Temporal range: Late Miocene to present
Opahs (also commonly known as moonfish, sunfish, kingfish, redfin ocean pan, and Jerusalem haddock) are large, colorful, deep-bodied pelagic lampriform fishes comprising the small family Lampridae (also spelled Lamprididae). Only two living species occur in a single genus: Lampris (from the Greek lamprid-, "brilliant" or "clear"). One species is found in tropical to temperate waters of most oceans, while the other is limited to a circumglobal distribution in the Southern Ocean, with the 34°S as its northern limit. Two additional species, one in the genus Lampris and the other in the monotypic Megalampris, are only known from fossil remains. The extinct family, Turkmenidae, from the Paleogene of Central Asia, is closely related, though much smaller.
Opah specimens are rarely caught by recreational anglers. They are prized trophies for deep-water anglers as their large size and attractive form lend themselves well to taxidermy. Opahs are frequently caught as bycatch in many longline tuna fisheries. Opah is becoming increasingly popular in seafood markets. It first became popular as a sushi and sashimi in the late 1980s and early 1990s. The meat is lightly flavored and lends itself well to a variety of preparations, principally sauté. Opah flesh has a light-pink to orange color, but turns white when cooked. It is popular in Hawaii, especially in restaurants. An average of 35% of an opah's weight is consumable, with the remaining 65% being bone and thick skin.
The opah is the first fish discovered to be whole body endothermic, in May 2015. The opah can keep its entire body consistently at a temperature above the water temperature. Other fish may have different types of warm-bloodednesses, for example the salmon shark has homeothermy in its stomach, but not for the entire body.
Opahs are deeply keeled, 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 scales and its silvery, iridescent guanine coating is easily abraded.
Opahs closely resemble in shape the unrelated butterfish (family Stromateidae). Both have falcated (curved) pectoral fins and forked, emarginated(notched) caudal fins. Aside from being significantly larger than butterfish, opahs have enlarged, falcated pelvic fins with about 14 to 17 rays, which distinguish them from superficially similar carangids—positioned thoracically; adult butterfish lack pelvic fins. The pectorals of opahs are also inserted (more or less) 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 snout is pointed and the mouth small, toothless, and terminal. The lateral line forms a high arch over the pectoral fins before sweeping down to the caudal peduncle. The larger species, Lampris guttatus, may reach a total length of 2 m (6.6 ft) and a weight of 270 kg (600 lb). The lesser-known Lampris immaculatus reaches a recorded total length of just 1.1 m (3.6 ft).
Only known warm-blooded fish
The opah is whole-body endothermic (warm-blooded), with a rete mirabile in its gill tissue structure, first described in 2015 as exhibiting counter-current heat exchange in which the arteries, carrying warm blood, from the heart, warm the veins in the gills carrying cold blood. The gills are cooled by contact with cold water. The opah's pectoral muscles generate most of its body heat. The opah retains heat with insulating layers of fat, which insulates the heart from the gills, and the pectoral muscles from the surrounding water. The opah can consistently keep its entire body around 5 degrees Celsius warmer than its environment. The opah is not homeothermic like birds and mammals.
Almost nothing is known of opah biology and ecology. They are presumed to live out their entire lives in the open ocean, at mesopelagic depths of 50 to 500 m, with possible forays into the bathypelagic zone. They are apparently solitary, but are known to school with tuna and other scombrids. The fish propel themselves by a lift-based labriform mode of swimming, that is, by flapping their pectoral fins. This, together with their forked caudal fins and depressible median fins, indicates they swim at constantly high speeds like tuna.
Lampris guttatus are able to maintain their eyes and brain at 2 °C warmer than their bodies, a phenomenon called cranial endothermy and one they share with sharks in the Lamnidae family, billfishes and some tunas. This may allow their eyes and brains to continue functioning during deep dives into water below 4 °C.
Squid and euphausiids (krill) make up the bulk of the opah diet; small fish are also taken. Pop-up archival transmitting tagging operations have indicated, aside from humans, large pelagic sharks, such as great white sharks and mako sharks, are primary predators of opah. The tetraphyllidean tapeworm Pelichnibothrium speciosum has been found in L. guttatus, which may be an intermediate or paratenic host. The planktonic opah larvae initially resemble those of certain ribbonfishes (Trachipteridae), but are distinguished by the former's lack of dorsal and pelvic fin ornamentation. The slender hatchlings later undergo a marked and rapid transformation from a slender to deep-bodied form; this transformation is complete by 10.6 mm standard length in L. guttatus. Opahs are believed to have a low population resilience.
Species and range
- Lampris guttatus (Brünnich, 1788) opah — from the Grand Banks to Argentina in the Western Atlantic; from Norway and Greenland to Senegal and south to Angola (also in the Mediterranean) in the Eastern Atlantic; from the Gulf of Alaska to southern California in the Eastern Pacific; in temperate waters of the Indian Ocean; and rare forays into the Southern Ocean
- Lampris immaculatus Gilchrist, 1904 southern opah — confined to the Southern Ocean from 34°S to the Antarctic Polar Front
Known fossil taxa
- 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 fragments, and the occasional headless specimens.
- Megalampris keyesi is an extinct species estimated to be about 4 m 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.
- Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. Retrieved 8 January 2008.
- 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.
- Bray, Dianne. "Opah, Lampris guttatus". Fishes of Australia. Retrieved 16 September 2014.
- Wegner, Nicholas C., Snodgrass, Owen E., Dewar, Heidi, John, Hyde R. Science. "Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus". pp. 786–789. Retrieved May 14, 2015.
- Pappas, Stephanie; LiveScience. "First Warm-Blooded Fish Discovered". Scientific American. Retrieved 15 May 2015.
- "Warm Blood Makes Opah an Agile Predator". Fisheries Resources Division of the Southwest Fisheries Science Center of the National Oceanic and Atmospheric Administration. May 12, 2015. Retrieved May 15, 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..."
- Yong, Ed. "Meet the Comical Opah, the Only Truly Warm-Blooded Fish". National Geographic. Retrieved 15 May 2015.
- Scholz et al., 1998.
- David, Lore Rose. 10 January 1943. Miocene Fishes of Southern California The Society