The ocean is home to an array of fish species, each having evolved specialized anatomical features for survival. Among these adaptations, the mouth is one of the most variable structures, tailored for everything from grazing on algae to ambushing prey in the deepest trenches. Determining which fish has the biggest mouth requires distinguishing between the largest measurable opening and the most extreme proportion relative to the animal’s overall body size. This comparison highlights the difference between the ocean’s largest filter feeders and the oversized predators of the deep sea.
Defining “Biggest”: Absolute Size Versus Body Ratio
Determining the “biggest” mouth requires defining the criteria, which splits into two main categories of biological measurement. The first category is absolute size, referring to the largest physical measurement of the mouth opening, such as its total width or diameter. This metric favors the ocean’s largest inhabitants, whose sheer mass dictates the size of their features. The second category is relative size, which compares the mouth’s dimensions to the fish’s overall body length or mass. This method highlights species where the mouth is disproportionately enormous. While absolute size identifies the physical record holder, relative size reveals the most extreme evolutionary adaptation.
The Record Holder: Fish with the Largest Absolute Mouth
The record holder for the largest absolute mouth size is the Whale Shark (Rhincodon typus), the world’s largest fish species. An adult whale shark can reach lengths of over 60 feet and possesses a transverse mouth that opens directly at the front of its broad, flattened head. This enormous opening can measure up to 5 feet (1.5 meters) across in large individuals. This immense size is a direct consequence of its filter-feeding lifestyle, allowing the shark to process enormous volumes of water. The mouth’s scale is supported by a large orobranchial chamber, which houses its ten gills and the specialized filter pads necessary for separating food from water. The Basking Shark (Cetorhinus maximus), the second-largest fish, is a close contender, with a mouth that can open over 3 feet (1 meter) wide, but it is eclipsed by the Whale Shark’s dimensions.
Mechanics of Megamouths: How Massive Jaws Operate
These gigantic mouths function as massive, water-processing funnels rather than being designed for biting. Whale sharks employ two main feeding strategies: ram filtration and active suction feeding. Ram filtration involves the shark swimming forward with its mouth held wide open, passively forcing water and prey over its gill structures. The jaw and mouth are adapted for this continuous flow, featuring filter pads that act as a sieve, trapping tiny plankton, krill, and small fish while allowing the water to exit through the gill slits. Whale sharks also engage in active surface suction feeding, tilting their body vertically to gulp large volumes of water and prey near the surface. This active gulping involves a pulsatile action, sucking in prey with force, and can process over 6,000 liters of water per hour.
Other Contenders: Fish with Disproportionately Large Mouths
Relative size record-holders primarily inhabit the nutrient-scarce deep sea. The Gulper Eel (Eurypharynx pelecanoides) is an extreme example, possessing a jaw that is disproportionately large compared to its slender body. Its loosely hinged lower jaw is attached to the base of its head, creating a pelican-like pouch that can expand to form a cavernous sac. The mouth can be about one-fourth the length of its entire body (typically around 3.3 feet long), allowing the eel to engulf prey much larger than itself. This is a crucial adaptation in the deep ocean where encounters with food are infrequent. The mouth’s small, needle-like teeth function as a net to contain the captured meal before it is swallowed whole into a highly expandable stomach.