< Guide to whale sharks
Whale sharks are captivating creatures, renowned as the world's largest fish. Exploring their unique body plan and adaptations helps us understand how these gentle giants have evolved to thrive in their environment. Here, we dive into the fascinating anatomy of whale sharks, discussing their extraordinary features and the reasons behind their various adaptations.
The Terminal Mouth and it’s Advantages for Filter-Feeding
Whale sharks boast a unique feature: their terminal mouth, which opens directly at the front of their head which is unlike any other shark species. This detail is crucial, as most sharks rely on their rostrum (snout) to create lift for level swimming. Whale sharks make up for their missing rostrum with large, broad pectoral fins and a flat underside of their head, keeping their planing surface balanced behind their mouth. Next time you spot a whale shark gracefully gliding through the water, take a moment to appreciate their one-of-a-kind anatomy, perfectly designed for their extraordinary lives.
Their terminal mouths are perfectly designed to facilitate filter feeding. This incredible adaptation allows them to actively engage in surface suction feeding, swimming at a 25° angle and gulping down mouthfuls of water from the surface layer, feasting on floating food items. While basking sharks also filter feed, they use a different method called subsurface ram feeding, simply swimming forward with their mouth wide open. Whale sharks can switch to this feeding style, but they prefer active surface suction feeding, making them specialists in dining on neuston (organisms floating on or just below the surface). Their distinctive mouth sets them apart from other filter-feeding sharks, showcasing the marvels of nature's adaptations.
Rows of Teeth and Their Role
Contrary to what you might think, whale sharks do have teeth—up to 300 rows of them, in fact, with each row containing up to 16 files (layers). These small, teardrop-shaped teeth are what inspired their genus name Rhincodon (Greek for "rasp-tooth"). While these teeth may seem insignificant, they played a vital role in naming and identifying whale sharks in the fossil record. Another fascinating feature in their mouths is the presence of several velums—transverse flaps of skin in both upper and lower jaws. When whale sharks close their mouths, these velums act as check valves, sealing the mouth and forcing water to flow out through their gills. This incredible adaptation showcases the intricate design of these gentle giants' anatomy.
Unique Nostals and Smelling Abilities
Whale sharks possess a truly unique mouth, stretching up to 1.5 meters wide, which has an interesting side effect: it widely separates their nares (similar to nostrils). Hammerhead sharks are famous for their widely separated nares on their distinctive "hammer" heads, which enhance their ability to determine the direction of smells—think of it as stereo-olfaction. However, the nares of whale sharks are actually larger and more widely separated than even the largest hammerhead. It's just less noticeable due to their less dramatic head shape. This suggests that whale sharks may have an even more impressive ability for directional smelling, making them unsung heroes in the realm of underwater olfaction.
Armored and Retractable Eyes
Unlike most sharks, whale sharks have eyes positioned near the front corners of their head, making them potentially vulnerable to impacts. To counter this, they've evolved two remarkable adaptations: armored and retractable eyes. While they lack nictitating membranes (or "3rd eyelids"), whale sharks are the only species with denticles (scales) on their eyeballs, providing extra protection against abrasion. When faced with more serious threats, whale sharks can retract their eyeballs into their head using a unique cartilage bar and attached muscles. This retraction also helps improve their forward vision when engaging in active surface suction feeding at an angle of more than 25°. However, they do have a blind spot directly in front, a small trade-off for their incredible eye adaptations.
The Enormous ‘Orobranchial’ Chamber
Whale sharks are practically a "swimming mouth," thanks to their enormous orobranchial (mouth and gills) chamber that spans roughly 30% of their body length. This spacious chamber houses their ten gills and 20 filter pads, which have evolved from gill rakers to ensure that only small particles can pass through. Anything entering their mouth must be swallowed, small enough to pass through the filters, or ejected through a cough-like motion. Although whale sharks have a spiracle (opening) just behind their eye, it's believed to be mostly non-functional, serving as a cozy home for small remoras instead. This unique filtering system highlights the whale shark's extraordinary adaptations to its underwater environment.
Unique Dorsal Fin Position
The first dorsal fin of whale sharks is set well back on the body at roughly the midpoint (compare this to many carcharhinids, in which the dorsal starts in the forebody, before the trailing edge of the pectoral fins). The body surface itself is distinguished by longitudinal ridges and valleys starting behind the head and continuous with the keels of the caudal peduncle (base of the tail).
Whale Sharks Thick Skin and Its Functions
Whale sharks' bodies are adorned with striking longitudinal ridges and valleys, starting just behind their heads and continuing along their tails. Their skin, one of their most extraordinary adaptations, is made up of a tough, pigmented outer layer (epidermis) with embedded denticles, and a remarkably thick, rubbery, white dermis composed of dense collagenous connective tissue. This incredible skin serves multiple purposes: providing structural support, insulation, and excellent defense against predators and well-intentioned researchers trying to attach satellite tags. Any predator attempting to take a bite out of a whale shark will likely end up with a mouthful of gristle, if anything at all.
The Polka Dot Pattern and Its Possible Function
There has been much speculation about the function of the polka dots on the skin, which are irregular anterior to the pectoral fins but give way to more regular alternating stripes and spots posteriorly. It is noteworthy that the zebra shark, Stegostoma tigrinum, is one of the whale shark's closest relatives and has both the ridges on the skin and the polka dot pattern, although in that case, it is dark spots on a light background, the opposite of the pattern in whale sharks. Perhaps their closest relative, however, the tawny nurse shark Nebrius ferrugineus, lacks both ridges and spots.
Conclusion
The whale shark's anatomy is not only captivating but also serves as a testament to the power of evolution in shaping the form and function of species. Understanding the whale shark's body plan and the reasons behind its various adaptations can help us appreciate the wonders of nature and the intricate complexities of the world's largest fish. As we continue to learn about these majestic creatures, we will be able to better conserve and protect them, ensuring their continued presence in our oceans for generations to come.
