Meet the 1,000 Spider Species Living in Caves. Some Feed on Fish and Frogs

A softball-sized spider known as Califorctenus cacachilensis, found in a cave in Baja California. San Diego Natural History Museum

It takes a certain nerve to write about cave spiders. They can't speak, after all; they cannot correct you if you make a mistake. There is something solemn about them. You don't want to let them down.

Like all arachnids, cave spiders are barely known. But the little we've uncovered is intriguing. So far, spelunking scientists have described more than 1,000 species that live in caves. These includes both the largest and the smallest ever described, says Marco Isaia, an arachnologist at the University of Torino. The most enormous, the giant huntsman spider (Heteropoda maxima), haunts the caves of Laos, with a leg-span of one foot. Though they are not deadly—like the vast majority of spiders—their bite "can cause swelling, nausea, vomiting and headaches," according to the Encyclopedia of Life. On the other end is our smallest spider, known by its Latin name Anapistula ataecina, which measures 0.4 millimeters in diameter. It's considered critically endangered by the International Union for the Conservation of Nature, but hangs on by a thread at four locations in the Frade cave system of Portugal. Twenty percent of this underground complex has been recently destroyed by quarries.

All these creatures live in pitch darkness, sometimes going months without eating. Many have no eyes and cannot see. A good number also have very long legs, all the better to sense the telltale vibrations of prey. These may include such large and mobile creatures as frogs and aquatic beasts. (For example, three species of subterranean arachnids in Mexico have been shown to feast upon cavefish, Gollum's favored food. Precious.)

Kryptonesticus eremita, a species commonly found in European caves. Francesco Tomasinelli

The most common type of spiders to pursue a subterranean life are sheet- and orb-weaving spiders, which catch prey in their webs, Isaia says. Jumping spiders are notably absent, as you might imagine, since they rely heavily on vision. Surprisingly, there is at least one species of wolf spider that is a troglobite (the animal form of troglodyte, meaning cave-dweller). These animals don't use webs but rather hunt down prey. When your life gets tough, and it will, just remember that you don't have to stalk and ambush prey in complete darkness.

Spiders are well-adapted to live in underground chambers. For one, they don't burn a lot of calories. That's good, because caves don't generally have a lot of inhabitants to eat. (With some notable exceptions, like the kind of tropical dives that support giant huntsman spiders.) They also can store a lot of food relative to their body weight in one feeding. "Their digestive tract is massive," says Eileen Hebets, an arachnologist at the University of Nebraska-Lincoln. This allows them to sup every last bit of succor from their prey. "If you ever dissect a spider, it's almost all gut," she adds. They also rely on other senses than vision, which wouldn't be an option for many animals.

In all, 48 of the 113 spider families have species specialized for cave life, as Isaia and colleague Stefano Mammola note in their comprehensive review of the current state of subterranean spider knowledge, published in April in Proceedings of the Royal Society B. That's more than 40 percent. "That's absolutely tremendous... it's bizarre to think how many families exhibit that [troglobite] lifestyle," says Chris Buddle, an arachnologist and dean of student at McGill University who like Hebets wasn't involved in the review. To an unparalleled degree, spiders are well-suited to such an existence.

A cave spider known as Troglohyphantes pluto photographed in a cave in the Ligurian Alps, in northwest Italy. Francesco Tomasinelli

But that dank cave life doesn't come easy. Caves, by their nature, are fragile places to live. Spiders, like other troglobites, are used to caves' unchanging climes, which reflect the average temperature above ground. They are finely tuned to live in specific places, which are sometimes quite isolated from other caves. That's why Isaia likens them to canaries in a coal mine; the creatures are incredibly sensitive to environmental disturbance and serve as harbingers of potential dangers to come for other beasts... and ourselves. "If you increase the temperature," as we're seeing with global warming, "all of these animals are going to disappear," Isaia says. "They are trapped."

Already, there are cave spiders that have gone extinct, or which almost have, like the world's smallest, A. ataecina. Marshal Hedin, a researcher at San Diego State University, has studied a species that lives in the San Antonio area "that is only known from a single cave—where the entrance has been filled in" by development, he says. The creature is known as the braken bat cave meshweaver (Cicurina venii). "We have no idea if it still exists," he says.

Isaia advocates for more exploration, and conservation of the spiders we know about. When people look, they tend to find. For example, researchers just published in April a study describing a new softball-sized spider that lives in the caves of Baja, California. And there are countless spiders out there aren't nearly so large and conspicuous.

"This is why we do these reviews," Isaia says, to encourage more research and exploration.

Somebody has to speak for the cave spiders.