NATURE NOTES:
The Salt Flat Spider: An Enigmatic Arachnid Finds Sanctuary in the Harshest Desert Places

“They create a desert and call it peace,” a first-century Celtic chieftain said of the Roman army devastating his land, rousing his warriors to resistance. Our species can indeed lay the Earth to waste – rendering places lifeless through plunder and violence. But natural deserts are never sterile. They’re places of austerity, but also of hidden life.

For an emphatic illustration of that desert truth, consider Saltonia incerta, the salt flat spider. Its story is difficult to fathom, and it’s largely thanks to one scientist that we know of its existence. But in isolated populations, in North America’s harshest places, it’s living its arachnid life.

Sarah Crews is an arachnologist at the California Academy of Sciences, in San Francisco. 

“My whole deal with science is – I kind of like to just poke around and find stuff,” Crews said. “I like hypothesis-driven work, but with something like spiders, there’s so little known it’s hard to even come up with hypotheses. You have to go into the field and watch them and see what’s going on.”

In the late 90s, Crews was working at Berkeley’s late-lamented Cody’s Bookstore, when she took home a scientific volume on spiders. At home, she spilled something on the book, and had to purchase and keep it – and her vocation was sealed. She contacted elders in the field, and ultimately completed a PhD focused on spiders.

Her work has taken her around the world, and much of it centers on spiders called selenopids, or “flatties.” In Australia, she’s identified more than 50 new selenopid species. But she said she’s most often asked about a discovery early in her career.

Saltonia incerta was first identified in 1942, at only two locations – the shores of California’s Salton Sea, and on an island near the Colorado River Delta, in the Gulf of California. After the first locale was inundated, the salt flat spider was widely assumed to have gone extinct.

While doing her master’s research in the Mojave Desert, Crews frequently found herself in the vicinity of salt flats. She decided to “poke around” for Saltonia, looking beneath woody debris and human trash on the lake beds, and below their hard, salty surfaces. She found the missing spiders first at the Salton Sea, and next at Soda Dry Lake, in Mojave National Preserve. And that was just the beginning.

“I’d see all of these salt lakes,” Crews said. “If they’re here and they’re here, maybe they’re everywhere. So I started looking – and found them everywhere.”

The salt flat spiders were present in the salt flats of Baja California, in Mexico. Crews found them in Death Valley, the continent’s lowest and hottest point. Then she journeyed 800 miles east – to the Chihuahuan Desert. In what’s now White Sands National Park, she discovered the spiders living in the salt flats of Lake Lucero.

Crews hasn’t had the resources to investigate them all – but it’s likely Saltonia is found on salt flats across the Southwest, perhaps including those near the Guadalupe Mountains in West Texas.

Salt flats are among the starkest of desert places. Few if any plants can grow here. The salt flat spider is somehow adapted to tolerate these harsh chemical conditions. In fact, the spider has never been found outside such salty environments.

The spiders spin “soft-looking” webs, Crews said, for use both as retreats and for capturing food. The creatures are tiny – measuring a centimeter or less – and a nondescript brown. Some spiders puncture their prey – and suck out its innards. But the salt flat spider does some “chomping” first, Crews said.

“The Saltonia are really small,” she said. “But I’ve watched them eat under a microscope, and they’re pretty vicious. So it’s probably better that they’re not very big.”

Much about the spider’s behavior is unknown – but Crews has seen their mating ritual in her lab. To announce his intention, the male plucks the female’s web with a leg or one of his “pedipalps” – facial protuberances that are often mistaken for fangs.

“It’s probably a ‘don’t eat me’ kind of thing,” Crews said, “and an ‘I like you’ kind of thing.”

But these spiders raise one confounding question in particular. How did they come to occupy their isolated salt flat abodes, separated as those habitats are by great distances?

One possibility is the spider behavior known as “ballooning.” 

We might think of the Wright Brothers’ first flight as a signal event in Earth’s history. But we aren’t the only wingless creatures that have attained to flight. 

In many spider species, juveniles setting out to find new territory can release silk in such a way that they are carried aloft. Such “ballooning” spiders can reach high altitudes, and cross entire oceans. Crews considered this to explain Saltonia’s distribution. But the odds of a ballooning spider being blown from one salt flat to another are slim, and prevailing winds here wouldn’t work to their benefit. 

From White Sands to Lake Bonneville in Utah, the salt flats of the Southwest are the remnants of massive lakes from the Ice Age past, when the region was wetter and cooler. Crews said this is likely the backdrop for Saltonia’s distribution. 

The salt flat spider may once have been widespread along the margins of those Ice Age lakes. As the lakes receded and disappeared, arid salt flats became the spider’s only refuge.  

Yet there are other findings that strain comprehension. Crews said the salt flat spider’s closest living relative appears to be a spider that occupies salty marshes in the Middle East. And related spiders have now been found on salt flats in Australia. How these kindred creatures came to occupy such far flung niches is a mystery.

It’s a reminder of how little we know about these ubiquitous creatures. Arachnids were among the planet’s first land-dwelling creatures – having adopted a terrestrial life at least 420 million years ago. Some 50,000 spider species have been described by science – but the total number of species is estimated at between 100,000 and 300,000.

They’re primal predators – and can inspire in some a primal dread. But they can also elicit fascination, Crews said.

“I always tell people,” Crews said, “when they say they don’t like spiders, that I bet we could find at least one that they like.”

Certainly the salt flat spider, as a singular desert survivor, merits our respect.