Guest Blogger: Ryan Stanley     

The following guest post was voted best in the class for an undergraduate assignment on fish adaptation from Humboldt State University’s Fall 2019 Fish Conservation and Management course taught by Dr. Andre Buchheister. The assignment required students to find a peer-reviewed article on a fascinating fish adaptation or species and to communicate the scientific information in a blog format to a general audience. 

The Hottest Place on Earth

If you have ever been to the Mojave desert in the southwestern United States, you immediately understand how the hottest place on earth got the appropriate name of Death Valley. The dry desert is expansive and intimidating, but the region is dotted with small pools that some pretty amazing fish, desert pupfish, call home. Pupfish are small, less than 3 inches, and can be very active swimmers. Males will often give chase to fleeing females after spawning and that seemingly playful, puppy-like behavior is the reason they got their name. But unlike dogs, pupfish come in a wide array of colors, ranging from silvers and tans, to bright blues and purples during breeding times. 

How did fish ever come to live in this hot, dry place? Ancient lakes were once part of this desert region, massive lakes that were formed during glacial periods. It is believed that the pupfish of today came from these lakes and when conditions were right, slowly spread out in the region when water connected the areas that are now separated by dry desert and mountains (Knott et al. 2008, Martin et al. 2016).

As the ancient lakes slowly dried, habitats became smaller and smaller. Groups of pupfish became isolated in their pools and overtime, genetically separated just enough to become the 30 or so unique pupfish species now found in the desert southwest (Jahren et al. 2002; Echelle et al. 2005). Although the area is much drier now, a large underground water network still exists and provides critical habitat throughout the region.

The Devil’s Hole pupfish (Cyprinodon diabolis) is just one of these species and has a modest habitat with just enough resources to support this small but hardy group (Jahren et al. 2002, Riggs and Deacon 2002). This species is a perfect example of just how adapted to surviving the pupfish of the desert are. The Devil’s Hole pupfish lives in the smallest known habitat of any vertebrate, reproduces and forages in an area about a ¼ of the size of a basketball court, and is often called the world’s rarest fish (Hausner et al. 2014; Martin et al. 2016).

Although habitats vary, life history and breeding habits are similar amongst the different species of pupfish. The average lifespan is only about 1 year, the breeding season is year round and allows for multiple generations to be produced in a year. Pupfish are not too picky or monogamous and some species are able to reproduce at only 4 weeks old (Moyle et al. 1995).  These rapid life cycles must have allowed individuals who were the best at surviving to quickly pass on those survival adaptations. One adaptation found in these fish is the ability to tolerate the extreme water temperatures found in the desert, sometimes up to 111°F (Martin eal. 2016). 

The Devil’s Hole pupfish and its neighboring species, like the Salt Creek pupfish (Cyprinodon salinus) and the Amargosa River pupfish (Cyprinodon nevadensis amargosae) all tolerate these extreme temperatures (Jahren et al. 2001, Lema et al. 2016). Handling the heat is what they do best! A recent study may have found a clue that could help explain how these fish can tolerate the hot temperatures and is, quite simply, breathtaking…

No oxygen? No problem?

Not only do these little fish have to deal with extreme temperatures, but they have adapted to live in water with very little oxygen. The warm waters of pupfish habitat, like Devil’s Hole, are naturally lacking in oxygen. So how do the pupfish do it? Bigger and more efficient gills to grab as much oxygen as possible? No, they just stop using oxygen for sometimes up to 2 hours! An action is termed “paradoxical anaerobism” (Heuton et al. 2015). Researchers have discovered that these fish have the ability to swap back and forth between periods of using oxygen to periods of not processing any oxygen. Now, these fish may not be using oxygen, but they are still “breathing.” Just without oxygen. It seems counterintuitive, but humans do something similar when we hold our breath or engage in strenuous (anaerobic) exercise. A big difference is these pupfish have become way more efficient and are still able to function fully during extended oxygen-free periods (Heuton et al. 2015; Heuton et al. 2018).

Everyone knows the feeling of trying to catch your breath after a hard workout. That rapid breathing is needed so we can break down something called lactic acid, a toxic compound that builds up in our muscles when they don’t get enough oxygen. Lactic acid is responsible for that burning sensation we get in our muscles that forces us to take a break. But, pupfish have found a workaround, they produce a far less toxic compound during anaerobic exercise, ethanol (Heuton et al. 2015 and 2018). Ethanol just might be the reason these little fish are so efficient and long lasting during the oxygen free periods…maybe their little fins don’t get that burning sensation!    

