that is borne out by measurements made by J. T. Corwin. He examined the ears of a number of species of sharks and found that the macula neglecta was most elaborate in the grey reef shark, an active predator, less elaborate in the dusky smoothhound and nurse shark, and still less elaborate in the rays, where directional hearing is of least importance for locating food.

Another curious feature of shark ears is a tiny duct -called the endolymphatic duct -which extends from the sacculus of both ears to small pores on the shark's head. The exact purpose of these ducts is as yet unknown.

Sharks also have one other sensory system which is still little understood. This consists of ~ large number of 'pit organs' scattered along the animals' bodies and around their lower jaws. These are usually protected by modified denticles, and closely resemble the taste buds of higher vertebrates. Opinion is divided on whether the organs have a taste function, whether they pick up vibrations, or even possibly sense movements in the shark's body.

 

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