Tuesday, August 14, 2007

ENV: Troglobites

Great article sent via Mike Quinn:

Eyeless spiders, translucent millipedes, 175-year-old crayfish, and other odd cave dwellers face an uncertain future
By Kevin Krajick, National Geographic

Cave creatures live buried alive. Troglobites—the technical name for these millipedes, spiders, worms, blind salamanders, and eyeless fish—are made to navigate, mate, and kill amid perpetual darkness, desperate starvation, poison gases, and endless labyrinths of stone. Evolved in isolation and unable to disperse, species often consist of just a handful of individuals in one cave, or one room of one cave. Their existence raises many questions. How did they get there, and when? How do they survive—and how much longer can they hang on? Increasingly, many are threatened by pollution, quarrying, and vandalism. Ultimately, they are connected to a surface ever more populated, and penetrated, by us. They are the wildest canaries in the coal mine.

Worldwide, perhaps 90 percent of caves lack visible entrances and remain undiscovered. Even in well-explored caves, troglobites are expert at hiding. The roughly 7,700 species known are probably only a small taste of what lives below.

To survive stagnant, low-oxygen air in dead-end recesses and months without food, many troglobites have super-slow metabolisms. And because they live slow, they live long. The Orconectes australis crayfish of Shelta Cave in Alabama may reproduce at 100 years, and live to 175. Many troglobites possess extraordinarily long legs (and lots of them) with spiky feet adapted for getting over rocky terrain and sticking to moist surfaces. Pigments (which protect surface organisms from ultraviolet rays) and eyes disappear; for some creatures, eyeless sockets serve as fat reservoirs. Instead of vision, many have elaborate appendages and beefed-up nerve centers to interpret slight air-pressure or temperature changes, sounds, and smells. This sensory equipment lets them travel, sense objects moving or still, ambush prey, and, according to a recent study of troglobitic fish, judge the size and suitability of prospective mates, sight unseen.

Cave biology might be dated from 1797, when foot-long Proteus salamanders were first seen in Slovenian caves. They were the first, and still among the largest, known troglobites; locals at first thought they were baby dragons. Since then, scores of blind salamander and fish species have turned up in places like Texas’ vast Edwards aquifer, where they sometimes shoot up in artesian wells tapping unseen watery caverns below.

Startling new discoveries are now coming out of Sequoia and Kings Canyon National Parks, in the Sierra Nevada mountains of California. Here, scientists recently announced the discovery of some 30 new invertebrate species—an extraordinary number for such a small area—all still undescribed and unnamed. Scientists are also finding new caves—255 at last count, a number that increases every few months.

California’s aboriginal Yokut people, who used some of these caverns for ceremonies, may have been the first to glimpse the Sierra troglobites; they left pictographs of scorpions and spiders at entrances. Starting in the 1970s, scientists spotted a few, including a blind harvestman (daddy longlegs) with gigantic jaws for seeking out prey in the dark. In 2002, after years of growing awareness of species diversity in the region, the parks commissioned a survey led by Jean Krejca, an Austin biologist, and the parks’ cave specialist, geologist Joel Despain.

One summer day last year, Krejca and Despain threaded their way down a remote, brush-choked canyon through poison oak and heaps of giant boulders toward Sequoia’s Hurricane Crawl Cave. Tucked into a cliff was a black, funnel-shaped hole, fringed with thimbleberry shrub. On an otherwise still morning, the shrub’s broad leaves were dancing: A breeze from the hole signaled it as an entry to another world, one with its own weather system. Krejca and Despain drew a deep breath, thrust their arms forward, and wriggled through 60 feet (18 meters) of twisting tube that pressed like a tight suit. They emerged in a tall, chill, shoulder-wide corridor floored with rubble.

Cut off from the fruits of photosynthesis, most caves are places of hunger. Yet, most depend indirectly on the sun. In some caves, like Hurricane Crawl, rootlets from trees far above dangle through cracks in ceilings, providing bug food. Leaves and twigs wash in on spring floods from nearby connected creeks. Rodents penetrate surprisingly far, bringing seeds and nesting material. Bats also come and go, leaving behind guano, and their dead. Occasional “accidentals”—big animals like raccoons or snakes—wander in but don’t wander out, providing banquets that may stoke the food chain for centuries (in one Sequoia cave, debris and bones are piled a hundred feet deep in a pit trap). Hurricane Crawl is named for 30-mile-an-hour (48-kilometer-an-hour) gusts that roar in through blowholes, driven by temperature fluctuations outside; these gusts may bear organic dust.

At least a dozen known caves from Romania to Wyoming have no ecological connection to the surface; they run on purely geologic substances such as sulfur compounds, methane, iron, and hydrogen eaten by specialized microbes, which in turn feed higher organisms. Israel’s Ayalon Cave, uncovered accidentally in May 2006 by excavation in a rock quarry near Tel Aviv, was probably sealed for millions of years. Warm groundwater laced with sulfur appears to be feeding microbes and, ultimately, at least ten previously unknown crustaceans and other creatures. The fact that Ayalon was found in an area inhabited by people since ancient times suggests that the world holds many more such caves.

In Hurricane Crawl, the scientists turned over rocks until something crawled out of a pore on the underside of one. It was a dipluran—a translucent, eyeless insect with eerily long appendages that waved slowly in a headlamp’s beam. It was almost certainly a new species, never seen by humans until that moment. For the next few hours, Krejca and Despain wove their conveniently slender bodies into tiny side passages like millipedes themselves, overturning rocks and scanning walls. Little more of note emerged. The problem is, there are two miles of known human-size passages in Hurricane Crawl, but vastly more unobservable “mesocaverns”: endless small crevices and tubes with multiple layers of floor rubble. That is probably where the real action is. Krejca’s advice: If you don’t have time to leave bait—rotting shrimp and blue cheese are her favorites—just stay put and wait.

