Giant oarfish

Species of fish

Speciesbox
| name = Giant oarfish
| image = Regalecus glesne, Naturhistorisches Museum Wien. jpg
| image caption = A taxidermied specimen of Regalecus glesne in Naturhistorisches Museum Wien
| status = LC
| status ref = Its coloration is silver and blue with spots of dark pigmentation, and its fins are crimson. Its physical characteristics and undulating mode of swimming have led to speculation that it might be the source of many "sea serpent" sightings.

R. glesne was first described by Peter Ascanius in 1772. The genus name, Regalecus, signifies "belonging to a king"; the specific epithet glesne is from "Glesnaes", the name of a farm at Glesvær (not far from Norway's second largest city of Bergen), where the type specimen was found. It has been categorized as oceanodromous, following its primary food source. It is thought to inhabit the sunlit epipelagic to dimly lit mesopelagic zones, ranging as deeply as below the surface.

This species is the world's longest bony fish, reaching a record length of It is commonly measured to in total length. Few R. glesne larvae have been identified and described in situ. These larvae exhibit an elongated body with rays extending from the occipital crest and a long pelvic fin. Unlike the adult form of the species, the skin of the larvae is almost entirely transparent with intermittent spots of dark coloration along the organism's dorsum. Adults have a ribbonlike shape that is laterally narrow, with a dorsal fin along its entire length from between its eyes to the tip of its tail. The dorsal fin rays are soft and number between 414 and 449 in total. This work was performed in 2020 and was the first time that the progression from fertilized eggs to larvae was observed. Post fertilization, the eggs took 18 days to hatch into larvae. They noted that the larvae appeared similar to other lampridiform larvae, facing downward with pectoral fins. The larvae then died four days later, so this study spanned only the early life cycle of the species.

Little is known about oarfish behavior. It has been observed swimming by means of its dorsal fin, and also swimming in a vertical position.