"MORE THAN MEETS THE EYE" by PI Clemency White

Sharks are pretty well-known for having fine-tuned sensory systems. More often than not, the first sense that you’d associated them would be smell; we’ve all heard exaggerated tales of a frenzied shark response to a single drop of blood in a Pacific Ocean of water… or something like that. But the true wonder of shark sensory systems is often sold short. As the title of Dr. Kara Yopak and colleagues fascinating 2015 paper states: “not all sharks are ‘swimming noses.’”

One sense that is widespread in its importance for sharks is vision. By maturity, all sharks have developed eyes that allow them to exploit the optic cues of their environment. The design of sharks eyes is not hugely dissimilar from the human eye: it comprises a cornea, iris, retina and lens. In additional, sharks possess a tapetum lucidum – something you may be familiar with if you have a pet cat. The tapetum lucidum is a layer of crystals that sits just behind the retina and reflects light back through the lens and out of the eye after initial entry. This allows sharks in low-light environments to get twice the amount of light from a single lumen than an eye without this feature, and is what gives cat’s eyes their glow in the dark.

[Photo by Chelle Blais]

Considering that sharks as a taxonomic group are found in a huge variety of environments – from the deep sea to murky estuaries, coral reefs to seamounts and mangrove nurseries to polar oceans – they also cover a huge array of different light environments. This is reflected in the vast array of different eye shapes, sizes and features possessed by sharks and rays. This diversity also indicates different function. While all sharks possess visual capabilities, they are often using it for different behaviours.

For example, while the release and detection of pheromones through the sense of smell plays a major role in mating displays in sharks, females of some species – such as the nurse shark, Ginglymostoma cirratum, will also give visual cues that they are receptive to mating such as arching of the body and cupping on the pelvic fins.

The white shark, Carcharadon carcharias, has been shown to demonstrate feeding behaviours linked to the amount and direction of light. Sharks will preferentially approach targets when the sun is directly behind them, which both makes the prey easier to detect and themselves as a predator harder to see. Juveniles will also dive deeper when there is a full moon, suggesting that they depend on the amount of available light to hunt at depth.

Even within a species, vision can change across an individual’s life history. This is true for the lemon shark, Negaprion brevirostris, which undergoes a shift in the visual pigments with ontogeny. As they shift from immature to mature, they also shift from bright, shallow nurseries to deeper open oceans with fewer visual features. This showcases the remarkable plasticity of sensory systems with ecology.

Now you know that sharks are watching you, you’ll never look at them the same way again!

[Photo by Chelle Blais]

References and further reading:

Yopak et al. (2015) Not all sharks are “swimming noses”: variation in olfactory bulb size in cartilaginous fishes. Brain Structure and Function. 220(2), pp.1127-43.

Claes et al. (2009) Bioluminescence of sharks: first synthesis.

Huveneers C, Holman D, Robbins R et al. White sharks exploit the sun during predatory approaches. Am Nat 2015; 185: 562–570.

Gardiner JM, Atema J, Hueter RE et al.Multisensory integration and behavioral plasticity in sharks from different ecological niches. PLoS ONE 2014; 9:

Pratt HL Jr, Carrier JC. A review of elasmobranch reproductive behaviour with a case study on the nurse shark, Ginglymostoma cirratum. Environ Biol Fishes 2001; 60:

Collin, SP (2018) Scene through the eyes of an apex predator: a comparative analysis of the shark visual system. Clinical and Experimental Optometry. 101:5, 624-640.

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