The world’s oceans harbor creatures of immense scale. To identify the absolute largest fish, one must look beyond whales, which are air-breathing, warm-blooded mammals. The true contenders are cold-blooded aquatic vertebrates that rely on gills to extract oxygen from the water. This biological distinction separates true fish from other large marine animals.
The Absolute Largest Fish Species
The largest fish species known to science is the whale shark, Rhincodon typus. This enormous creature belongs to the class of cartilaginous fish, meaning its skeleton is made of flexible cartilage rather than rigid bone.
The largest reliably measured individual reached a length of 18.8 meters (61.7 feet). While many reports of greater sizes exist, this figure is the most accurately documented maximum length. The largest scientifically recorded specimen weighed an estimated 21.5 tonnes (47,000 pounds).
Defining the Scope of “Fish”
The classification of “fish” is based on biological characteristics that exclude whales. Unlike marine mammals, fish breathe using gills, are cold-blooded, and typically reproduce by laying eggs. Whales possess lungs, are warm-blooded, and give birth to live young.
Fish are broadly divided into two major classes: Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). Cartilaginous fish, like the whale shark, have a skeleton composed entirely of cartilage and possess exposed gill slits. Bony fish, which represent the vast majority of species, have a skeleton made of calcified bone tissue. They typically use a gas-filled swim bladder to maintain neutral buoyancy and have a hard, bony plate called an operculum that covers their gills.
Ecology and Behavior of the Record Holder
The whale shark is a cosmopolitan species, inhabiting tropical and warm temperate waters across the globe, generally between the 30°N and 30°S parallels. They are often found near coastal regions where nutrient-rich upwellings promote high concentrations of their food source. Satellite tracking shows these animals are highly migratory, often traveling thousands of miles to follow seasonal plankton blooms and fish spawning events.
Despite its immense size, the whale shark is a docile filter feeder that poses no threat to humans. Its diet consists almost entirely of tiny organisms, including zooplankton, small crustaceans, and fish larvae. The shark feeds using ram filtration, swimming forward with its enormous mouth open and passively forcing water over specialized filtering pads.
The mouth can be over a meter wide. The filtering apparatus is composed of unique pads with openings averaging about 1.2 millimeters in diameter. A large individual can filter hundreds of cubic meters of water per hour, efficiently separating microscopic prey. They also occasionally use an active suction-feeding technique when targeting dense, localized patches of food.
The Biggest Bony Fish
While the whale shark is the largest fish overall, the title of the largest bony fish belongs to the giant sunfish, Mola alexandrini. This species is the heaviest known within the Osteichthyes class. The fish has a distinctive, flattened, and truncated body shape.
The largest recorded specimen was discovered in 2021 off the Azores, weighing 2,744 kilograms (6,049 pounds) and measuring 3.25 meters in length. This massive weight makes it nearly a ton heavier than the previous record holder. Sunfish are pelagic creatures found in tropical and temperate oceans worldwide.