Having undergone several hundred million years of evolution - honing their bodies and brains to perfection - sharks are magnificently well-adapted predators. One of the most mind-blowing facts about sharks is that they have seven senses - two more than human beings. This means that sharks can absorb a huge amount of information about the world around them and a large proportion of their brain is devoted to constantly absorbing and processing this information, so that they can respond accordingly. So what senses to sharks have? Do sharks perceive the world in a similar way that we do? And how do their unique sensory capabilities work?
The Magnificent Seven
It's often said that sharks have seven senses and whilst this is true, it is also an oversimplification. In reality our senses are not always completely distinct - we taste at smell at the same time and we can feel falling both with our ears and by the movement of our bodies, so do we feel it or hear it!?
Sharks have all the same senses that human beings are familiar with: "Chemoreception" in the form of taste and smell, "mechanoreception" via touch and hearing, and "photoreception" aka sight. However, they also have two additional sensory organs that allow them to detect other stimuli: their "lateral line" and their "ampullae of Lorenzini" mean they can detect pressure and electromagnetism respectively (Gardiner et al, 2014; Abel et al, 2020; Ebert et al, 2021).
That means that there are things in the environment that are completely invisible and undetectable to us, but sharks would be able to perceive them. How mind-boggling is that!?
The scale over which sharks are able to detect different things in their environment depends on the sense. This means that they use some of their senses over long ranges and others for shorter distances (Gardiner et al, 2014; Abel et al, 2020; Ebert et al, 2021).
Hearing
The longest range sense that sharks use is their hearing because sound travels much frther in water than air. You might be surprised to hear that sharks have ears, but they do, and they can hear very well. You could be forgiven for not noticing them though because the external openings - called "endolymphatic glands" - are only very tiny holes that are on the top of their heads (Casper, 2006; Abel et al, 2020).
The larger openings on the side of some shark's heads, behind their eyes, are not their ears. They are called "spiracles" and are actually involved in respiration (Casper, 2006)
There are two different components to sound: pressure waves (far field sound) and particle displacement (near field sound). Sharks can detect both components using different organs (Casper, 2006).
Thanks to their ears, sharks can hear sounds between 10-100Hz, but are most sensitive to low frequency sounds, like those produced by a struggling fish. It's thought they can hear these sounds as far as 1.5 kilometers away and they can be attracted to sounds in the range of 20 - 1000 Hz over hundreds of metres (Casper, 2006).
Smell
The second long-range sense that sharks use is their sense of smell, also known as "olfaction". Sharks have a remarkable sense of smell compared to our's and can detect odour plumes from hundreds of metres away (Abel et al, 2020; Ebert et al, 2021).
Sharks are able to detect an odour particle to several million or billion parts water - the equivalent of one drop of blood in an Olympic sized swimming pool. But it is important to note that their olfaction is actually not really significantly better than many other species of fishes (Abel et al, 2020; Ebert et al, 2021).
How far away they can smell something depends on the strength of the signal (how much of the smell is being produced at the source), the distance it can travel (if there are strong currents or not) and how much the odour disperses (directional currents that transport the odour a long way, versus mixing currents that spread and diffuse the odour particles in multiple directions). Generally sharks can smell up to a kilometer away (Abel et al, 2020; Ebert et al, 2021).
Sharks' nostrils are called "nares". They have two separate openings, so water flows in one way and out the other, so they can effectively smell things twice. They detect odours via their "olfactory sacs" inside the nares. The distance between these openings varies between species, but longer distances make for a heightened ability. The longest are found in hammerheads, thanks to their exaggerated, extended head, known as a "cephalofoil", so they have an especially excellent olfactory abilities (Abel et al, 2020; Ebert et al, 2021).
Water Pressure Detection
Sharks and other fish are able to sense vbrational particle displacement via their "lateral line". This sensory organ, clearly visible down each side of their body, is made up of specialised hair-like cells called "neuromasts" that are fixed just below the skin. Displacement of these cell by moving particles allows sharks to sense a kind of vibrational touch about 15 metres around their bodies (Abel et al, 2020; Ebert et al, 2021).
