Sharks and rays possess highly acute senses to interpret their surroundings. As a group they have maximized their potential habitats by adapting their senses to function in often adverse conditions such as dark or turbid water. Their sensory organs fit (sometimes loosely) into the six categories of sight, hearing, taste, touch, smell, and electroreception.
The olfactory ability in sharks is well known, although the idea that sharks are able to detect and follow a drop of blood diluted in the ocean over many miles is an exaggeration.
Sharks possess paired nares; nostril-like holes that are located just under the leading edge of the snout. Each nare is divided by a nasal flap into two openings. Water is channeled into the incurrent aperture and having passed over the olfactory lamellae, exits the olfactory sac through the excurrent aperture. The olfactory lamellae are a series of folds on the interior surface of the olfactory sac which greatly increase the surface area, giving the shark more opportunity to register smells. Odors passing over the olfactory lamellae stimulate cilia-like endings of neuro-sensory cells. What actually happens is, dissolved molecules transported along in the water column bind to the receptor cells which then send a signal to the brain. This process is referred to as a chemosensory function.
Sharks have olfactory bulbs which are anterior extensions of the large olfactory lobe in the fore-brain that lead directly into the olfactory sacs. fibers in the olfactory bulb continue directly into the olfactory tract. With such advanced hardware it is not surprising that sharks are able to detect fish extracts in concentrations lower than 1 part in 10 billion!
Once a shark picks up a scent trail it swims up the trail moving its head from side to side (which is its natural swimming motion). As its snout passes back and forth through the scent trail it is able to determine the direction from which the odor is emanating. If the scent is lost or if the slick is too wide to use for navigation, the shark may swim forward in an exaggerated S pattern until it can pick up the direction again.
The nares are completely separate from the mouth and throat and therefore do not aid in respiration. In order to detect odors, sedentary species are able to pump water over the nares whilst resting on the sea floor. Not all sharks require a highly developed olfactory system. Whilst sense of smell may be the prime hunting tool for an Oceanic whitetip shark swimming along in the featureless ocean, an angel shark laying in wait for a meal, is primarily a visual predator and consequently its olfactory sense is not so finely tuned. However, odor detection is not only used in hunting. Sharks and rays produce pheromones which signal their reproductive state to potential mates and smell may also be used in navigation.
Oceanic whitetip sharks have been seen with their snouts out of the water supposedly sniffing the air. This may allow them to detect odors over far greater distances than would be possible under water.
Many sharks depend heavily on their vision whilst hunting although there are occasions where the eyes play little or no role at all such as when a hammerhead is searching for a stingray which is completely buried. In the majority of species, the eyes are well developed, large, and complex structures. Some species are also able to sense light and dark through a thin area of skin on top of the head which leads directly to the pineal gland in the brain. As there is no lens to focus the light there is no shape perception but these species may be able to measure changes in light levels to time their nightly vertical migrations, or perhaps even to sense predators of schools of fish that block out the light.
Many shallow water sharks are able to regulate the amount of light entering their eyes by dilating or contracting their pupils in the same manner that we do. In some species this is done laterally whereby the sharks eyes resemble those of a cat. However in some skate and ray species the pupil contracts into a horseshoe shaped slit. This may create a kind of double vision which the skate or ray could use to determine distance. Some bottom dwelling species of sharks and rays have a ragged protective flap which slides over the pupil effectively blocking out much of the light entering the eye. This flap produces a fragmented image that may aid in depth perception.
Focusing in sharks is accomplished with the aid of the rectus muscles which pull the lens closer to or further away from the retina, just as a camera lens focuses. This differs from higher vertebrates in which the lens is distorted in order to focus light from different distances. These muscles combined with the oblique muscles also control the direction that the eye faces.
Due to the poorly lit environment in which some deep water sharks live, they lack the ability to stop light from entering their eyes. This inability to shroud their eyes from intense light partially explains why sixgill sharks are seen in shallower water more regularly in the summer, when higher concentrations of suspended plankton near the surface create a darker shallow water environment. Sixgills and other deepwater species such as the Bigeye thresher shark have very large eyes and (not surprisingly) no cones for sensing colour. There are sharks that do possess both rods and cones in their eyes. But, the question regarding a shark’s ability to tell one color from another is still undecided. Dr. Eugenie Clark conducted a test on lemon sharks that she had trained to push a white panel lowered into their pen at which point they were rewarded with food. When she changed the colour of the panel to yellow, the approaching Lemon shark almost back-flipped in the water and refused to ever eat again. Whether this proves that the shark was able to determine the change of colour or whether the slight change of brightness was responsible for its behavior is not clear.