This ability to switch-off the use of oxygen might seem useful but scientists are still unclear at what triggers the oxygen/no oxygen swap. The exchange has been observed to happen when there is plenty of oxygen available, so it is believed that there is a connection between pupfish breathing and how these fish conserve energy to survive in the hot waters of the Mojave and Sonoran deserts (Lema et al. 2016; Heuton et al. 2018). Maybe this adaptation is something that continues to be finely tuned as the climate warms and pupfish habitat becomes increasingly warmer and threatened. Sure, these fish live in hot water, but small changes to their delicate environment affects eggs, newly hatched fish, and reproducing adults in a negative way (Hausner et al. 2014). Hopefully, the individuals that do survive will pass on those hot water survival skills to the next generation.

Fish living in the hottest place on earth seems like a bad idea, but the pupfish of the desert have a very unique adaptation that has enabled them to succeed through time. Ancient lakes have become isolated habitats, with each group of pupfish just as unique as the place they live. The warming climate and habitat fragility look to be the biggest threats to these survival experts, and their skills will be put to the test. I’m a sucker for an underdog, and I’m rooting for the pupfish!

About the Author

For other Humboldt State student posts, please see:

• GOOD ADDITION? OR BAD INVADER? CHINOOK SALMON IN SOUTH AMERICA by Alexander Eaton
• CLEARLY, THE COOLEST FISH IN THE SEA by Justin Miller
• A PLATE OF LIES by Meghan Fox
• DANANANANANANANA BAT RAY by Angela Schmidt
• FROM REEF BANK TO FISH TANK: HOW THE AQUARIUM TRADE CAN IMPACT CORAL REEF CONSERVATION by Josh Cahill
• THE PEACEFUL BETTA by Joelle Montes

Literature cited:

Echelle, A. A., Carson E. W., Echelle A. F., Van Den Bussche, R. A., Dowling, T., and A. Meyer. 2005. Historical Biogeography of the New-World Pupfish Genus Cyprinodon (Teleostei: Cyprinodontidae). Copeia 2005(2): 320–339.

Hausner, M. B., Wilson, K. P., Gaines, D. B., Suárez, F., Scoppettone, G. G., and S. W. Tyler. 2014. Life in a fishbowl: Prospects for the endangered Devils Hole pupfish (Cyprinodon diabolis) in a changing climate. Water Resources Research 50(8): 7020–7034.

Hausner, M. B., Wilson, K. P., Gaines, D. B., Suárez, F., and S. W. Tyler. 2013. The shallow thermal regime of Devils Hole, Death Valley National Park. Limnology and Oceanography: Fluids and Environments 3(1): 119–138.

Heuton, M., Ayala, L., Burg, C., Dayton, K., McKenna, K., Morante, A., and F. Van Breukelen. 2015. Paradoxical anaerobism in desert pupfish. The Journal of Experimental Biology 218(23): 3739.

Heuton, M., Ayala, L., Morante, A., Dayton, K., Jones, A. C., Hunt, J. R., and S. Hillyard. 2018. Oxygen consumption of desert pupfish at ecologically relevant temperatures suggests a significant role for anaerobic metabolism. Journal of Comparative Physiology B 188(5): 821–830.

Jahren, A. H., and K. L. Sanford. 2002. Ground-water is the Ultimate Source of the Salt Creek Pupfish Habitat, Death Valley, U.S.A. Journal of Arid Environments 51(3): 401–411.

Knott, J. R., Machette, M. N., Klinger, R. E., Sarna-Wojcicki, A. M., Liddicoat, J. C., Tinsley, J. C., III, and V.M. Ebbs. 2008. Reconstructing late Pliocene to middle Pleistocene Death Valley lakes and river systems as a test of pupfish (Cyprinodontidae) dispersal hypotheses. in M. C. Reheis, R. Hershler, and D. M. Miller, editors.Late Cenozoic Drainage History of the Southwestern Great Basin and Lower Colorado River Region: Geologic and Biotic Perspectives.

Lema, S. C., Chow, M. I., Resner, E. J., Westman, A. A., May, D., Dittman, A. H., and K.M. Hardy. 2016. Endocrine and metabolic impacts of warming aquatic habitats: Differential responses between recently isolated populations of a eurythermal desert pupfish. Conservation Physiology 4(1).

Martin, C. H., Crawford, J. E., Turner, B. J., and L.H. Simons. 2016. Diabolical survival in Death Valley: Recent pupfish colonization, gene flow and genetic assimilation in the smallest species range on earth. Proceedings of the Royal Society B: Biological Sciences 283(1823).

Moyle, P. B., Yoshiyama, R. M., Williams J. E., and E. D. Wikramanayake. 1995. Fish species of special concern in California, 2nd edition. Department of Wildlife and Fisheries Biology. Davis, California

Riggs, A. C., and J. E. Deacon. 2002. Connectivity in Desert Aquatic Ecosystems: The Devils Hole Story. In Proceedings: Spring-fed Wetlands: Important Scientific and Cultural Resources of the Intermountain Region., Las Vegas, Nevada. D.W. Sada and S.E. Sharpe, editors. DHS Publication no. 41210