Seeing Spots
Updated: Dec 30, 2021
Whale sharks (Rhincodon typus) are undeniably beautiful. The ornate patterns of spots and stripes on their skin make them immediately recognisable and particularly charismatic. It is the reason that whale sharks and their relatives were dubbed the 'carpet sharks'. But, besides being lovely to look at, what is the function of these markings? What purpose do they serve?
The process of evolution involves random mutations (aka changes in the genetic code), which confer some kind of physical change to how the organism lives. Sometimes these mutations create positive changes; meaning they are more well adapted to their environment. Therefore, these individuals tend to do well and go on to produce many offspring and pass their excellent DNA on to them. On the other hand, sometimes mutations bring about negative effects; meaning the animal is poorly adapted to the environment. These individuals either will not survive or will not be able to breed, meaning their maladaptive mutation dies out with them. Over many millions of years, this process eventually cause animals to change to be more well suited to the environment. Thus, evolution does not make pretty patterns on the skin just for the fun of it - every feature must serve some purpose.
Whale sharks have a white underbelly, and intricate patterns of blue / grey colouration, broken up by checkerboards of white spots and stripes on their back. There are several different theories about why whale sharks have such intricate patterns on their skin...
Firstly, it is thought that these markings might be advantageous because they camouflages the shark against the watery background. Despite being the largest fish in the ocean, whilst they are young and small, whale sharks are very vulnerable to predation - many other species of shark will snack on whale shark pups! So, their mottled colouration probably offers them some protection (Wilson & Martin, 2003).
Whatsmore, the light belly and darker back means that whale sharks are difficult to see from both above (against the dark background of the ocean depths) and below (against the bright background of the surface). This is known as "countershading" and is a feature of many sharks, as it allows them to blend in to their surroundings (Wilson & Martin, 2003).
It is also thought that the skin patterns might be useful for communication between whale sharks (aka "visual cues"). Many sharks recognise other members of their species based on markings on their fins, and others use the markings to recognise members of the opposite sex, when looking for a mate. So it is likely that whale sharks are able to use the markings to recognise individual sharks, much like how we use height, hair colour and eye colour to recognise people we know (Wilson & Martin, 2003).
Furthermore, it is also possible that whale sharks also use their patterns for more complex communication; like "threat displays". Animals which do not make many sounds, often use visual displays to let others know they are annoyed. For example, when whale sharks encounter each other and are trying to use the same resources (like a good patch of food), they will bend round to show their competitor their back and force them to swim in tight circles until they flee. Usually the larger shark is the victor. Scientists think this is a way of communicating a threat without actually fighting and, as the sharks show their patterned side, the markings are probably a way of communicating their impressive size to a smaller individual (Wilson & Martin, 2003).
Whale sharks are born with their beautiful markings and as they age, the patterns change very little. In fact, these patterns are as unique to each whale shark as a human fingerprint (Wilson & Martin, 2003).
In the whale shark's close relatives, like the zebra sharks (Stegostoma fasciatum), the juvenile animals have very different skin patterns compared to the adult sharks. It is thought this metamorphosis (known as an "ontogenetic change") happens because the two different life stages occupy very different habitats; the young animals in very shallow, coastal waters, and the adults in deeper waters around reefs. As whale sharks occupy similar habitats as juveniles and as adults (out in the open ocean), this could explain why their skin patterns do not change much as they age (Wilson & Martin, 2003).
But what is great for us, is that these consistent patterns allow scientists to identify individual whale sharks! Images of whale shark skin patterns can be loaded into software very similar to that which is used in forensics when examine fingerprints. This allows scientists to create databases with known whale sharks' "bodyprints", which can be used to identify an animal when it is seen again at a later date. This means we are able to track individuals around the world as they undergo their migrations, monitor them as they grow, and even track changes in abundance.
There is certainly a lot more work to be done to be sure what the function of whale sharks' markings are and it is very possible we will never know... and that is one of my favourite things about these animals, and marine biology in general... It is a wonderfully mysterious world and we can only hope to touch the surface when trying to understand their bewildering and alien secret lives.
If you would like to help scientists learn more about whale sharks, you can submit any sightings and images of these animals (lucky you!) at WildBook for Whale Sharks.
And don't forget to celebrate International Whale Shark Day every 30th August!
References
Wilson SG & Martin RA (2003). Body markings of the whale shark: Vestigial or functional? Western Australian Naturalist.
By Sophie A. Maycock for SharkSpeak.