Smell

Sharks are able to smell a drop of blood in an Olympic sized swimming pool. Sharks have nostril like holes under their snout called nares. Each nare is split by a nasal cap into two openings. The olfactory sac contains a series of folded olfactory lamellae to increase the surface area, allowing the shark more opportunity to register the odour. Odours that pass over the olfactory lamellae stimulate the endings of neuro-sensory cells. Dissolved water molecules flow into the incurrent opening, pass over the olfactory lamellae, bind to the receptor cells which then send a signal to the brain, and then exit the olfactory sac through the excurrent opening. Nares are not used for respiration as they are separate from the mouth and throat. A shark will swim moving its head from side to side once it has picked up a scent and on the hunt, this allows the snout to move backwards and forwards through the scent helping the shark determine the direction of the source. If the shark loses the scent it will swim in a wide zig-zag pattern until the scent is picked up again. Some shark species are able to pump water over the snares while resting on the ocean floor. Some species, like the angel shark, rely less on smell and more on vision in order to locate prey and will have less sensitive olfactory sacs. Sharks don’t only use their sense of smell for hunting - they also release pheromones which signal their reproductive state and potential mates will use their sense of smell to locate that shark.

Touch

Most sharks nose an object before biting it to determine whether it is edible or not. Biting is also considering an extension of touch as well as taste. Sharks are also able to pick up distant touch through a series of interconnected canals that start at the back of their head and end on the upper lobe of the tail; this is called their lateral line. On the sharks head is the infraorbital canal (extends ventrally from behind the eye and along the snout), the supraorbital canal (passes above the eye towards the snout) that connects to the infraorbital canal and the supratemporal canal (leads back over the top of the head). Tiny pores that allow water to penetrate open the canals to the outside of the skin. As other animals move in the sharks surroundings, small waves are sent over the hairs along the lateral line canals which then send a signal to the brain. The sharks own movement will create waves that hit obstructions and return, this creates a vibration echo map for the shark and helps the shark determine whether there are any sick or injured animals around.

Taste

Sharks have small pits that contain rod-shaped gustatory sensory cells lining the mouth and throat. A signal of whether to reject or consume reaches the brain after the dissolved chemicals from the bitten object attach to the gustatory cells. Other shark senses play a more vital role in locating prey and are therefore more adapted than the taste organs. A very small amount of sharks rely heavily on taste, an example would be the Nurse shark which use their whisker-like barbells on the upper lip to rake through the sand to taste for a possible meal. Great whites require a high quantity of fat in their diet and that is why three out of four shark attacks on humans aren’t fatal, because the shark will take a bite then realize that there won’t be enough fat in the meal and swim away.

Vision

Requiem sharks have no eyelids over their eyes but rather a protective membrane covering called the nictitating membrane. The nictitating membrane is a toughened layer that rises from below the eye, covering it during feeding. Other sharks, like the Great White shark don’t have nictitating membrane but can roll their eyes back into their sockets, exposing a hardened pad, to protect their pupils. Their retina contains only rods (in most sharks), making sharks adapted to dim light. Some sharks do rely on vision for hunting and most species the eyes are well developed, large complex structures. Many shark species have a thin area of skin on top of the head leading directly to the pineal gland in the brain; this allows them to sense light and dark. They don’t possess lenses to focus the light so there is no shape perception, but the shark will use the differing light levels to plan its movement throughout the day. Many shallow water sharks are able to dilate their pupils to control the amount of light entering their eyes. Sharks are able to focus when the rectus muscles pull the lens closer or further away from the retina. A shark’s focus is different to higher vertebrates which have distorted lenses making them able to focus light from different distances. Sharks that live in the deeper, darker waters may lack the ability to stop light entering their eyes; for example sixgill sharks. About twenty shark species have rods and cones in their eyes, but whether they are able to see colour is questionable. The tapetum lucidum structure in their eyes is a layer of mirrored crystals behind the retina that can be adjusted to reflect light back into the retina, amplifying the strength of the image; this gives sharks an excellent vision in low light conditions. An interesting fact about Great White sharks is that they have been observed sticking their heads out the water and look at their surroundings, but it is not proven that they are adapted to site above water.

Electroreception

Ampullae of Lorenzini are sensory organs around the nose that pick up fluctuations in electric fields. They are large pores filled with a gelatinous substance; cylindrical canals which store the gelatinous secretions are connected to the pores. Sensory nerves that transports the electrical senses to the brain are found at the base of each pore. Sharks that actively hunt may have up to 1500 ampullae around their snout while the rest may have just a few hundred. Sharks like hammerheads rely on these organs to pick up electrical signals coming from prey hiding in the sand. The electrical sense of sharks takes over from sight or smell once they are close to their prey. Wounded animals give off erratic electrical signals causing sharks to react aggressively – this is the reason that sharks will continually bite and attack victims while remaining disinterested in the rescuers swimming next to the victim. It has amazed scientists how sharks can migrate extreme distances and the conclusion is that many senses are in use during navigation. One theory is that the combination of the saline water and the magnetic fields around the earth creates an electrical gradient that helps lead the shark in the correct direction.

Hearing

It is unclear whether sharks have the same hearing as humans. They have internal ears that are embedded within the chondrocranium (frontal skull). The spiracles which are used in respiration are often confused for ears as they are holes behind the eyes. Sharks use their ears to remain in balance and equilibrium. A sharks swaying is picked up on its sensory hair cells which will send signals to the brain, the shark will then correct the imbalance.