True Colours

Sharks and their relatives live in a huge range of dynamic and complex habitats, in which their array of incredible senses are continuously monitoring and assessing their world. On top of taste, touch and hearing, sharks have the added ability to detect electromagnetic fields, changes in water pressure and to smell over incredible distances, in order to identify threats and opportunities. And - despite what you may have been told - sharks have very well developed eyes and excellent vision. Yet, exactly how shark vision really works and whether they can perceive different colours, is one of the greatest mysteries baffling scientists. So, how good is a shark's eyesight? Are they colour blind? And if not, what colours can they see?

Scientists suspect that some sharks are able to perceive colour, but maybe not necessarily in the same way that we do (Image Credit: mr_tigga / Shutterstock)

Feel Your Way

Sensory systems allow organisms to sense and interpret signals that are available in the world around them, allowing them to navigate, avoid obstacles, assess habitat quality, detect prey, find mates and avoid potential threats. These signals might be mechanical (like touch, hearing and balance) or they may be chemical - like smell (aka "olfaction") and taste (aka "gustation"). To learn more about sharks' incredible array of different sensory systems, check out Sixth Sense  (Ritter, 2004; Collin et al 2015; Abel & Grubbs, 2020).

Sharks use their vision to catch prey, avoid predators and to navigate around their environment (Image Credit: NatalieJean / Shutterstock)

Photo Opportunity

Vision is a form of "photoreception" - the ability to perceive light. Believe it or not, the eyes are not the only organs able to detect light. In fact, sharks (along with many other creatures, including us!) have a specialised part of their brain, known as the "pineal gland", that can sense light and they also have light-sensitive cells (aka "photoreceptors") embedded in their skin. To learn more about this, check out Light It Up (Collin et al 2015; Abel & Grubbs, 2020; Ebert et al, 2021).

Just like us, a shark's eyes are the organs responsible for "image-forming photoreception" (aka vision). Contrary to common belief, sharks actually have very good eye sight and rely hugely on their vision. In some species their contrast vision is incredibly acute - much more so than many other animals! In fact, the majority of sharks - especially those that live in clear waters - are primarily visual predators over close ranges. For example, white sharks (Carcharodon carcharias) rely almost exclusively on their sight in order to hone in on their prey close-up (Ritter, 2004; Meredith et al, 2022).

Hammerhead sharks' eyes are positioned at each end of the cephalofoil, so they have very wide visual fields (Image Credit: Martin Voeller / Shutterstock)

Eyes in the Back of My Head

Sharks have a very wide visual field. As their eyes are located on the side of their heads sharks can see above and below them, and some can even see behind the back of their heads! This means that they only have a small, funnel-shaped blind spot near their second dorsal fin that widens towards their tail. Many (but not all) species also have a visual field overlap in front of their faces (known as the "funnel of focus") where both eyes can see, so they have binocular vision. The largest binocular overlap is found in the fastest swimming species, allowing them to be more accurate and nimble (Abel & Grubbs, 2020).

The shape of the cephalofoil affects how wide each hammerhead species' visual field is, with the widest in those with longer cepahalofoils (Image: McComb-Kobza, 2009)

The width of the visual field depends on the species, as each different type of shark will have a different head shape, and neck flexibility, and the position of the eyes will vary in order to maximise the efficacy of their vision in their particular habitat. For example, in wobbegongs (Family Orecolobidae) and angel sharks (Order Squatiniformes) the eyes are directed upwards, so that they can see far and wide whilst they are lying in wait for prey on the ocean floor (Ritter, 2004; Abel & Grubbs, 2020; Ebert et al, 2021).

Without a doubt the widest visual fields are found in the hammerheads (Family Sphyrnidae). All of these sharks have almost 360 degree vision and five species have been shown to have no blind spots behid them at all. Additionally, the winghead shark (Eusphyra blochii) has a four times larger binocular overlap in front of them compared to other requiem sharks (Order Carcharhiniformes). This is thanks to the position of the eyes right out on the end of that iconic "cephalofoil" (McComb-Kobza et al, 2009; Abel & Grubbs, 2020; Ebert et al, 2021).

The size and position of the eyes varies depending on the habitat and lifestyle of the different species of sharks e.g. a. sandtiger, b. sicklefin lemon shark, c. western wobbegong, d. Port Jackson shark (Source: Lisney et al, 2012)

Cat's Eyes

Just like in humans, the "duplex retina" at the back of the eye is where all the magic happens. This tissue layer contains two types of specialised photoreceptors that can detect different types of light; "rod cells" and "cone cells" (Ritter, 2004; Abel & Grubbs, 2020; Hart, 2020; Ebert et al, 2021; Meredith et al, 2022).

Under a microscope we can see the photoreceptor cells in the retina of the bull shark (Source: Hart, 2020)

Rod cells function under dim light conditions, and as sharks have so many of them, they are able to see with great acuity even when it is relatively dark. Increasing this contrast sensitivity is a special layer behind the retina called the "tapetum lucidum". This membrane reflects light back through the retina, so that sharks get a second chance to detect all the light that they possibly can. Also found in cats and other nocturnal mammals, it's the tapetum lucidum that makes it look like sharks' eye are glowing in the dark when a torch is shone on them (Ritter, 2004; Abel & Grubbs, 2020; Ebert et al, 2021; Meredith et al, 2022).