Most sharks possess excellent vision in low light conditions. The structure in the eye responsible for this is called the tapetum lucidum. This is a layer composed of mirrored crystals which lay behind the retina, that can be adjusted to reflect light back onto the retina amplifying the strength of the image. Sharks share this ability with some nocturnal mammals and reptiles which require extremely sensitive vision to hunt.
Unlike bony fishes, sharks do possess eyelids (both upper and lower) but they are fixed and unable to cover the eye. Requiem sharks have developed a toughened layer known as a nictitating membrane that rises from below the eye to completely cover it during feeding rendering the shark blind from a purely sight oriented perspective. White sharks and some others that do not possess a nictitating membrane are able to roll their eyes back in their sockets protecting the pupils and exposing a hardened pad on the back of the eye.
White sharks have been seen to lift their heads out of the water far enough to get a look at their topside surroundings. This behavior has been witnessed in the presence of boats but possibly developed from the sharks desire to determine whether seals or sea lions are present in the area. It is unclear how well their eyes are adapted to above water vision.
Eye of a Porbeagle shark
Eye of a Banded Guitarfish
Eye of a Pacific sharpnose shark
Touch can be split into actual contact and distant touch experienced through the lateral line system. Sharks will often nose an object prior to biting it in an effort to establish its edibility. The bite itself is also an exploration of touch as well as taste.
A more alien concept for us to relate to, is the ‘distant touch’ that is picked up through the lateral line canals; a series of interconnected canals that run from the back of the head all the way to the upper lobe of the tail in a distinct line. On the head itself, is the infraorbital canal which extends ventrally from behind the eye along the snout, the supraorbital canal which passes above the eye towards the snout eventually connecting to the infraorbital canal, and the supratemporal canal which leads back over the top of the head. Secondary canals may also be present around the head and jaw. The canals open to the outside of the skin by means of tiny pores which allow water to penetrate. As water is displaced by the movements in the sharks surroundings, small waves are created which move away from the disturbances like ripples on a pond. When these pressure waves pass over the hairs which line the lateral line canals, the hairs are disturbed and send signals to the brain. The sharks own movement also creates these waves which then bounce off obstructions and return to the shark creating a kind of echo map of its surroundings. The frequency or erratic nature of the vibrations indicate whether any nearby animals are sick or injured.
Sharks also possess a row of pores around the pectoral fins and gills, known as pit organs. These contain neuromast-like cells that are believed to aid the shark in the detection of temperature changes. Temperature is an important factor for migratory sharks. It is possible that some sharks seek out particular temperatures where they know that a particular prey species is most likely to be present.
At some point during the evolution of elasmobranchs the lateral line pores around the snout developed a sensitivity to fluctuations of the electrical fields in the shark’s environment. These modified sensory organs are known as the ampullae of Lorenzini. They consist of relatively large bulbous pores filled with a gelatinous substance. Connected to the pores are cylindrical canals in which the gelatinous secretions are stored. At the base of each pore is a sensory nerve which transports the electrical signals (which are collected by sensory cells lining the pore) to the brain. Actively hunting sharks may have as many as 1500 ampullae around their snout and head whilst more sedate species may only have a few hundred. The ampullae also react to a lesser degree to temperature and pressure changes.
The ability of sharks and rays to detect weak electrical signals in their surroundings may be one of the greatest factors relating to their survival through the millennia. The organs are sensitive enough for hammerheads and some other sharks to detect the small electrical signals put out by fishes hidden below the sand. In fact, the ampullae are so sensitive that they can pick up fluctuations of just 10 millionths of a volt or the equivalent of the electrical gradient of a AA battery with wires put into the sea 1 mile apart. The widened heads of the hammerhead family are probably an adaptation to increase the triangulation capabilities of their electroreception.
When sharks are very close to prey, it appears that their electrical sense takes over from sight or smell. This explains why sharks will sometimes bite the propellers and other metal objects rather than the bait which has been put in the water in front of them. Photographers (and I can testify to this) may have the unnerving experience of having a shark maul their underwater camera strobes because they emit strong electrical fields. Sharks will also respond more aggressively to the erratic electrical signals emitted by a wounded animal. This may explain why shark attack victims are repeatedly bitten whilst rescuers swimming next to them often remain completely unscathed.
Some sharks migrate across thousands of miles of bottomless ocean. Other fish which do this are believed to navigate by sensing the polarization of the earths magnetic fields through the presence of magnetite in their bodies. Sharks however do not possess magnetite and it has baffled researchers how a shark can navigate effectively across huge expanses of open water. Many senses may play a part in this type of navigation but one interesting idea is that the saline environment that they swim through combined with the magnetic fields around the earth may create a strong enough electrical gradient to lead the shark in the right direction; the equivalent to having a permanent North Star in the shark’s head.
