When my Texas friends asked me whether I planned on snorkeling in the West Texas flatlands, I thought they were joking. But a few days later I found myself rushing to my rental car to search the desert shops for mask and snorkel.

The occasion was a road trip from Austin, where I visited friends, to Mexico, where I rejoined my sailboat, Honu. After a long day of driving, I stopped at a Texas state park called Balmorhea. There an underground spring pumps 22 million to 28 million gallons of fresh water into the desert each day, creating a natural oasis.

Oases in deserts aren’t unusual but this one is. In the 1930s, workers with the Civilian Conservation Corps dredged the spring and built limestone walls around it. When finished, the structure became the largest spring-fed swimming pool in the world. From 5 to 25 feet deep, the pool holds about 3.5 million gallons of cool water. Temperatures range from 72 to 76 degrees.

This looks like a normal swimming pool (although much bigger) until you walk to the edge. The bottom consists of natural rocks and rubble of the area, and the water teems with fish.

Inspecting the overflow of this wondrous pool, I found a wetland area, re-created in the 1990s for native aquatic wildlife. Among these are two endangered fish species with colorful Southwestern names: the Comanche pupfish and the Pecos mosquitofish.

Although it sounds like an oxymoron, pupfish are strictly desert fish. The 2-inch-long fish adapt well to harsh desert conditions, tolerating wide ranges in temperatures and salinities. The Comanche pupfish, found only in Balmorhea, is greenish above, whitish below, and has orange fins. Females lay about 30 eggs, which hatch in five days.

The other endangered species, the Pecos mosquitofish, doesn’t lay eggs at all. Females give birth to live young.

The Pecos mosquitofish is one of about 40 species of fish known as mosquitofish because they eat, among other things, mosquito larvae. Most mosquitofish are freshwater species, but some can survive in brackish, ocean or supersalty water. One species has been found thriving in Hawaii in water about 6 percent salt, twice the salinity of sea water.

Mosquitofish were introduced to Hawaii in the early 1900s to control mosquitoes, but whether they do this is debatable. Studies show that the presence of mosquitofish can actually increase the number of mosquitoes in an area because the fish also eat the larvae of natural mosquito predators, such as beetles and dragonflies.

Mosquitofish are prolific. A female can store enough sperm from one male to fertilize several broods of offspring per year. As a result, a single female can colonize an entire wetland area. This is a problem in Hawaii where the alien fish prey on native species, such as gobies and our tiny, red anchialine shrimp.

The Pecos mosquitofish and its pool mate, the Comanche pupfish, have dwindled in numbers, however, because people diverted and pumped water for years from the fishes’ springs. Now, with the re-establishment of three acres of wetlands at Balmorhea, and increased awareness of these remarkable fish, Texas citizens are giving back and sharing water with their aquatic native species.

After realizing I could swim with these rare fish, I drove from shop to shop to rent snorkel gear. But the season is over and all were closed.

I didn’t stay disappointed for long because the next day, in bright sunlight, I could easily see the little fish through the clear spring water.

I plan to visit this popular dive site again, and next time I’ll bring the gear to appreciate it properly. I want to say I went snorkeling in West Texas — and not be joking.

Susan Scott can be reached at www.susanscott.net.