The opah, also commonly known as moonfish, sunfish (not to be confused with Molidae), cowfish, kingfish, and redfin ocean pan are large, colorful, deep-bodied Pelagic zone lampriform Fish comprising the genus Lampris, of the small family Lampridae (also spelled Lamprididae).
Species
Two living species were traditionally recognized, but a
taxonomic review in 2018 suggests the idea of splitting
L. guttatus into several species, each with a more restricted geographic range, bringing the total to six. The six species of
Lampris have mostly non-overlapping geographical ranges, and can be recognized based on body shape and coloration pattern.
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Southern Spotted Opah ( Lampris australensis): Found in the Southern Hemisphere, primarily in the Pacific Ocean and .
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North Atlantic Opah ( Lampris guttatus): Potentially located in the Northeastern Atlantic, including the Mediterranean Sea.
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Southern Opah ( Lampris immaculatus): Geographical range extends from 34° S to the Antarctic Polar Front.
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Smalleye Pacific Opah ( Lampris incognitus): Found in eastern and central North Pacific Ocean.
Extinct species
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† Lampris zatima, also known as Diatomœca zatima, is a very small, extinct species from the late Miocene of what is now Southern California known primarily from fragmentary fossils, and occasional headless specimens.
[David, Lore Rose. 10 January 1943. Miocene Fishes of Southern California The Society]
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† Megalampris keyesi is an extinct species estimated to be about in length. Fossil remains date back to the late Oligocene of what is now New Zealand, and it is the first fossil lampridiform found in the Southern Hemisphere.
[Gottfried, Michael D., Fordyce, R. Ewan, Rust, Seabourne. Journal of Vertebrate Paleontology. " Megalampris keyesi, A Giant Moonfish (Teleostei, Lampridiformes), from the Late Oligocene of New Zealand". pp. 544–551.]
Description
Opahs are deeply keeled, laterally compressed, discoid fish with conspicuous coloration: the body is a deep red-orange grading to rosy on the belly, with white spots covering the flanks. Both the median and paired fins are a bright
vermilion. The large eyes stand out as well, ringed with golden yellow. The body is covered in minute
Cycloid scale and its silvery,
Iridescence guanine coating is easily abraded. The snout of the opah is pointed as the
lateral line (the sensory system in fish) forms a high arch over the pectoral fins before sweeping down to the
caudal peduncle. The
Lampris species vary in size. For instance, the larger
Lampris species,
Lampris guttatus, has a small, terminal, and
Tooth mouth and can reach a total length of and a weight of , but has been reported up to , while the lesser-known
Lampris immaculatus reaches a recorded total length of just .
Opahs closely resemble the shape of the unrelated
Stromateidae (family Stromateidae). Both have falcated (curved)
Pectoral fin and forked, emarginated (notched)
Caudal fin. Aside from being significantly larger than butterfish, opahs have enlarged, falcated
Pelvic fin with about 14 to 17 rays, which distinguish them from superficially similar
Carangidae—positioned thoracically; adult butterfish lack pelvic fins. The pectorals of opahs are also inserted horizontally rather than vertically. The anterior portion of an opah's single
dorsal fin (with about 50–55 rays) is greatly elongated, also in a falcated profile similar to the pelvic fins. The
anal fin (around 34 to 41 rays) is about as high and as long as the shorter portion of the
dorsal fin, and both fins have corresponding grooves into which they can be depressed.
The genome of Lampris megalopsis was analyzed in 2022.
Endothermy
Opah fishes use two methods for
thermogenesis: pectoral muscle metabolism and specialized tissues in their brain. The opah is the only fish known to exhibit whole body
endothermy where all the internal organs are kept at a higher temperature than the surrounding water.
This feature allows opahs to maintain an active lifestyle in the cold waters they inhabit.
[ "Warm Blood Makes Opah an Agile Predator". Fisheries Resources Division of the Southwest Fisheries Science Center of the National Oceanic and Atmospheric Administration. 12 May 2015. Retrieved 15 May 2015. "New research by NOAA Fisheries has revealed the opah, or moonfish, as the first fully warm-blooded fish that circulates heated blood throughout its body..."] Unlike birds and mammals, the opah is not a
homeotherm despite being an endotherm: while its body temperature is raised above the surrounding water temperature, it still varies with the external temperature and is not held constant.
In addition to whole body endothermy, the opah also exhibits regional endothermy by raising the temperature of its brain and eyes above that of the rest of the body.
[ Regional endothermy also arose by convergent evolution in tuna, Lamnidae and where the swimming muscles and cranial organs are maintained at an elevated temperature compared with the surrounding water.
]
The large muscles powering the pectoral fins generate most of the heat in the opah. In addition to the heat they generate while moving, these muscles have special regions that can generate additional heat without contracting.
In addition to the rete mirabile in its gills, the opah also has a rete in the blood supply to its brain and eyes. This helps to trap heat in the cranium and further raise its temperature above the rest of the body. While the rete mirabile in the gills is unique to the opah,[
]
Behavior
Much research explores the endothermic attributes of the Opah but knowledge is limited to the specific aspects of the Lampris genus, and live specimens are rarely observed.
The opah can be found worldwide in temperate and tropical oceans.
Lampris guttatus are able to maintain their eyes and brain at warmer than their bodies, a phenomenon called cranial endothermy and one they share with sharks in the family Lamnidae, Billfish, and some tunas. This may allow their eyes and brains to continue functioning during deep dives into water below .
Large pelagic sharks, such as great white sharks and Mako shark, are the primary predators of the Opah as shown from archival transmitting tagging operations. The opah fish eats a lot of food, typically, smaller fishes, invertebrates like squid and euphausiids (krill) – that make up the bulk of the opah diet – and large pelagic organisms. Opah fish also carry many parasites such as the Tetraphyllidea tapeworm, which has been found in L. guttatus, which may be an intermediate or paratenic host.
The Plankton opah larvae initially resemble those of ribbonfishes (Trachipteridae), but are distinguished by the lack of dorsal and pelvic fin ornamentation. The slender hatchlings later undergo a marked and rapid transformation to a deep-bodied form; this transformation is complete by standard length in L. guttatus.
