The giant oarfish (Regalecus glesne) is a species of oarfish of the family Regalecidae. It is an oceanodromous species with a worldwide distribution, excluding polar regions. Other common names include Pacific oarfish, king of herrings, and ribbonfish. R. glesne is the world's longest ray-finned fish. Its shape is ribbon-like, narrow laterally, with a dorsal fin along its entire length, stubby pectoral fins, and long, oar-shaped pelvic fins, from which its common name is derived. 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. In 2016, the TV show River Monsters concluded that the most likely origin of the sea serpent myth is the oarfish; host Jeremy Wade was filmed diving with two living, healthy giant oarfish in the Mediterranean Sea.
Taxonomy R. glesne was first described by Peter Ascanius in 1772. The genus name, Regalecus (from Latin 'regalis' meaning royal), 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. Its "king of herrings" nickname may derive from its crownlike appendages and from being sighted near shoals of herring, which fishermen thought were being guided by this fish. Its common name, oarfish, is probably an allusion to the shape of its pelvic fins, or else it may refer to the long slender shape of the fish itself.
Distribution The giant oarfish has a worldwide distribution, having been found as far north as 72°N and as far south as 52°S, but is most commonly found in the tropics to middle latitudes, 180°W to 180°E. It has been categorized as oceanodromous, following its primary food source. It can be found in both the Atlantic and Pacific Oceans, though it is more widely distributed in the Atlantic. The fish is thought to be cosmopolitan in distribution, though it is not found in the polar regions. It is thought to inhabit the sunlit epipelagic to dimly lit mesopelagic zones. The deepest verified account of R. glesne is 463–492 metres (1,519–1,614 ft) from the Gulf of Mexico, as part of the Gulf SERPENT project.
Description
This species is the world's longest bony fish, reaching a record length of about 7–8 metres (23–26 ft), possibly up to 11 metres (36 ft), and a maximum record weight of 272 kilograms (600 lb). Older, much longer estimates, such as a believed to be 17 metres (56 ft) long, are now considered "very likely inaccurate." It is commonly measured to 3 m (9.8 ft) 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, identical to that of the adult fish. 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 and head. This dark pigmentation is presumably an adaptation developed for counter-shading when the adult fish is vertical in the water column. Additionally, the larvae possess a caudal fin with four fin rays, which is a trait not present in the adult form of the species. In some larger juvenile specimens, body coloration similar to that of the adult form was observed. Observations of larvae specimen of Regalecus glesne captured off the island coast of Palagruža analyzed the size of these specimens. The larvae specimen was measured to be 103.4 millimetres (4.07 in) with a body height of around 7 millimetres (0.28 in). Adults have a pale silver ribbonlike body shape that is laterally compressed and extremely elongated with a dorsal fin along its entire length from between its eyes to the tip of its tail, ranging in color from faint pinkish to a bright red. The body often has dark wavy markings resembling spots or stripes. There is a black coloration of the membrane between the opercle and the other head bones. A series of faint horizontal stripes is evident in some specimens, while absent in others. The skin is scaleless, with extensive tuberculation. The dorsal fin rays are soft and number between 414 and 449 in total. At the head of the fish, the first 10–12 of these dorsal fin rays are lengthened, forming the distinctive red crest associated with the species. Its pectoral and pelvic fins are nearly adjacent. The pectoral fins are stubby while the pelvic fins are long, single-rayed, and reminiscent of an oar in shape, widening at the tip. There are no anal fins. The caudal fin is usually under 2 m (6.6 ft) in length, with most well under 1 m (3.3 ft) and has four rays. In most specimens, the caudal fins are badly broken or absent entirely. Its head is small with the protrusible jaw typical of lampriformes. The species has 33 to 47 gill rakers on the first gill arch, no teeth, and the inside of the mouth is black. It has a pair of large eyes just above the mouth. The organs of the giant oarfish are concentrated toward the head end of the body, possibly enabling it to survive losing large portions of its tail. It has no swim bladder. The liver of R. glesne is orange or red, the likely result of astaxanthin in its diet. The lateral line begins above and behind the eye then, descending to the lower third of the body and extending to the caudal tip. There is a postabdominal gastric caecum, a tube which extends from the end of the stomach to the end of the body. The function of this structure is unknown, as no food items have been observed within it. It is not necessary for vital functions, as Regalecus have lost half or all of the caecum and survived without it.
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