What Makes Utah’s Desert Potholes So Mysterious—And Miraculous

It carries the wonder of a miracle, and some of its mystery, the little pool cupped in the rock of a Utah desert. Mere inches deep and a few feet across, the water that rain has brought will evaporate in a few days or weeks. Yet while it lasts, fairy shrimp and a host of other aquatic creatures thrive in it.

But how did such life come to occupy this precarious niche, and how can it hope to survive the desiccation ahead?

"Maybe the colonies are remnants of a wetter age," says Dr. Lewis T. Nielsen of the University of Utah. "Maybe they began with eggs borne by the wind or birds. No one can say for sure."

Adds Steven V. Romney, "They survive through amazing versatility in adapting to change."

I met the two biologists on a Utah field trip while Steve was working on his doctoral dissertation; Lew was his faculty adviser. Steve's subject area: desert potholes. And any conversation with him is a revelation.

For example, "There is a pothole gnat in West Africa," he says, "whose larva, and sometimes its pupa, can survive for unknown lengths of time after losing most of its body moisture—as much as 92 percent. Immersion brings full recovery, sometimes in a few minutes. We have a similar form here."

I was entranced by visions of "instant life" through the miracle of water and shared my enthusiasm with the Geographic's natural science photographer, Robert F. Sisson.

Soon Bob was photographing these fairy shrimp, here enlarged 15 times, and we were compiling their biographies. The two at top are males; they bear antennae that have become modified for clasping females. The three below are females; they bear egg sacs to help assure the next generation.

The female turned on her side at left reveals a food groove on her belly that collects plankton as she swims. The others swim upside down to our eye but normally for them.

Gill-like plates on eleven pairs of legs capture life-sustaining oxygen from water or from the air above it and give the shrimp another name: branchio-pods—"foot-breathers."

Fairy shrimp hedge their survival bets by laying two types of eggs. So-called summer eggs will hatch the first time the pothole refills. But winter eggs may lie in the pothole's sandy substrate through repeated fillings, over one season or many, hatching at different times.

Seeking more facts on desert potholes, I found that literature on the subject is virtually nonexistent. In this little water world of wonders, we probe at the edges of man's knowledge.

The promise of life exists anywhere water collects, but that promise in a desert pothole is doubtful at best. Even when the hole contains water, the amount, temperature, and chemistry may change radically in a matter of minutes.

"The stresses on life posed by the pothole are among the most extreme in the world," comments Lew Nielsen, here dipping specimens with Bob Sisson's son, Robert. They investigate a pothole in Canyonlands National Park, Utah.

In shallow pools, Lew explains, the turn from day to night can bring a 30° drop in temperatures, a variation virtually unheard of in larger bodies of water.

Sudden downpours can lower temperatures a degree a minute, at the same time radically altering the alkalinity and oxygen content.

Where natural drainage lines feed a pothole such as this one, torrential runoff can refill a basin in seconds, or scour through it with such force that its small residents—such as the 1⅛-inch-long tadpole shrimp—are flushed away to certain death.

Ultimately the pothole goes dry. Using a ladder to lug cameras, lights, and dissecting kit to the shelflike pothole, Bob Sisson combs dry sands where Lew, just days before, had dipped up wriggling life.

Later Bob laid a thermometer on the sand, and the mercury rose off its scale at 124° F. Estimates of pothole surface readings range to 150°. In winter, temperatures in this mile-high plateau country plunge to below zero.

"You've just got to have respect for eggs that can survive all that," Bob concludes.

From egg to adult and back to egg, the life cycle of the pothole mosquito may take as little as six days. In a realm where the water may vanish at any time, such speed is a decided asset and one that most life forms share. But unlike other residents of the pool, which can occupy a variety of habitats, the pothole mosquito lives only in potholes—an all-or-nothing gamble on little pockets of water.

"The eggs hatch early in the pothole's life cycle, well ahead of other mosquito species," explains Steve Romney, whose research focused on the pothole mosquito. "Mating and feeding occur in the cooler, moister air of night or dawn, and the egg-laying female seeks out sheltered cracks or niches at the waterline, where the eggs escape the worst of the heat and cold and wind."

Here two larvae try their new world after popping the caps off eggs to emerge. Among their hazards: a parasitic fungus that is usually fatal. Discovered by Steve during his study, the parasite may prove to be a control agent harmless to other life.

In as little as four days the larvae become pupae, with long bowed tails and huge dark eyes topped by little breathing horns.

In as little as 36 hours, the pupa turns dark brown, the long tail straightens just under the surface of the pool, and the adult begins to escape from a split along the top of the pupal case.

For a vulnerable two or three minutes, the adult labors to extract its long wings and legs, still soft and easily mangled. Now it has no defense against hungry toads, spiders, or dragonflies.

Finally free, a female stands briefly on the surface tension of the pool (right). Soon she will hasten the hardening of her wings by gliding about the pool like an ice skater. Then she will fly away to fulfill her mission:

Find a male with which to mate and a warm-blooded creature to bite, essentials for the fertilization and nourishment of her eggs.

"Water tiger!" The larval form of a pothole beetle earns its name. Multiple eyes, swift legs, and a rapacious appetite make it one of the fiercest predators in a pool. Its mouth parts include hypodermic-like jabbers for injecting substances that turn the insides of victims into suckable juices.

Cunning hunter, the adult beetle first bites off the tail of a hapless tadpole, which otherwise might have a chance of escape. Then, in a leisurely ten minutes, the diner reduces its dinner to a sleeve of sagging skin.

An air breather, the adult beetle totes a bubble under its wing case, now and then taking in new air and jettisoning the old. It also has wings for flying, permitting escape from a dying pothole, and specially adapted rear legs for swimming.

The mating of the clam shrimp is a joyous ballet to our eyes, yet it is actually a dance of death.

Their little rock-pool home has shrunk steadily during days of unrelenting sunshine. Since early this September morning, Bob Sisson, Lew Nielsen, and I have kept watch beside the diminishing water. Before the sun sets, we calculate, the last of it will evaporate. For the little pool's thousands, this is doomsday.

By 10:30 a.m., as the heat mounts, the pothole's populace shows agitation. Snails burrow into sand, where they have hope of surviving while the dampness lasts. Tadpole shrimp wallow downward into the bottom mud, as if in retreat from the pool's lowering surface. In the shallow fringes fairy shrimp by the hundreds flail and writhe, their wet bodies reflecting dancing bits of sunlight. Marooned by retreating water, they can only die, together with myriads of speck-size seed shrimp, copepods, and other tiny crustaceans.

In the narrowing, warming pool the clam shrimp—up to now given to only occasional mating—become possessed by the drive to reproduce. A score or so of pairs, most of the clam shrimp populace, swim locked in a T embrace, the male grasping the belly of the female, whose translucent form bares orange masses of eggs.

They weave a giddy course of dives and soaring climbs, turns and glides, impeded now and then by lone males trying to cut in. At last they part, the male to burrow into the sand, the female following after a moment's rest.

Next day we return to find only damp sand in the pothole. So fragile are all its tiny dead creatures that we are hard put to discover a carcass intact. Sun and wind have already dried and crumbled the tiny victims.

But in the sand the snails still live, and in thousands of eggs and in the gnats' pupae and larvae, the seeds of a new generation await only the coming of rain. Then the hole will fill once more, the water will warm, and the sparkling pool will teem again with the vigor of the life force as on the first day of the world.