Life on Earth is both incredibly diverse and numerous. According to a study from Camilo Mora et al., there are likely 8.7 million eukaryotic species. With so many species populating our globe, the job of taxonomy, or the classification of organisms, is incredibly difficult. While there are many approaches to taxonomy, one common method is using traits to divide species into groups. However, due to convergent evolution, many species with different ancestors have similar traits because these traits allowed them to best adapt to their environment. While convergent evolution is well known, the recent example of a fully endothermic fish was a surprise because thermoregulation is a trait with a large impact for the organism, and there has been little deviation found across large groups of animals.
Thermoregulation is an organism’s ability to keep its body temperature within certain boundaries. One way organisms do this is through ectothermy, which is the use of an external source of temperature to maintain an organism’s internal temperature. The second class of thermoregulation is endothermy, where an organism creates most of its heat using metabolic processes which it then seeks to conserve. Endothermy has many advantages including increased aerobic performance, muscle power, and reaction rates. However it also costs great amounts of energy, meaning that endotherms need to eat at a much higher rate than other similarly sized ectotherms. The reason thermoregulation is such a key trait is because it neatly separated birds and mammals from almost all other animals. Birds and mammals are both endotherms, and use their increased levels of activity to consume more food to fuel their higher metabolic rate. However, food in the open ocean is far less abundant than on land, and many animals must travel far to find just enough food to survive. In these conditions, it seems unlikely that an increase in performance alone would allow any creature to consume enough food for it to truly make sense to be endothermic. Because of these challenges, very few fishes have acquired even the ability to to retain some internal heat.
The few fishes that are partially endothermic use this advantage to hunt in the cold nutrient rich waters of the deep ocean. Unlike on land, where the most nutrient are in the warmer climates, in the ocean it is the colder water that has the most nutrients. The accumulated detritus in the ocean tends to sink to deep water, and a fish that can spend more time in these waters will have an edge over other ectothermic fishes of similar size. A recent publication in Science by Wenger et al. mentioned many examples of regionally endothermic fishes including some tunas, lamnid sharks, and billfishes. Regional endotherms differ from full endotherms because the organism only warms a specific area or organ instead of the whole organism. These fishes cannot achieve full endothermy due to their body structure. The organs to be warmed are located towards the endotherm’s midline allowing these organs greatest amount of insulation from the outside environment and from the gills, which are the primary location of heat loss in fish. While this serves to warm individual organs, this does nothing to insulate the rest of the fish. Moreover, in order to reduce heat loss from the gills these endoderms create counter-current heat exchanges in order to use warm deoxygenated venous blood to heat the cold arterial oxygenated blood returning from the gills. High performance fishes only have these heat exchanges in connection with certain muscle groups and organs, and not the entire respiratory system.
In contrast to these fishes, the Lampris guttatus, also known as the opah, is fully endothermic. The opah has many unique mechanisms which allow it complete endothermy. Unlike other fish, which move using an undulating motion, the opah uses large pectoral muscles that serve as the primary site of metabolic heat creation. These pectoral muscles account for 16% of the total mass of the opah, making the pectoral muscles proportionally higher in the opah than most fish including other high performance partial endotherms. The heat is then conserved using a thick layer of connective fatty tissue. However, the most unique part of the opal’s thermoregulation is how it isolates its gills from the rest of its body. The opah developed retina mirabilia in each gill arch, which allow cold oxygenated blood to be heated by warm deoxygenated blood in a highly insulated structure. This differs from the other high performance fish which only form countercurrent heat exchanges around specific organs. The opah has these exchanges directly in the gills allowing the entire fish to remain a warmer temperature from the environment and be isolated from the primary location of heat loss.
Because of these many adaptations, the opah is the first, and so far only, fully endothermic fish. Like high performance fish, endothermy allows the opah to have enhanced physiological function, but the opah’s full body endothermy importantly allows the fish to warm its heart. Partially endothermic tunas and lamnid sharks are limited in their performance and time spent in nutrient rich cold waters due to their inability to warm their hearts. However, the opah has been found to spend far more time in deep and cold waters suggesting that the full-body endothermy and especially the warming of the heart allows the opah to surpass these limitations. The opah’s full body endothermy is also hypothesized to have many effects beyond mere enhanced physiological function including enhanced temporal resolution, neural conductance for eye and brain, faster food digestion and assimilation, and reduced effect of cold on organ performance. All of these advantages start to explain why the extremely costly trait of endothermy evolved in the opah. Despite its high metabolic cost, endothermy allows the opah to have a huge competitive edge and therefore was selected through years of evolution.
With all of this talk of evolution, it makes sense to compare the history of the high performance fish and the opah. These high performance fishes came from tropical ancestral species and developed regional endothermy to expand their hunting to deep seas. On the other hand, the opah likely came from an ancestral deep ocean fish that developed its endothermy to exploit those deep waters while being faster and have higher level of performance than other predators at that same level. Because the opah spends much of its life in these cold waters, the high price of endothermy has been selected for because it can compete better in the highly nutritious deep ocean environment. The opah is a great example of how traits shows up in unexpected places. While the opah is just one exception to the overarching trends of thermoregulation, there are millions more species to account for and study. With an ever-growing phylogenic tree, we can be confident that more unique species will be found with adaptations and traits that once again make the news and change how scientists approach a group.
1 Mora, C., Tittensor, D. P., Adl, S., Simpson, A. G. B., & Worm, B. (2011). How Many Species Are There on Earth and in the Ocean? PLoS Biol, 9(8), e1001127. http://doi.org/10.1371/journal.pbio.1001127
2 Wegner, N. C., Snodgrass, O. E., Dewar, H., & Hyde, J. R. (2015). Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science, 348(6236), 786–789. http://doi.org/10.1126/science.aaa8902