Largest bony fish ever
Leedsichthys problematicus
16.76 metre(s)
Not Applicable ()

The biggest bony fish ever known to have lived is a specimen of the marine fossil species Leedsichthys problematicus. Dating back some 165 million years, it is estimated to have reached around 55 ft (16.76 m) in length. Its species, first made known to science in the 1800s, belongs to an extinct group of bony fishes known as the pachycormids, but it is believed to have been a plankton feeder, comparable to the basking shark and also the baleen whales. The fish had previously been thought to have been far longer, but its speculated length was downsized after new findings announced by University of Bristol palaeontologist Jeff Liston in August 2013 at the 61st annual Symposium on Vertebrate Palaeontology and Comparative Anatomy in Edinburgh, UK.

This remains a necessarily speculative measurement (as indicated by the species' Latin name). No complete specimens of Leedsichthys problematicus have been found. Much of its skeleton would have been made of cartilage, which does not become fossilized. The fossils studied by Liston and his colleagues had estimated lengths of between 26 ft and 55 ft (7.92–16.76 m).

These size estimates also make it one of largest fish ever recorded overall, but it's likely it was exceeded by the megalodon (Otodus megalodon; formerly Carcharodon megalodon), a cartilaginous fish (Chondrichthyes) which is believed to have reached at least 15–18 m (50–59 ft). Based on a new method of extrapolating body size from tooth width (as opposed to length) published in 2021, megalodon may have even grown up to 20 m (65 ft) long. It lived in Earth's oceans (particularly warmer waters) during the Early Miocene to the Pliocene, between 23 and 3.6 million years ago.

Previously thought to be related to the great white shark (Carcharodon carcharias) – which is today's largest predatory fish – the megalodon has now been designated into its own family, Otodontidae. As a size comparison, megalodons reached around three times the size of the largest ever documented great white.

The largest extant fish overall is the filter-feeding whale shark (Rhincodon typus), which grows on average to 4–12 m (13–39 ft) long, though one exceptional female specimen caught in the Arabian Sea off Veraval in Gujarat, India, on 8 May 2001 is reported to have measured 18.8 m (61 ft 8 in).

Among bony fish, the current crown for the biggest goes to two species. Based on length, the oarfish (Regalecus glesne), also called the "King of the Herrings", prevails. In c.1885, a 7.6-m-long (25-ft) example weighing 272 kg (600 lb) was caught by fishermen off Pemaquid Point in Maine, USA. Another oarfish, seen swimming off Asbury Park, New Jersey, USA, by a team of scientists from the Sandy Hook Marine Laboratory on 18 July 1963, was estimated to measure 15.2 m (50 ft) in length. When it comes to weight though, the heaviest extant bony fish are the sunfish (genus Mola), with adults obtaining an average weight of around 1,000 kg (2,200 lb) and length of 1.8 m (6 ft). The heaviest specimen on record is a bump-head sunfish (M. alexandrini) caught off Kamogawa, Chiba, Japan, in 1996; it weighed 2,300 kg (5,070 lb) and measured 2.72 m (8 ft 11 in) in total length.