The whale shark, Rhincodon typus, is the largest fish in the world’s oceans, with individuals reaching up to 18 meters in length. It is a gentle, slow-moving filter-feeder, relying on plankton and small nektonic prey for sustenance. Its anatomy is adapted for navigating the open ocean while processing enormous volumes of water.
Total Number and Types of Fins
The whale shark possesses eight fins, consisting of two paired sets and four unpaired fins. The paired fins are the pectoral fins, situated behind the gill slits, and the pelvic fins, located on the underside of the body toward the rear. The unpaired fins include two dorsal fins along the midline of the back, a single anal fin positioned ventrally near the tail, and the large caudal fin (tail fin).
The first dorsal fin is significantly larger than the second dorsal fin, helping to differentiate the species from other sharks. The anal fin is situated between the pelvic fins and the caudal fin. The caudal fin itself is semi-lunate in adult sharks, meaning it is crescent-shaped with two lobes of nearly equal size.
How the Fins Aid Movement
The primary source of forward thrust for the whale shark is its powerful caudal fin, which moves in a side-to-side, or lateral, motion typical of bony fish and sharks. This contrasts with the up-and-down movement of the horizontal flukes found on marine mammals like whales. The pectoral fins, which are large and broad, function much like the wings of an airplane, generating lift to counteract the shark’s negative buoyancy and helping it maintain a stable depth.
The pectoral fins also allow the whale shark to steer and make subtle adjustments to its direction as it glides through the water. The two dorsal fins work together to provide stability, preventing the body from rolling side-to-side during travel. This combination of specialized fins enables the shark to move efficiently over long migratory distances as it tracks seasonal blooms of its prey.
Distinctive Anatomical Features
Beyond its eight fins, the whale shark has several unique physical traits. Its skin features a distinctive “checkerboard” pattern of light spots and stripes over a dark gray or blue background, unique to each individual. The head is noticeably broad and flattened, ending in a wide, terminal mouth positioned at the very tip of the snout.
This cavernous mouth, which can measure over a meter wide, is used for filter-feeding, allowing the shark to gulp or ram-filter water for plankton. The shark also has five large gill slits on each side of its head, which are internally modified into filtering screens to strain its food. Furthermore, three prominent longitudinal ridges run along the sides of the body, offering a streamlined profile for movement.