Rows of Ampullae of Lorenzini (electro-receptive pores) on the snout of a blue shark.
Taste is another chemosensory function similar to smell. Sharks have small pits in the lining of the mouth and throat that contain rod shaped gustatory sensory cells. Once dissolved chemicals from the bitten object attach themselves to the gustatory cells a signal passes to the brain which is instrumental in determining whether the shark rejects the meal or not. The taste organs are not as highly adapted as other shark senses as they do not play a role in locating prey. The exception to this may be in species such as nurse sharks that possess whisker-like barbells protruding from the upper lip that can be raked through the sand possibly to taste for a meal.
There has been a lot of speculation on the bite and release behavior exhibited by white sharks. Repeatedly in attacks on humans white sharks have been noted to take an initial chomp after which they often swim away. Partially due to this behavior 3/ 4 of white shark attacks are not fatal. One theory that may explain this is the idea that once the shark tastes the person and determines that it is not the high fat mammal that it was expecting, it does not waste its time eating and trying to digest a meal which will not provide the energy that it needs. Contradicting this is evidence that occasionally white sharks do partially or completely devour humans. An equally reasonable theory suggests that white sharks take an initial debilitating bite and leave the victim to bleed to death. This makes sense for a shark that preys on sea lions that possess a formidable set of teeth of their own.
Porbeagle shark jaw used for feeling as well as tasting
Whether sharks actually hear in the same sense that we do is unclear. The ears of sharks are completely internal. They are embedded within the chondrocranium (frontal skull). What looks like ear openings behind the eyes of sharks and rays are actually spiracles which primarily aid in respiration.
As in the inner ears of higher vertebrates the ears of sharks are responsible for balance and equilibrium. The inner ear of sharks consists of a series of ducts and sacs collectively known as the membranous labyrinth. These cavities are filled with endolymph which in sharks is mainly sea water that enters through the endolymphatic ducts. Around the membranous labyrinth is another series of fluid filled canals known as the cartilaginous labyrinth that are supplied through the perilymphatic ducts. The cartilaginous labyrinth protects and supports the more sensitive membranous labyrinth within. The sacculus is a large inner chamber into which the endolymphatic ducts lead. Within this is an area called the lagena that has been shown to receive sound waves in some fishes. Lining the walls of the sacculus are patches of sensory epithelium covered with sand grains and calcareous deposits known as otoliths. As the shark banks the endolymph fluid and otoliths lag behind slightly. Their swaying registers on nearby sensory hair cells that send signals to the brain resulting in sensations of imbalance that the shark then corrects. Imagine the importance of good balance in an environment where there are sometimes no visual cues at all to tell you which way is up.
At one time scientists thought that sharks may be able to hear with their entire bodies using their lateral line systems. This idea has since been discredited but the principle remains a reasonable one. Sound waves are after all little more than vibrations that fall within an animals auditory range. And, considering the importance of hearing it would make sense that such a successful group of animals as elasmobranchs would make use of it in one way or another.
Sharks have fairly immobile pectoral fins that do not aid in locomotion but serve in guidance, stability, and lift, in the same manner as airplane wings.
All of a shark’s forward propulsion comes from its powerful caudal fin (tail fin). Most sharks have heterocercal tails (long upper lobe, short lower lobe) but the fastest swimmers such a s mako sharks, have crescent shaped tails because they provide the greatest speed. Makos and other lamnid sharks also have caudal keels; lateral ridges positioned slightly forward of the tail that provide extra stability and agility at great speed.
Although the momentum comes from the caudal fin, the body plays an important role in locomotion too. As sharks swim, they move their heads from side to side. The back and forth motion of the head creates areas of high and low pressure which the rest of its body essentially slips into. The tail then gives the final flick from within this low-pressure space allowing the shark to slide through the water with the minimum possible effort.
Rays have evolved a variety of methods to get from A to B. Some use their tails, some use their pectoral fins, and a few actually walk along the bottom.
Electric rays, guitarfishes, and sawfishes, have retained somewhat shark-like torsos with powerful muscles in their flanks, so they are able to use tail propulsion for forward momentum.
Eagle rays, mobulas, stingrays, and butterfly rays have vestigial, whip-like or non-existent tails that may be used for balance or defence, but do not have the power of propulsion. Instead, these families have highly flexible pectoral fins. They are able to roll their pectoral fins through the water either backwards or forwards, creating a pressure wave that pulls/pushes them slowly forward or backwards.