This allows sharks to sense the second component of sound - particle displacement - and also means they can feel water displacement and changing pressure. So they basically feel a fish trashing and moving the water from several metres away (Abel et al, 2020; Ebert et al, 2021).
This sense even allows some species to detect the change in barometric pressure associated with hurricanes, so they can get out of the way of the oncoming storm. To learn more check out The Eye of the Storm.
Sight
Over relatively short ranges - approximately 25 metres depending on the conditions - sharks use their sight. In fact, many species of sharks, including the great white (Carcharodon carcharias) are primarily visual predators and favour their sight to hone in on their prey when they get close (Abel et al, 2020; Hart, 2020; Ebert et al, 2021).
The anatomy of sharks' eyes is very similar to that of many other vertebrates, including us - they have a clear region at the front of the eye where light can pass through (known as the pupil) and there are photosensitive cells on the back of the eye (the retina), which detect light and transmit the signal to the brain (Abel et al, 2020; Hart, 2020; Ebert et al, 2021).
Sharks also possess both "rod" and "cone cells" in their retina, as we do, which are capable of detecting photons and different colours respectivley. Many sharks, especially deep-sea species, that live in dark environments, have an especially high concentration of rod cells, so they can maximise the amount of light they are able to detect. Sharks also have a structure called a "Tapetum Lucidum" behind the photosensitive region at the back of the eyes. This membrane reflects light after it has passed through the retina, to bounce it back through the photosensitive cells a second time. This maximises the amount of light they can see. It is also what makes their eyes glow in the dark - the same as cats' do (Abel et al, 2020; Hart, 2020; Ebert et al, 2021).
Human beings have three different types of cone cells for detecting different colours. However, how many cone cells sharks have and therefore how much colour they see, varies between species. Many rays and reef sharks have a higher variety of cone cells, to allow them to see the full range of different colours in their habitats. However, some sharks only have one type of cone, so have limited colour vision (Abel et al, 2020; Hart, 2020).
Sharks eyes also differ in how they focus. They have spherical lenses in their eyes. In human eyes, the lenses are more ovoid and muscles attached to each lens, stretch it to varying degrees in order to focus. In sharks the lens does not change shape. Instead muscle contractions actually move it forwards or backwards within the eyeball in order to focus near or far away (Abel et al, 2020; Hart, 2020; Ebert et al, 2021).
As shark's eyes are located on the side of their head, they have almost 360 vision. This is especailly pronounced in the hammerheads thanks to thelarger distance between the eyes created by the "cephalofoil" (hammer). Sharks only blinds spots are directly in front of their snout and behind their tails. Yet there is also a region of cross-over in front of them - where both eyes can see - allowing them binocular vision and excellent depth perception (Abel et al, 2020; Hart, 2020; Ebert et al, 2021).
Electromagnetic Sensitivity
Over intermediate ranges, sharks rely on their supersense - electrosensitivity - especially when they are hunting. Sharks, rays and chimaeras (and only very few boney fishes) have specialised organs called the" ampullae of Lorenzini". These gel-filled pores - clearly visible as black dots scattered around the snout and face - can sense minute electrical currents (Kempster et al, 2009; Abel et al, 2020; Ebert et al, 2021).
As they can detect as little as 0.1 micro volts per cm, this means sharks can sense static build up and biological electrical fields, like those generated by muscle contractions. The electrosense is especially important for smaller, bottom-dwelling species that have a blind spot below them because their eyes are situated on the top of their heads. It allows sharks to track down their prey even if it busied under the sand, out of sight. It is also very useful if they live in turbid water where visibility is poor (Abel et al, 2020; Ebert et al, 2021; Pinheiro & Reis, 2024).
The electrosense also has a role in predator avoidance, because the ampullae can detect when an animal is swimming past. Bamboo shark embryos are able to use this sense to evade predators before they have even hatched out of their egg case (Kempster et al, 2009). For more detail, you can check out He Can't See Us If We Don't Move.
