Identifying the largest fish that has ever existed requires exploring oceanic history and understanding how the fossil record informs our estimates. This quest reveals creatures of astonishing size. The significant difference between the giants of the past and the largest fish of the present illustrates how much marine ecosystems have changed over millions of years.
Clarifying the Term “Fish”
Determining the largest fish requires establishing a scientific boundary that separates true fish from other large marine animals. This distinction excludes marine mammals, such as the Blue Whale, which is not a fish. Whales are warm-blooded, air-breathing vertebrates classified as mammals. True fish are aquatic, gill-breathing vertebrates that are not tetrapods. The two primary groups that qualify are the Chondrichthyes (cartilaginous fish like sharks and rays) and the Osteichthyes (bony fish).
The Extinct Record Holder
The title for the largest fish known to science is frequently attributed to Leedsichthys problematicus, a massive filter-feeding bony fish from the Jurassic period. This extinct giant is estimated to have reached lengths of up to 55 feet (16.5 meters), though size estimates vary due to the fragmentary nature of its fossil record. Leedsichthys existed approximately 165 million years ago, making it the largest ray-finned fish known to have ever lived.
This behemoth was a plankton-feeder, utilizing an immense gill-raker system to strain tiny organisms from the water, much like modern baleen whales. While Leedsichthys is often cited as the largest fish overall, the extinct megatooth shark, Otodus megalodon, holds the record for the largest predatory fish. Megalodon was a cartilaginous fish that swam the oceans, reaching lengths estimated between 50 and 65 feet (15 to 20 meters), potentially exceeding Leedsichthys in mass. However, because the skeleton of Megalodon was primarily cartilage, its size is extrapolated almost entirely from its enormous teeth.
Largest Living Fish Species
The largest fish swimming in the world’s oceans today is the Whale Shark (Rhincodon typus). This cartilaginous giant can typically grow to lengths of about 40 feet (12 meters) and weigh over 20 tons. The maximum recorded length reaches nearly 62 feet (18.8 meters), though specimens of that size are extremely rare. The Whale Shark is a gentle filter-feeder, consuming plankton and small fish by straining water through its enormous mouth. It inhabits tropical and warm-temperate waters globally.
The Largest Known Bony Fish
The classification of fish into bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes) highlights a difference in maximum potential size. The extinct Leedsichthys problematicus remains the largest bony fish in history, demonstrating that bony fish once achieved dimensions comparable to the largest sharks.
Among the bony fish alive today, the largest is generally considered to be the Ocean Sunfish (Mola alexandrini). This species can weigh up to 6,000 pounds (2,744 kilograms) and reach a length of over 11 feet. The Giant Oarfish (Regalecus glesne) holds the record for the longest modern bony fish, with unconfirmed reports of individuals reaching 45 feet in length.