They can also flap their pectoral fins in essentially the same way that birds do.
Most skate species use the same method of propulsion as stingrays but many also have highly adapted pelvic fins that they use to ‘walk’ with. This is especially true among the leg skates.
Fish sink. Fish are denser than the water around them and therefore need help to stay motionless in the water column.
Most teleosts (bony fishes) have solved this problem by evolving swim bladders; gas filled chambers in their torsos that add positive buoyancy to help them remain neutrally buoyant.
The drawback of having a swim bladder, is the inability to quickly undergo significant depth changes. Time is required to ‘on/off gas’ so that the gas filled chamber does not collapse or balloon as the fish descends/ascends. This inability to maneuver makes them vulnerable to predation.
Sharks and rays do not have swim bladders. Instead, they possess highly enlarged, oil filled livers. Although their livers are less efficient than swim bladders for maintaining neutral buoyancy, there are advantages. Sharks are able to make fast vertical ascents without having to worry about the gasses in their bodies expanding too rapidly. This makes them more efficient predators.
The trade-off is that sharks also have to swim forward relying on the additional lift created by their pectoral fins to avoid sinking.
As sharks grow bigger, their weight increases significantly. This requires a lot more buoyancy, so the liver size of large sharks is often massive; up to a quarter of their overall body mass. Consequently, large mature sharks tend to look extremely stocky.
The pectoral fins of fusiform sharks are perfectly shaped to add lift and stability. In particular, oceanic wanderers such as blue sharks and oceanic whitetip sharks have evolved extremely long pectoral fins that help them glide with minimum effort.
It was once thought that the seemingly hydrodynamic shape of the hammerhead’s cephalofoil added extra lift as well, but recent flow chamber tests have shown that the hammer actually creates ten times more drag than having a traditionally pointed snout like other sharks (Gaylord, Blade, and Parsons 2020).
It is now thought that the hammer’s primary function is simply to spread their electro-receptors over a wider area to aid in locating prey under the sand. Some hammerheads also use their cephalofoils to pin prey (stingrays) to the seabed.
Sandtiger sharks (Odontaspidae) have developed a unique solution to the problem of negative buoyancy. Unlike other species, they are able to levitate in mid water, seemingly defying gravity.
To achieve this, sandtigers periodically swim to the surface and swallow air. This essentially turns their entire stomach cavity into a swim bladder; similar to the ones that bony fishes have. This solution allows them to hover near the reef without expending any energy. Less energy expenditure means less time spent hunting.
Their docile behavior (and ferocious looking dentition) has made sandtigers very popular in public aquariums where they seem content to remain static without bumping the glass.
Turbulence is a major factor in the quest for speed. All objects disturb the medium through which they pass. In air this a minor consideration (at most speeds) but water is 800 times denser than air, so the turbulence created is enough to slow a moving object to a crawl. As in air, when an object moves increasingly faster the drag increases at a compound rate until a speed is reached where the force required to increase the speed any further becomes impractical. The fusiform bodies of sharks are designed to cut cleanly through the water, but body shape alone does not explain some shark’s ability to exceed all expectations based on a size to speed ratio. So how do large sharks such as the Mako attain speeds fast enough to catch a fleeing tuna? The answer lies in the dermal denticles that are imbedded in the shark’s skin. Incredibly, the skin of many sharks has evolved to channel the water smoothly along the sharks body without creating back-eddies or waves which the shark would have to push against. These minute channels (known as riblets) are so efficient at reducing drag that racing yachts have successfully employed the same technology to increase their speeds.
The unmistakable jaw of a Great white shark. This large adult specimen measuring about 60cm across is perfectly constructed to do the job for which it was designed, i.e. to quickly sheer through the tough hide of pinnipeds causing devastating wounds and rendering them immobile. The jaw characteristics that support this diet include both the heavy built cartilaginous jaw itself that will not buckle under the enormous force exerted by the surrounding heavy musculature, and the wide based serrated teeth in both the upper and lower jaw. When closed these strong cutting tools meet like interlocking steak knives. In younger white sharks the lower teeth are narrower reflecting the difference in their more fish based diet.
There is no stereotypic shark jaw that is representative of the group as a whole. This is because of the vastly different diets and lifestyles of the various groups. Most sharks and rays roughly fit into the categories of ‘crushers’, ‘biters and shakers’, and grazers. Many sharks sit on the fence when it comes to a dietary group for example tiger sharks (which are obviously biters) will also happily slurp up a conch shell if they need some food. Some sharks such as horn sharks have even developed different teeth from front to back much like our own dentition allowing them to grasp and crunch as the food demands. Horn sharks constitute the taxonomic order Heterodontiformes and the family Heterodontidae, which literally means different teeth.
