The whale shark, a large filter-feeding animal with a distinctive spotted pattern, navigates the world’s tropical oceans. Its substantial dimensions, reminiscent of large marine mammals, often lead to questions about its biological classification.
The Direct Answer
Despite its common name, the whale shark is not a whale; it is a fish. It holds the distinction of being the largest known fish species. This classification places it firmly within the realm of aquatic vertebrates that respire using gills and are typically ectothermic. Its immense size, with individuals reaching up to 18.8 meters (61.7 feet) in length and weighing over 26,000 pounds, contributes to the common misconception about its identity.
Why Whale Sharks Are Fish
Whale sharks possess several distinct biological characteristics that unequivocally classify them as fish.
Respiration
They breathe using five large pairs of gills, which extract oxygen directly from the water as it passes through their mouths and out these slits. Unlike marine mammals, they do not have lungs and cannot breathe air.
Skin and Thermoregulation
Their skin is covered in dermal denticles, which are tiny, tooth-like structures, rather than the hair or fur found on mammals. These denticles reduce drag and protect their thick skin, which can be up to 4 inches thick. Whale sharks are ectothermic, meaning their body temperature is primarily regulated by the external environment. While their large size provides some thermal inertia, allowing their body temperature to change more slowly than the surrounding water, they do not internally generate heat to maintain a constant body temperature as mammals do.
Reproduction
Whale sharks are ovoviviparous, with eggs hatching inside the mother and live young being born. A single pregnant female was found carrying approximately 300 pups, which develop nourished by a yolk sac inside the mother, without a placental connection typical of mammals.
Distinguishing Them From Marine Mammals
Marine mammals exhibit a different set of biological traits that differentiate them from whale sharks.
Breathing
Marine mammals breathe air through lungs and must periodically surface to do so, often through a blowhole located on top of their heads. This is a conscious action, unlike the continuous, involuntary respiration of fish through gills.
Body Temperature
Marine mammals are endothermic, meaning they maintain a relatively constant internal body temperature regardless of the external water temperature. They achieve this through a high metabolic rate and a thick layer of blubber for insulation.
Other Key Differences
Marine mammals possess hair at some point in their lives, unlike fish. They give birth to live young that are nourished with milk produced by mammary glands, a characteristic absent in fish like the whale shark.