What's So Hot About the Opah? by Tanner Cullop

Lampris guttatus, commonly known as the Opah, is the first fish confirmed to be warm blooded. The Opah can be found in most all tropical oceans around the world excluding the Indian Ocean. What advantages and disadvantages are there to being a warm-blooded fish in relation to its ability to catch prey and evade predators? The Opah is able to warm blood within their eyes and brain to give them advantages that many of their prey and predators will not have when plunging into deep colder waters (Runcie et al.  2009). The explanation for cranial endothermy in the Opah is most likely due to the increased ability to expand its niche vertically in order to more efficiently feed.

Opah Lampris guttatus Photo by Fishcare Victoria   Creative Commons Flickr

            Little is known of the biological and ecological role of the Opah, based on captured and studied specimen it is thought that their diet mainly consists of squid and smaller fish (Jackson et al. 2000). They are a rare catch for many recreational and commercial fishermen as they are mostly found in deeper waters miles off-shore. Most Opah that are harvested are caught as by-catch from tuna longlines and are sold as one of the most sought-after sushi and sashimi fishes. The Opah is considered a trophy fish to commercial and recreational fishermen alike. They are described as a large, colorful, deep-bodied pelagic lampriform fish. There are only two species in the lampris genus, one living in local tropical temperature waters of most oceans, while the other can be found anywhere in the southern hemisphere.

            In 2015 the Opah was discovered to be the first warm-blooded fish (Pappas, 2015). This gives the ability for the fish to swim faster and more efficiently in deeper, colder waters. The Opah has the ability to dive to depths of 1300 feet in the search of its favorite prey, squid. These quick and drastic changes in depth and water temperature are accomplished by a process called counter current circulation. Cold oxygenated blood is passed along warm deoxygenated blood in the blood vessels warming the oxygenated blood throughout the fish. The ability to warm its blood gives the Opah a great advantage in the colder waters over its prey and most all of its predators making it easier for them to avoid predation. There is not a lot of research that has been completed on the warm-blooded features and anatomy of the Opah. Although there is readily available research on the cranial endothermy of the fish.

Cranial endothermy has evolved independently in lamnid sharks (family lamnidea), billfishes (families Xiphiidae and Istiophoridae) and in tunas (family Scombridae) is thought to decrease the effects of repeated deep-water dives into colder waters (Runcie et al. 2009). Researchers have taken localized temperature samples of regions of the cranial cavity of the Opah that is believed to be the source of their ability create cranial warmth. The researchers found that areas behind the eye were on average 2.1^oC and as much as 6.3^oC warmer than the myotomal muscle temperatures (Runcie et al. 2009). Observed in a laboratory the location of the heat source was determined to be the lateral rectus extraocular muscle which is located posterior to the eye and separated from the brain by a thin layer of bone (Runcie 2009).

Temperature profile of the opah with ambient temperature of 10.5 C.  Wegner et al. (2015).

            This concept of tissue specialization in the production of heat is not common in the animal kingdom. The ability for the Opah to be able to perform this endothermy is a key role in its survival. With the Opah’s main food source being squid that live in the deeper waters of the ocean having the ability to dive to these deeper, colder waters to feed is vital. The Opah keeps its brain and eyes warm by conserving metabolic heat with countercurrent heat exchangers in the circulation between the eyes and the brain (Block, 1986). The lateral rectus extraocular muscle has developed a morphology where the coratid rete is a counter current heat exchanger and prevents the loss of heat being produced (Block, 1986). The conservation of heat is accompanied by a layer of fat that is surrounding this muscle.

            The Opah, like most fish with cranial endothermy, is a predatory fish which moves vertically in the water column and experience drastic temperature changes quickly when moving into deeper waters (Runcie et al. 2009). This gives the Opah an advantage that many other fishes do not have allowing it the ability to go into these deeper, colder waters to feed. There are some studies being done now to determine if the Opah and its cranial endothermy might also advance its ability to see better when moving quickly in the water. The mechanics of this theory are thought to be similar to that of the blue marlin (Runcie et al. 2009). The ability for these fish to enter waters also give it the advantage of evading predators that are unable to swim to water with these conditions. 

            In conclusion, the Opah has been announced to be the first warm-blooded fish to be positively identified. Although there is not a lot of data yet released on the understanding and the science behind this warm-blooded lampris it is still yet fascinating to understand. The understanding of how the Opah uses its cranial endothermy to more efficiently hunt for prey and evade predators is most likely the backbone to understanding it being a warm-blooded fish. Cranial endothermy is a key morphological trait which is vital to the survival of the Opah, allowing it to repetitively dive into deep cold water to feed.

           

References

Block, B.A.  1986. Structure of the brain and eye heater tissue in marlins, sailfish, and spearfishes. Journal of Morphology 190: 169–189.

Jackson, G, Buxton N, George M (2000) Diet of the southern opah Lampris immaculatus on the              Patagonian Shelf; the significance of the squid Moroteuthis ingens and anthropogenic plastic. Marine Ecology Progress Series 206:261–271.

Papas, S (2015) First Warm-Blooded Fish Discovered.  Scientific American.  Available at:

 https://www.scientificamerican.com/article/first-warm-blooded-fish-discovered/.  Accessed 2 Apr 2018

Runcie, R.M., H. Dewar, D. R. Hawn, L. R. Frank, K. A. Dickson. 2009. Evidence for cranial endothermy in the opah (Lampris guttatus). Journal of Experimental Biology 212:461–470  

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(6236):786-789.