Warm Blooded Opah, by Kieran Barlow

The Lampris guttatus is a unique marine fish of many names: Moonfish, Kingfish, Arokura, Cravo, Gudlax, Jerusalem Haddock, but the most common name is the Opah (Smith-Vaniz et al. 2015). There are many interesting features to this fish, such as its strange body shape and bright coloration; however, the most striking difference in the Opah, is that it’s the only completely endothermic fish. In this essay, I will define the complex morphological structures and behaviors of the species to present a deeper understanding of the Opah itself, along with how its endothermic body effects its lifestyle.

NOAA Fisheries researcher Nick Wegner holds an Opah.   Photo Credit:  NOAA Fisheries.

Opahs are ray-finned fishes (Class Actinopterygii) of family Lampridae. There were thought to be only two living species in their Genus Lampris, the Lampris guttatus and Lampris immaculatus. Although, according to a recently published taxonomic review of the Opah, there are six species that have since diverged from the Lampris guttatus species, with their own specific geographic ranges (Underkoffler et al. 2018). However, for clarification, even though all six species are Opahs, we are mostly looking at the species Lampris guttatus specifically. As the taxonomic study was published fairly recently and ichthyologists have not been able to study these independent species in depth due to a lack of data.

Lampris guttatus is a pelagic fish species that is steel blue with white spots, minute cycloid scales, very deeply compressed disc-like body shape, red dorsal, anal, caudal, and large falcate pectoral fins, along with a pointed snout and distinctly convex head (Bray 2018). Size of adult Opahs can vary, but the average for the Lampris guttatus is 2 meters (or 6.6 feet) and 140 kilograms (or 308 pounds), about the size and weight of a manhole cover.  Not much is known about the behavior or ecology of these fishes. Lampris guttatus do not school with other Opahs, yet are seen to school with other scombrid fish species, and Opahs regularly dive to depths from 50 to 500 meters (possibly into the edges of the bathypelagic zone) to feed on mesopelagic fishes, squids, and crustaceans (Runcie 2009). Despite the Opah’s round and un-streamlined appearance, they are expert swimmers, using labriform (lift-based) locomotion through the “flapping” of their pectoral fins along with their sharply forked caudal fin to swim at a consistent high speed. While moving through considerable depths of water, the Lampris guttatus uses a similar mechanism as lamnid sharks and billfishes, raising the temperature behind the eyes in lower temperatures to regulate cranial and eye functioning in harsh conditions (Nordahl et al. 2018).

Opah swimming in near surface waters.  Photo credit: Ralph Pace. 

The Opah is well known by ichthyologists because it is the first fish, and only fish so far, discovered to maintain a habitable core temperature through its entire body, regardless of outside temperature changes, Opahs are the only wholly warm-blooded fish. Most fishes become sluggish in colder temperatures in order to conserve energy, but due to the Opah’s endothermy, Lampris guttatus can keep quick movement, metabolism, and reaction times, making this fish a very active predator in even colder water temperatures (Wegner et al. 2015). But how do these fishes function endothermically and why don’t other fish use the same process? The main reason is the Opah’s unique gill structure and respiration technique. The gills of Lampris guttatus are very densely packed with networking veins and arteries, the vasculature of the gill tissue (rete mirabile) uses counter-current heat exchange, using heat generated from the movement of pectoral fin muscles that gets passed to the Opah’s gills through deoxygenated blood.

The heat is transferred to oxygenated blood flowing through the arteries, which then flows to the rest of the body, keeping the Lampris guttatus at 5 degrees Celsius (or 41 degrees Fahrenheit) warmer than its surroundings (Mohapatra et al. 2018). There are a few fish species that have regional endothermy (such as lamnid sharks, billfishes, and some salmon species). This is due to their rete mirabile warming only specific muscles and organs (excluding the heart), typically aiding in propulsion and sight, but with all other organs and muscles remaining colder. Although, Lampris guttatus avoids this by having the rete mirabile within the gill arches insulated by fat storages, therefore keeping heat in the body for an extended period of time while completely heating the heart and other muscles of the Opah.

Opah with temperature sensor attached.  Photo by NOAA Fisheries. CC BY-NC-NC 2.0.

Sadly, the Lampris guttatus is an understudied fish. As the Opah are often arduous to catch due to them being quick and deep swimmers. Opah is not a major commercial seafood, despite the fact Opahs can be sold for a high price for consumption (sold for $3 million in 2012 Hawaiian Opah market). The reason there is so much information for Lampris guttatus now is mostly due to their full-body endothermy (Lee et al. 2016).  The Genus Lampris itself, as I stated previously, has a lack of information regarding the independent species due to the newest taxonomic split, but it is assumed to be very similar to the guttatus. I hope to see more studies in the future about this fascinating fish species to both expand our knowledge of fish species as a whole and to preserve the Opah for future generations.

References

Bray, D.J. 2018, “Lampris australensis in Fishes of Australia,” Accessed 06 Apr 2019, http://fishesofaustralia.net.au/Home/species/1870.

Lee, J.J. 2016. “Rarely Seen Moonfish, Size of Manhole Cover, Caught on Camera.” National Geographic, National Geographic Society, 8 May 2016, news.nationalgeographic.com/news/2015/02/150205-opah-fish-sushi-ocean-animals-science/.

Underkoffler, K.E., M.A. Luers, J.R. Hyde, M. T. Craig. 2018. "A taxonomic review of Lampris guttatus (Brünnich 1788) (Lampridiformes; Lampridae) with descriptions of three new species". Zootaxa  doi:10.11646/zootaxa.4413.3.9.  

Mohapatra, A.. Roy, A.. “Opah: The World’s First Warm-Blooded Fish.” Odisha Bigyan Academy. Science Horizon, 2 May 2018. http://www.orissabigyanacademy.nic.in/pdf/May_2017_Science_Horizon-2882017.pdf#page=32http://science.sciencemag.org/content/348/6236/786.

NOAA Fisheries. “Seafood Industry and Scientists Team Up to Make the Most of Opah.” NOAA Fisheries, October 15, 2018. https://www.fisheries.noaa.gov/feature-story/seafood-industry-and-scientists-team-make-most-opah.

Oscar, N., et al. “Sun-basking fish benefit from body temperatures that are higher than ambient water.” Proceedings. Biological Sciences vol. 285,1879 (2018): 20180639. doi:10.1098/rspb.2018.0639.

Runcie, Rosa M et al.  2009. Evidence for cranial endothermy in the opah (Lampris guttatus).  The Journal of Experimental Biology 212 (Pt 4): 461-70. doi:10.1242/jeb.022814

Smith-Vaniz, W.F., Collette, B., Moore, J., Polanco Fernandez, A., Russell, B. & McEachran, J.D. 2015. Lampris guttatus (errata version published in 2017). The IUCN Red List of Threatened Species 2015: e.T195038A115338069. http://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T195038A19929436.en. Downloaded on 28 March 2019.

 Warm-Blooded Makes Opah an Agile Predator.Fisheries Resources Division of the Southwest Fisheries Science Center of the National Oceanic and Atmospheric Administration. May 12, 2015.

Wegner, N. C., O. E. Snodgrass, H. Dewar, and J. R. Hyde. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science 348:786–789.