Opah

Opah Temporal range: Late Miocene–present PreꞒ Ꞓ O S D C P T J K Pg N [1]
Lampris guttatus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Lampriformes
Family:	Lampridae Gill, 1862
Genus:	Lampris Retzius, 1799

Opahs, also commonly known as moonfish, sunfish (not to be confused with

In 2015, Lampris guttatus was discovered to have near-whole-body endothermy^[3][4][5] in which the entire core of the body is maintained at around 5 °C above the surrounding water. This is unique among fish as most fish are entirely cold blooded or are capable of warming only some parts of their bodies.

Species

Two living species were traditionally recognized, but a taxonomic review in 2018 found that more should be recognized (the result of splitting L. guttatus into several species, each with a more restricted geographic range), bringing the total to six.^[6] The six species of Lampris have mostly non-overlapping geographical ranges, and can be recognized based on body shape and coloration pattern.^[6]

• Lampris australensis Underkoffler, Luers, Hyde & Craig, 2018 Southern spotted opah – Southern hemisphere, in the Pacific and Indian oceans.^[6]
• Lampris guttatus (Brünnich, 1788) North Atlantic opah – formerly thought to be cosmopolitan, but now thought to be restricted to the northeastern Atlantic including the Mediterranean Sea.^[6]
• Gilchrist, 1904 southern opah – confined to the Southern Ocean from 34° S to the Antarctic Polar Front
  .
• Lampris incognitus Underkoffler, Luers, Hyde & Craig, 2018 smalleye Pacific opah – central and eastern North Pacific Ocean.^[6]
• Mediterranean, Azores and Canary Islands.^[6]
• Lampris megalopsis Underkoffler, Luers, Hyde & Craig, 2018 bigeye Pacific opah – cosmopolitan, including the Gulf of Mexico, Indian Ocean, the western Pacific Ocean and Chile.^[6]

Extinct species

• †
  extinct species from the late Miocene of what is now Southern California known primarily from fragments, and the occasional headless specimens.^[7]
• †
  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.^[2]

Description

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

cycloid scales and its silvery, iridescent guanine

coating is easily abraded.

Opahs closely resemble in shape the unrelated

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,

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).

Endothermy

The opah is the only fish known to exhibit whole body

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.^[8] 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.^[8] Regional endothermy also arose by convergent evolution in tuna, lamnid sharks and billfishes

where the swimming muscles and cranial organs are maintained at an elevated temperature compared to the surrounding water.

The large muscles powering the

counter-current heat exchanger

.

In addition to the rete mirable 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,[3] the cranial rete mirable has also evolved independently in other fishes. Unlike in billfish which have a specialised noncontractile tissue that functions as a brain heater, the opah cranium is heated by the contractions of the large eye muscles.^[8]

Behavior

On July 18, 2021, a 3-1/2 foot-long opah, weighing 100 pounds, was found washed up on the Northern Oregon coast. Heidi Dewar, a research biologist with the National Oceanic and Atmospheric Administration (

NOAA) Fisheries, was quoted as saying that the stranding was "unusual", and that climate change may have played a role in it.^[10]

Almost nothing is known of opah

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 family Lamnidae, billfishes, and some tunas.^[11][12] This may allow their eyes and brains to continue functioning during deep dives into water below 4 °C.^[11]

paratenic host.^[13]

The

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.

References

1. ^ Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology. 364: 560. Archived from the original on 20 February 2009. Retrieved 8 January 2008.
2. ^ ^{a b} 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.
3. ^
   PMID 25977549
   . Retrieved 18 February 2021.
4. ^ Pappas, Stephanie; LiveScience. "First Warm-Blooded Fish Discovered". Scientific American. Retrieved 15 May 2015.
5. ^ ^{a b} "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..."
6. ^
   PMID 29690102
   .
7. ^ David, Lore Rose. 10 January 1943. Miocene Fishes of Southern California The Society
8. ^
   PMID 19181893
   . Retrieved 18 February 2021.
9. PMC 7017440
   . Retrieved 20 February 2021.
10. ^ Firozi, Paulina (19 July 2021). "Massive and mysterious, a 100-pound fish washed ashore. Scientists hope to learn its secrets". Washington Post. Retrieved 19 July 2021.
11. ^ ^{a b} Bray, Dianne. "Opah, Lampris guttatus". Fishes of Australia. Archived from the original on 18 May 2015. Retrieved 16 September 2014.
12. OCLC 52386194
    .
13. ^ Scholz et al., 1998.