Every single physical feature on every animal has evolved over many millions of years, to be become better adapted to the environment... Sharks have become more streamlined to help them swim efficiently, they have developed very strong jaws to chomp through their prey and they have evolved incredible senses which allow them to detect minute signals in their environment... All of these adaptations are quite obvious, but have you ever wondered about the colour of sharks? Many large sharks are quite a similar colour; grey on the top, fading into a white belly. But why is this? What is the evolutionary incentive behind this pattern that makes it so ubiquitous?
What is "Countershading"?
Fishes that have a dark colour on their "dorsal side" ( back), which shifts into a lighter colour on their "ventral side" (underside) are said to have "countershading". This is a common feature, especially amongst fishes which do not live near to the ocean floor (aka "pelagic") (Rowland, 2009).
There are many different theories about why countershading evolved and what benefits it might offer. The most common is that countershading makes animals more difficult to see as they swim through the water, for several different reasons...
Does Countershading Counteract the 'Self-Shadow Effect'?
Whenever an animal swims through the water, light coming from above causes the animal's own shadow to be cast on the underside of their body. This is known as the "self-shadow effect" (Rowland, 2009).
Having a lighter colour on their underside compared to their back, allows these animals to counteract the self-shadow effect and makes their 3-dimensional shape look more flat in the water. This can make them much more difficult to see (Rowland, 2009).
Is Countershading for Matching the Background?
When you are underwater and look up you can see the light filtering through from the surface, but when you are at the surface looking down, deep water looks very dark. This is because light can only penetrate so far through water (Rowland, 2009).
It is thought that pelagic fishes like sharks might be countershaded so that they become less visible against the background. From the bottom looking up, the white belly is harder to see in silhouette against the bright surface and from the surface looking down the darker colouring on their back blends in with the shadowy depths. Once again this makes the shark harder to see (Rowland, 2009).
Why is it Useful to be Invisible?
There are different reasons why being difficult to see can be handy, depending on the shark's niche in their ecosystem.
A predatory shark would find this camouflage very useful when hunting because it would make them invisible to their prey. For example, the great white shark (Carcharodon carcharius) approaches it's prey from great depths (where it cannot be seen thanks to it's countershading) and rushes up to take a seal at the surface (Klimley et al, 1996).
Comparatively, countershading camouflage is also very handy to sharks lower down on the food chain. Many sharks are themselves prey to higher predators, but their background matching allows them to hide from hunters. For example, smaller sharks like blacktip reef sharks (Carcharhinus melanotperus), which have many natural predators, would be harder to see from both above and below thanks to their countershading, making it easier to hide (Rowland, 2009).
Does Countershading Protect from UV?
More recently, it has also been suggested that there might be another reason why some species of sharks are countershaded... some scientists wonder if it helps to protect them from ultraviolet light (UV) damage (Burtt, 1981). Just like human beings, if sharks get too much sun it can be bad for their health! We know that some sharks have evolved some natural UV defences, with some species developing more melanin in their skin when they are exposed to UV (just like us getting a tan!), which helps to protect their skin from burning. So it is possible that countershading - with darker skin on the back where it is more exposed to the sun - has evolved as a type of natural sunscreen (Burtt, 1981; Lowe & Goodman-Lowe, 1996; Wilson & Martin, 2004).
For example, the dark back of juvenile scalloped hammerhead sharks (Sphyrna lewini) gets darker in response to high UV light. Scientists think this has evolved because it protects them from sun damage during their younger years, spent living in shallow water (Lowe & Goodman-Lowe, 1996; Wilson & Martin, 2004).
On the other hand, there are examples where this theory does not seem to fit... For instance, the whale shark (Rhincodon typus) spends much of it's life right at the surface of the water, where exposure to UV is high. These sharks have countershading within their beautiful spotted pattern, and whilst the darker regions would have melanin sunscreen, the lighter regions right next to these would not. This suggests countershading does not offer UV protection in this species. But counteracting the self-shadow effect and matching the background also do not seem to make sense for the whale shark - why would an 18 metre long, filter-feeding shark need camouflage?
Is Countershading a Vestigial Trait?
Scientists now wonder if countershading actually has no function in some species of sharks! Sometimes certain traits were useful in the past, but as the animal evolved into a new species, are no longer useful. Evolution will pressurise traits to disappear if they have a negative effect on the animal, but if the trait has no effect at all (neither a help or a hindrance) then sometimes the trait remains. This is known as a "vestigial feature". The appendix in is a vestigial organ in human beings, for example (Wilson & Martin, 2004).
We may never know exactly why so many different sharks are countershaded. It could well be a combination of all of the reasons mentioned and / or offer different advantages to each individual species... That is the wonder of evolution... There is so much that we do know, and there is so, so much more that remains (and may always remain) a complete mystery!
References
Burtt EH (1981). The adaptive-ness of animal colors. BioScience, 31: 723–729.
Klimley AP, Pyle P. & Anderson SD (1996). The behaviour of white sharks and their pinniped prey during predatory attacks. In: Klimley, A.P. & Ainley, D.G.(Eds.). Great White Sharks The Biology of Carcharodon carcharias. Academic Press, San Diego, p. 175-191.
Lowe C & Goodman-Lowe G (1996). Suntanning in hammerhead sharks. Nature. 383:6602, 677. Access online.
Rowland HM (2009). From Abbott Thayer to the present day: what have we learned about the function of countershading? Philosophical Transactions of the Royal Society B: Biological Sciences, 364:1516, 519–527. Access Online.
Wilson SG & Martin AR (2004). Body markings of the whale shark, vestigial or functional. West. Aust. Nat, 124, 118–134. Access Online.
By Sophie A. Maycock for SharkSpeak