Many bottom feeding elasmobranchs live on hard shelled animals such as crabs and snails. If you can imagine how difficult it would be to eat a walnut (still in its shell) with a knife and fork, you can see the need for adaptation in order to utilize this source of nourishment. Just as we have developed nut cracking tools for that very specific purpose, so too have sharks and rays that depend on hard shelled organisms for sustenance. The teeth of most rays and some sharks have evolved into what appears to be a roughened pad that has high friction surfaces capable of hanging onto awkward smooth objects such as shelled mollusks. Close inspection reveals that these pads are actually made up of hundreds of tiny flattened teeth. This form of dentition is referred to as molariform.
Many of the worlds pelagic sharks have very sharp, pointed teeth that are specifically designed for piercing through fish in order to hold onto them as they struggle. The classic example of this dentition pattern is the Mako shark which has wickedly sharp, inwardly curving, spindle shaped teeth protruding from both jaws. The Mako hunts fast swimming tuna and other pelagic fishes. Its ability to punch through the skin of a fleeing tuna and hold tightly onto it as it weakens has an obvious benefit for this hunting strategy.
Some sharks have teeth designs that are more blade like as in the above white shark or the tiger. These teeth are perfect for slicing and are capable of severing large pieces of flesh and bone from a prey animal. Although the sharpness of the serrated teeth is often enough to completely remove chunks of flesh, these sharks also employ another strategy for tearing away food. Once the jaw is closed onto the desired area, the shark thrashes its head from side to side. The resistance of the water and the force of the shark’s head swinging, aid to saw through the remaining flesh allowing the shark to remove enormous crescents out of large prey animals. One of the times that this behavior becomes most evident is when viewing sharks feeding on a whale carcass. The sharks can be seen thrashing their bodies from side to side wildly at the surface while their jaws are latched onto the whale’s body.
All sharks are carnivores but not all hunt large prey. Ironically, the largest of the sharks and rays are plankton feeders. This includes the Whale shark, Basking shark, Megamouth shark, Manta ray, and Mobula rays. Each species has a different method for collecting plankton but the resultant supply is the same. Some of these species also occasionally supplement their planktonic diet with small fishes. Plankton filtering sharks do not require teeth to collect their food. Basking shark for example, possess a line of thick brush-like strands that cover the gill openings and keep any food from escaping (not unlike the baleen in some whales). These ‘gill rakers’ which get battered throughout the summer feeding season are shed during the winter months only to be regrown before the plankton blooms again in the spring. Interestingly, these plankton feeders possess rows of tiny residual teeth that at first sight appear useless. It has now been suggested that these tiny teeth are used to help the male gain purchase on the female during mating. It must be quite a sight to see two basking or whale sharks joined in such a way.
At the other end of the spectrum one of the smallest ‘biters’ in the shark world is the ectoparasitic cookiecutter shark. This little monster which matures at a little over 30cm, is responsible for the round hollows of flesh that are sometimes missing from large bony fishes and cetaceans. There are actually at least 3 members of the Dalatiidae (kitefin sharks) family that feed this way, the largest being the Kitefin shark itself (Dalatias licha) which reaches almost 2 meters in length. Most sharks have more pointed teeth in their lower jaws for grasping and sharper sawing teeth in the upper but this is reversed in the cookiecutters. It is assumed that the cookiecutter first bites down with the top jaw to gain a secure purchase. Then it slices upwards with the lower more knife like teeth while simultaneously twisting its body around. This maneuver cores out a neat plug of flesh rather like the indent left when scooping out a spoonful of ice cream.
Most carnivorous, terrestrial mammals are able to seize a prey animal with their canine teeth and then hold it below their paws while they rip off pieces of flesh. Sharks do not have the luxury of appendages to hold down their prey so they need to keep their teeth sharp enough to ‘saw’ cleanly through their lunch. They do this by replacing their entire set of teeth continuously throughout their lifetimes. There are pockets in the jaws just behind each tooth where new teeth ‘bud’ and as they grow, they move forward to replace the worn or missing teeth that fall outwards or are ripped away during feeding or mating. It has been estimated that many sharks can produce 20,000 (or more) teeth during their lives. It is this loss of teeth which often leaves clues to the culprit in shark attacks. Shark teeth are quite different from species to species so if a tooth is left at the scene of a bite, then the shark can be easily identified.
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