Great whites rely heavily on their vision for hunting, so when they feel t risk, they will roll their eyeball back in their head in order to protect the delicate structures (Image Credit: Bernard DUPONT / WikimediaCommons)

Cone Off

Cone cells assist in the detection of light in dim conditions, but in many species (including us!) they also perceive colour when conditions are brighter, thanks to the presence of visual pigments. As each type of cone cell has a single specific pigment, they can only detect certain wavelengths of light, that is to say, they only sense specific colours, like red or blue. This means there will need to be several different types of cone cells present in the retina in order to see all the colours of the rainbow (Collin et al 2015; Meredith et al, 2022).

The retinas of rays contain rod cells to detect dim light, and also two different types of cone cells to detect different wavelengths of visible light (aka colours) (Source: Hart, 2020)

In many species of sharks, we only find one type of cone cell in the retina, so we assume that these sharks are colour blind (aka "monochromats"). However, from looking at their retinas, we do know that some species of rays (close relatives of the sharks) are "dichromats" or "trichromats". For instance, ocellate river stingrays (Potamotrygon motoro) possess two different cone cell types and they are definitely able to distinguish between colours in field trials. So they are certainly not colour blind (Hart et al, 2011; Brown & Schuessel, 2023; Meredith et al, 2022).

Ocellate river stingrays are able to see multiple colours (Image Credit: Karelj / WikimediaCommons)

Out of the Blue

But this is where things get really interesting... Whether cone cells are actually responsible for the observation of colours in sharks and their relatives is up for debate and there is some dispute about whether some monochromats are actually, truly colour blind (Lisney et al, 2012; Meredith et al, 2022).

As sharks only have one type of cone cell in their retinas, the wavelengths of visible light they can detect is limited, so scientists have presumed they are colour blind (Source: Hart, 2020)

The contention comes from scientists finding that some species of sharks seem to be able to respond to different colours despite the fact that they do not have the range of cone cells necessary for colour vision. In fact, there are many examples of sharks responding to different colours: sandtiger sharks (Carcharias taurus) can discern between nets of different shades and bull sharks (Carcharhinus leucas) are known to avoid bright yellow nets. Similarly, many species of oceanic sharks are highly attracted to brightly coloured objects or equally repelled by certain colours (Myrberg, 1987; Bres, 1993; Hart et al, 2011).

Despite being monochromats, silky sharks avoid taking baits from orange hooks (Image Credit: Marc Taquet / WikimediaCommons)

But how can these sharks perceive colour without the cone cells necessary?

Scientists now suspect that sharks' perception of coloured light might be completely different to ours. Rather than cone cells activating, scientists wonder if there may be some kind of cognitive processing that humans lack, that allows sharks to see in colour, despite being monochromats. This would mean that their light detection and visual mental processing systems follow a completely different path to ours, and our understanding of how their vision works is massively limited. It is a wonderful mystery that is yet to be solved. We'll just have to watch this space... (Lisney et al, 2012).

The shape of a shark's pupil depends on the species; it may be round, oval, or a vertical or horizontal slit (Image Credit: Kurit afshen / Shutterstock)

References

Abel DA & Grubbs RD (2020). Shark Biology and Conservation: Essentials for Educators, Students and, Enthusiasts. Johns Hopkins University Press: USA.  

Bres M (1993). The behaviour of sharks. Reviews in Fish Biology and Fisheries, 3, 133-159. Access online.

Brown C & Schluessel V (2023). Smart sharks: a review of chondrichthyan cognition. Animal Cognition, 26:1. Access online. 

Collin SP, Kempster RM & Yopak KE (2015). How elasmobranchs sense their environment. In Shadwick RE, Farrell AP & Brauner CJ (Eds.). Fish Physiology (Volume 34). Academic Press: UK, pp 19-99. Access online.

Ebert DA, Dando M & Fowler S (2021). Sharks of the World: A Complete Guide. Princeton University Press: USA. 

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.

Hart NS, Theiss SM, Harahush BK & Collin SP (2011). Microspectrophotometric evidence for cone monochromacy in sharks. Naturwissenschaften, 98. Access online.

Lisney TJ, Theiss SM, Collin SP & Hart NS (2012). Vision in elasmobranchs and their relatives: 21st century advances. Journal of Fish Biology, 80:5, 2024-2054. Access online.

McComb DM (2009). Visual adaptations in sharks, skates and rays. Florida Atlantic University Doctoral Thesis. Access online.

McComb DM, Tricas TC & Kajiura SM (2009). Enhanced visual fields in hammerhead sharks. Journal of Experimental Biology, 212:24. Access online.

Meredith TL, Kajiura SM, Newton KC, Tricas TC & Bedore CN (2022). Advances in the sensory biology of elasmobranchs. In: Carrier JC, Simpfendorfer CA, Heithaus MR & Yopak KE (Eds.). Biology of Sharks and their Relatives, third Edition. CRC Press: USA. pp. 143-176.  

Myrberg Jr AA (1987). Understanding shark behavior. In: Cook S (Ed.), Sharks: an inquiry into biology, behavior, & fisheries, and use. Proceedings of the Conferences Portland, Oregon, pp. 41-83. Access online.

Ritter E (2006). Understanding Sharks: The Fascinating Behavior of a Threatened Hunter. Krieger Publishing Company: USA. 

By Sophie A Maycock for SharkSpeak