A review of giants: Examining the species identities of the world's heaviest extant bony fishes (ocean sunfishes, family Molidae).

The world's heaviest extant bony fish has long been considered the ocean sunfish Mola mola (family Molidae). However, recent taxonomic research suggests the world record specimen (2300 kg) was actually a misidentified giant sunfish M. alexandrini. Moreover, the history of taxonomic confusion in the genus Mola, combined with anecdotal size information perpetuated in both media and literature, has resulted in uncertainty in regard to species identity and actual weights of very large Mola specimens. In this study, we searched for records of molid specimens with reported body mass over 2000 kg, with a focus on the north-west and south-west Pacific. For each specimen, we reviewed the species identification and assessed the reliability of the information, including if the specimen had actually been weighed. Of the 15 specimens uncovered, we identified one M. mola and 14 M. alexandrini, and found that only five had actually been weighed (one with the viscera removed). Furthermore, we collected length-weight data for verified M. alexandrini specimens from the literature and museum specimens, and established the first length-weight relationship for this species [body mass (kg) = 1.1 × 10 -5  × total length (cm) 3.3248 (29-330 cm total length, n = 20)]. These findings are discussed in the context of other extant bony fish species, which allegedly also exceed 2000 kg in body mass, i.e., other Molidae species and the beluga sturgeon Huso huso. Overall, we conclude that M. alexandrini is the only extant bony fish species we could confirm that exceeds 2000 kg, and that basic biological information is generally not collected when very large molid specimens occasionally are stranded or are caught. Finally, we confirm that the world's heaviest extant bony fish specimen actually weighed was a female M. alexandrini (2300 kg, 272 cm total length) captured from Kamogawa, Chiba Prefecture, Japan, on August 16, 1996.