Park Mysteries

Instant Life: Just Add Water

In the harsh arid landscape of Canyonlands National Park, some peculiar organisms spring to life after only a few inches of rainfall.

Peer into any fleeting water hole in the desert Southwest, and you’re likely to find it teeming with all kinds of critters. That might come as quite a surprise, since the same hole was bone dry just days before. Water comes and goes quickly in the Southwest, so the region is dotted with temporary, or ephemeral, water sources. The weather’s extremes yield potholes in sandstone and other rock surfaces—shallow impressions just a few feet across. Scoured by wind in drier days, those potholes often fill with water and dry out several times during the summer monsoons common in the Southwest.

Temperatures can swing from 140 degrees Fahrenheit in summer to below freezing in winter. And in this setting, a whole host of critters seem to come and go as quickly as storm clouds. Is it magic? Not quite. It’s actually a finely tuned biological adaptation achieved by some of the oldest creatures on Earth. Fairy shrimp, tadpole shrimp, and clam shrimp are all capable of this impressive feat, a skill not uncommon among a category of animals called branchiopods. As a group, branchiopods are united by the presence of gills on their legs and various appendages used for swimming. But each of the shrimp-like members of the group looks a little different. Tadpole shrimp have a carapace—the part of the shell that covers the head—and fairy shrimp (including brine shrimp) lack it. The clam shrimp carapace is doubled over so it resembles a little clam, as its name suggests.

All the shrimp-like branchiopods feed themselves by filtering algae and detritus out of the water. Their eggs can survive years of drought and hatch within days of rain. Most are less than half an inch long, but at least one species of fairy shrimp can grow to six inches—big enough to eat its smaller kin.

Tim Graham, a research ecologist with the U.S. Geological Survey in Canyonlands National Park, has been studying the park’s fairy shrimp, tadpole shrimp, and clam shrimp since the late 1980s. Although he doesn’t play in these miniature “desert tidepools” as much as he’d like, he’s spent more than a few hours staring into the water and recording as many species as he can find. All that experience has shaped his understanding of their tenuous existence, and the likelihood they’ll survive a future that may include drastic climate change.

Evolution has not been especially kind to branchiopods to this point. Newer species— including predators such as fish and aquatic insects—have evolved to live in the same habitats. Simultaneously, branchiopod populations have declined. Some studies show this can happen in short order: Introduce fish, and the shrimp quickly disappear.

These days, branchiopods fare best in pools that periodically dry up, making it harder for those predatory aquatic insects to get established. “The insects have to get up and leave,” Graham explains. “They can fly from pool to pool, but they can’t run around on the bottom of a hot, dry sandy pothole.”

Indeed, generations of branchiopods never have to leave the pool at all, which means they’ve got the home-court advantage. Their eggs can lose up to 92 percent of their water content and still survive. As a sort of evolutionary hedge against unpredictable weather, each female lays eggs that hatch according to different plans: some after a first rain, others after subsequent showers. And they lay a lot of eggs—up to 200 per day. One generation of branchiopods can hatch, grow, and lay the next generation before their predators have even matured. By then, they’ve already learned to avoid predators quite effectively.

Branchiopods’ hardiness serves them in some extreme—and unlikely—conditions. In 1980, one researcher glued brine shrimp cysts to a spacecraft and retrieved them after the space flight; they hatched into living brine shrimp a short time later. Scientists are still working to discover all the factors that trigger hatching. Some species respond best to warm water, and won’t hatch after a cold rain. Other mysteries remain unsolved.

“The community we see after a particular rain event is not as predictable as we would like,” says Graham “There are some situations where you might find only one species [even though] there are cysts from both species in the same pothole.”

Until ecologists can tease out the reasons, it’s not clear how branchiopods are likely to fare in the face of climate change. Christopher Rogers, an invertebrate ecologist with the consulting firm EcoAnalysts in Woodland, California, points to recent findings that increased ultraviolet light may cause more mutations in such animals, possibly speeding their ability to adapt to different habitats.

“If temperatures are going up and more things are going dry, it’s just going to mean more temporary wetlands for them,” says Rogers. “On the flip side, the Ice Ages pushed many species further south,” which means global warming might push these species north. And that means branchiopods will need to continue evolving or face an uncertain future.

Anne Minard is a freelance writer and journalism teacher who lives in Flagstaff, Arizona.