Author: Angela Warrior
Researchers in Japan have recently discovered that whale sharks (Rhincodon typus) have a peculiar trick up their sleeve, a strange alternative to an eyelid. Their eyeballs are covered in tiny “teeth”, otherwise known as dermal denticles. However, these teeth-like structures are not used for biting but they are employed as a “defense mechanism” researchers explained. Many other shark species have adapted an extra eye protection, a third eye lid known as a nictitating membrane. This tough inner eyelid, which is also covered in dermal denticles, fully protects the eyeball from eye injury, particularly during a feeding event, whereby the prey may cause damage to the eye whilst trying to escape. Whale sharks do not have this adaptation, furthermore their eyes are situated at the corner of their heads (Picture A and B) rather than at the front, like most sharks. This means the eyes are more exposed to the elements and are more vulnerable to mechanical, chemical and biological damage to the eye surface.
(Tomita et al PLOS One, 2020)
The researchers have now discovered that the entire casing of the eye is covered in dermal denticles that are of similar shape to human molars. Dermal denticles are also known as placoid scales (the denticles are rooted in a layer of skin called the dermis-hence the name dermal denticles). These tough scales are structurally very similar to teeth, including an inner core of pulp, a layer of bony tissue and enamel protection covering the outer layer. Whale sharks also have the ability to retract their eyes backward, rotating the entire eyeball back into the eye socket.
Observations of kinematics (properties of motion in an object) of the eye showed the eyes of whale sharks retract into the orbit for less than one second when an object approaches. During this time the white connective tissue partially fills the space where the eye once was. Generally, retraction lasts for a short time, however authors Tomita et al state that long-term retraction was once observed in a captive whale shark in the Georgia Aquarium. The 5-meter female whale shark kept its eyes retracted for a total of ten days!! Amazingly she had no problem navigating the tank with her retracted eyes until the day they spontaneously “popped” back to their normal position. These highly protective features highlight the importance of vision when the whale shark is evaluating its environment. This contradicts past assumptions that the whale shark depends very little on vision and more on its other senses such as olfaction (smell).
Eye denticles of the whale shark (Tomita et al PLOS One, 2020)
Whale sharks, like other sharks, also have dermal denticles on their body. The thickness makes it very difficult for other sharks to bite them, which is particularly important for females when they are performing courtship rituals. These rituals can be quite aggressive, whereby females often end up with “love bites” along their fins and back, this doesn’t hurt them as their skin is much thicker than the males and covered in more denticles.
Dermal denticles are also known to reduce the frictional drag that sharks experience as they swim through the water. The denticles are shaped like small riblets (see photo below) and are aligned in the direction of the flow. They are thought to reduce drag by managing the water flow closest to the skin. The denticles also help regions of low-pressure swirling water stay close to parts of the shark’s body, creating more suction and forward thrust. As a result of this amazing discovery, Olympic swimsuits were developed to mimic shark skin. One female competitor described her self as “gliding through the water with ease, just like a shark”. The same competitor went on to become the first swimmer in the U.S to swim a 100-yard breaststroke in under 59 seconds wearing one of these suits. The suits were soon banned as the were deemed to provide an unfair advantage to the wearer.
Scale patterns of fast sharks (Fu et al, ScienceDirect, 2017)
Sharks are incredible. Just when we thought we knew everything there was to know about them, we discover another cool adaptation to add to their plethora of survival adaptations. They have been around for more than 400 million years, they outlived the dinosaurs, survived mass extinctions and and they are still here, flying high at the top of the food chain. Their ability to adapt to suit their underwater environment and exploit different parts of the marine ecosystem means they are more likely to survive changes in their environment, they also have very few natural predators, other than other sharks and killer whales. However, despite these adaptations, sharks are now facing many threats such as overfishing and recreational fishing. Sharks are the apex predators of the ocean, their role is to regulate the oceans and maintain a healthy balance by removing the sick and diseased fish and preventing a population explosion of any single species, this is crucial to maintaining a healthy ecosystem. However, plenty of specie are now facing extinction, similar to the apex predators of the continents, the wolves, the tigers, and the bears, the sharks are now disappearing, but sadly most people have not even noticed.
Sharks need our support more than ever now. If you are interested in learning more, please contact me. There are some great resources out there and lots of it is free, that’s the best bit! I am also an Ambassador for Sharks4Kids, a non-profit focused on creating the next generation of shark ambassadors and they have some amazing resources on-line, so there is no excuse to not be fintastic and jawsome and help support sharks!!
Find out more!!