Seeing Spots
Updated: 18 hours ago
Whale sharks (Rhincodon typus) are undeniably beautiful. Their ornate markings of spots and stripes make them immediately recognisable and particularly charismatic. These markings are the reason that whale sharks and their relatives are commonly called the 'carpet sharks'. But, besides being lovely to look at, what is the function of these markings? What purpose do they serve? And how might scientists use these markings to study whale sharks?
Everything Happens for a Reason
The process of evolution involves random mutations - 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.
Whaley Beautiful
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 for camouflage. 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 helps the small shark to blend in with their watery background and their "countershading" (white belly fading into a darker colour on their 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). To learn more about countershading, which is common in sharks, head over to Shady Characters (Wilson & Martin, 2003).
It has also been suggested that the skin patterns might be useful for communication between whale sharks. 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 as markings as "visual cues" to recognise individual sharks, much like how we use height, hair colour and eye colour to recognise people we know (Wilson & Martin, 2003).
Some experts also wonder if 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. These "agonistic displays" are communicate that they pose a threat without actually having to fight. Whale sharks might use their patterns to show how big they - to intimidate smaller sharks (Wilson & Martin, 2003).
Growing Old Gracefully
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; Speed, 2006).
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).
Spot the Difference
As whale sharks' patterns never change throughout their lives, they can be used by scientists to identify individual whale sharks! By loading images of whale shark patterns into specialised software (not dissimilar to that which is used in forensics when examining fingerprints), scientists can create databases with known whale sharks' "bodyprints", which can be used to identify a specific animal when it is seen again at a later date.
This means we can track individuals around the world as they undergo their migrations, monitor them as they grow, and even track changes in abundance (Wilson & Martin, 2003; Speed, 2006; Norman et al, 2017). To learn more you can check out Fingerprinting.
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
Norman BM, Holmberg JA, Arzoumanian Z, Reynolds SD, Wilson RP, Rob D, Pierce SJ, Gleiss AC, de la Parra R, Galvan B, Ramirez-Macias D, Robinson D, Fox S, Graham R, Rowat D, Potenski M, Levine M, Mckinney JA, Hoffmayer E, Dove ADM, Hueter R, Ponzo, Araujo AG, Aca E, David D, Rees R, Duncan A, Rohner CA, Prebble CEM, Hearn A, Acuna D, Berumen ML, Vázquez A, Green J, Bach SS, Schmidt JV, Beatty SJ & Morgan DL (2017). Undersea constellations: the global biology of an endangered marine megavertebrate further informed through citizen science. BioScience, 67:12. Access online.
Speed CW (2006). An information-theoretic assessment of spot-pattern matching software and its application to population estimates of whale sharks (Rhincodon typus) (Doctoral dissertation, BSc thesis, Charles Darwin University, Darwin). Access online.
Wilson SG & Martin RA (2003). Body markings of the whale shark: Vestigial or functional? Western Australian Naturalist. Access online.
By Sophie A. Maycock for SharkSpeak