The ampullae of Lorenzini are also thought to sense magnetic fields and geomagnetism, so they have some role in navigation, but this is still being studied. To learn more see Animal Magnetism.
Taste
Just like us, sharks can taste things in their mouths. This is important because it allows the shark to determine if they should eat something or not. For instance, besides the common misconception, great white sharks (Carchcharodon carcharias) are actually very picky eaters and will spit things out if they don't like the taste. Any prey which is too low in fat or protein is simply not worth their time (Abel et al, 2020).
However, unlike us, their mouths and nostrils are not connected, so sharks' senses of taste and smell are more distinct that our's (Abel et al, 2020; Ebert et al, 2021).
Sharks also do not have a moveable tongue like we do, which aids to focuses our taste buds. Instead the taste buds are spread throughout the mouth and most densely concentrated behind the teeth (Abel et al, 2020; Ebert et al, 2021).
Touch
Finally, like many living creatures sharks are able to sense when something makes contact with their skin - they know when they are being touched and they feel pain, just like we do. This means sharks can discern when something might injure them and they can feel when they are reaching their "thermal tolerance", so they avoid entering (or remaining in) waters that could cause them physiological damage because they are too hot or too cold) (Abel et al, 2020; Ebert et al, 2021).
This is where the senses start to get a bit criss-crossed because mechanoreception also occurs in a shark's (and our!) ears. The displacement of fluids within a the inner ear allow sharks to perceive their orientation and assess their body's position, in order to maintain equilibrium and balance (aka "proprioception"). In fact, in reality none of the senses act alone, but instead they all work continuously in cohert to create a cohesive picture of the outside world (Abel et al, 2020).
Thinking & Feeling
Every single nanosecond, a shark's brain is processing all this incoming sensory information in order to assess the environmental conditions and bring about a behavioural response. You might not have ever thought about it before, in our modern world, but this is actually a matter of life and death! The senses help to avoid harm, track down food and locate a safe habitat. Therefore, (like us!), huge proportions of a shark's brain is devoted to receiving and processing sensory information... There is a reason these amazing animals are sometimes known as 'swimming noses' though - in some species as much as 2/3 of the total mass of their brain is devoted to smell alone (Gardiner et al, 2014; Yopak et al, 2015). What a richly smelly world it must be for sharks.
References
Abel CD, Grubbs RD, Pullen E & Dando M (2020). Shark Biology and Conservation: Essentials for Educators, Students and Enthusiasts. Johns Hopkins University Press, ISBN: 9781421438368.
Casper BM (2006). The hearing abilities of elasmobranch fishes. University of South Florida Tampa Graduate Theses and Dissertations. Access online.
Ebert DA, Dando M & Fowler S (2021). Sharks of the World: A Complete Guide. Wild Nature Press, ISBN: 9780957394605.
Gardiner JM, Atema J, Hueter RE & Motta PJ (2014). Multisensory integration and behavioral plasticity in sharks from different ecological niches. PLoS One, 9:4, e93036. Access online.
Hart NS (2020). Vision in sharks and rays: Opsin diversity and colour vision. In Seminars in Cell & Developmental Biology, 106, pp 12-19. Academic Press. Access online.
Kempster R, Hart Nathan & Collin S (2009). Survival of the stillest: predator avoidance in shark embryos. PloS One, 8, e52551. Access online.
Pinheiro MH & Reis RE (2023). High resolution in turbid waters: Ampullae of Lorenzini in the Daggernose Shark Carcharhinus oxyrhynchus (Valenciennes, 1839)(Elasmobranchii: Carcharhinidae). Journal of Fish Biology. Access online.
Yopak KE, Lisney TJ & Collin SP (2015). Not all sharks are “swimming noses”: variation in olfactory bulb size in cartilaginous fishes. Brain Structure and Function, 220, 1127-1143. Access online.
By Sophie A Maycock for SharkSpeak
