In Death Valley, life can be difficult. One might think that such a dry area would be a bad place for fish to live, and it is. But that is exactly why it is such a great habitat for this particular fish, Cyprinodon salinus, as well as the other desert pupfishes.
The salt creeks and pools of the California desert evaporate quickly, making their salinity change day by day. In the winter some creeks will be essentially freshwater, while in the hottest parts of the summer the water can become twice as salty as the ocean. Because the desert pupfish can handle this kind of fluctuation, which would kill most of the rest of us, they usually get the creek to themselves, with no other competing fish.
Some desert pupfishes in South America even live in ponds that dry up entirely during the summer. They lay their eggs in the mud before it dries, then when the rain starts to fall again, the population is reconstituted and the eggs begin to hatch.
This past March, while visiting Death Valley with his family, Casey Dunn, the principle investigator of our lab at Brown University, visited a salt water creek and found these pupfish spinning around each other while mating. The females are the smaller ones, and they lay one egg at a time. A male will swim up next to her, they will both curve their bodies into an S shape, the female drops an egg into the male’s fin, he fertilizes the egg, then drops it on the floor of the creek. In this clip the males are tagging off, each taking turns fertilizing eggs as they come out of the females.
This video is released under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 license. Thanks to Maria Dzul for pointing me towards some information about desert pupfish. Here is a paper about pupfish and the fluctuating salinity of their water, here is a description of C. macularius‘ mating behavior, and here is a nice book about California fish, which might be at your local library.