Troglobites tend to be lean, ready for action. One day in nearby Crystal Cave, members of the team crawled through a low, lifeless passage floored with sand. They came upon a single acorn, probably brought in by a pack rat, and a squirt of rodent poop. These objects had sprouted whole ecosystems: cotton candy gardens of multicolored fungus, near-microscopic springtails, detritus-eating beetles, and quarter-inch Taiyutyla and Striariidae millipedes. The innards of a see-through Striariidae told the short story of its life: The yellowish blotch in its midsection was part of its gut; the brownish stuff farther back, its latest meal. Nearby lurked tiny predators: Nesticus spiders spun slender webs, and pale, venomous centipedes peculiar to Crystal darted with startling speed. Often caves contain a weird overabundance of predators; this suggests that prey often runs out, leaving the hunters to go after anything alive or dead, including each other. Up another sandy passage in Crystal Cave, one very lost acorn had used its stored energy to sprout a six-inch, pure white tendril with minuscule leaves—its last gasp before being devoured in this alien world.

Another morning the team hopped a rushing creek and climbed up to Kaweah Cave, whose airy cliff entrance was camouflaged with buckeye trees. Krejca disappeared through a tiny hole in back, and an hour later emerged with a prize: a new species of pseudoscorpion she had just discovered under a rock 200 feet (61 meters) back. Eyeless and gray, it looked like a scorpion minus the stinging tail; these creatures inject their venom instead with their sharp claws. These were wildly outsize compared with the body, and covered with fine sensory cilia that waved independently. Krejca breathed gently on it. It ran backward, then raised its poisoned hands as if to strike. Luckily, it was no bigger than a letter on a page of National Geographic magazine.

Being small is an advantage if you live in small spaces, and many cave critters are. However, some go the other way. Recently researchers in Venezuela reported seeing Scolopendra centipedes nearly a foot long devouring whole roosting bats. Titiotus spiders of Kaweah Cave, blackish and bigger than silver dollars, are more than twice the size of their surface relatives. They do not spin webs but simply run down prey and grab them with their spiny legs. These spiders are found mainly near entrances and still have eyes and pigments, so they are classified as troglophiles—troglobites in training, retaining enough surface characteristics that they might also live under rocks or in soil burrows.

Scientists believe that virtually all terrestrial troglobites evolved from such animals, pre-adapted as they are to cool, moist, confined conditions. It is thought that at some point they moved farther down and stayed, either because they liked it or because they were confined there by climate swings on the surface.

But how long ago particular creatures went underground is rarely clear, for the immediate surface ancestors of most seem to have gone extinct. Scientists think aquatic organisms in the Edwards aquifer are descended from marine creatures stranded there some 60 million years ago when shallow seas receded. Remipedes, Earth’s most primitive living crustaceans, dwell in saltwater coastal caves across the globe. They may have started out over 100 million years ago, when the supercontinent of Pangaea was breaking up. Today related remipedes are scattered from the Caribbean to Australia, possibly brought there by eons of continental drift.

Palmer Cave, the oldest dated cavern in Sequoia-Kings, goes back some 4.7 million years, but the local troglobites could be far older. The Sierra have been uplifting and eroding for tens of millions of years, and mountains long gone may once have held caves; when they wore out, the occupants could just have moved downstairs.

On the other end of the evolutionary timescale are endemic diplura and harvestmen in near-freezing Panorama Cave, in the alpine zone at 10,600 feet (3,230 meters). This area was glaciated only 10,000 years ago, and it is hard to believe anything survived under a mile of ice and meltwater. Like Darwin’s finches, these creatures must have arrived and evolved not over eons, but in human time.

Unfortunately, the futures of many troglobites may be shorter than their pasts. Just 41 species are on federal endangered or threatened lists, but the Nature Conservancy says 95 percent of the thousand species known in the United States are actually imperiled. Caves provide ready conduits for seeping pesticides and sewage from cities and farmlands; troglobites are exquisitely sensitive to such poisons. Entire aquifers such as the Edwards are fast being drained, and as the water disappears, so does aquatic habitat. Some caves are simply excavated out of existence for roads and buildings. The Kauai wolf spider, which inhabits lava tubes in Hawaii, is facing competition from a new invader: the brown recluse spider. In Sequoia-Kings, officials worry about airborne fertilizers and pesticides from the heavily farmed adjoining San Joaquin Valley—not to mention scores of marijuana growers who have invaded the parks’ backcountry and apply the same stuff.

Even scientific expeditions, it must be admitted, can be a hazard. More than once, team members lightly brushed a tiny bug while turning over a rock, or crawling along: end of troglobite.

In Crystal Cave, they peered respectfully at—but did not touch—a sink-size series of rimstone pools supplied with water dripping from the ceiling. A few grayish, jellylike things the size of string snippets cruised the surface’s underside—newly discovered aquatic flatworms known from this spot, and this spot only, on Earth. One must be careful: Hairs, dandruff, and lint shed by humans may provide food sources that nourish competing alien surface mites, fungi, and bacteria, which also hitchhike in on humans. Finally, climate change—the force that may have helped create many troglobites—could help destroy them. Most caves have cool, constant temperatures that reflect the yearly mean outside. The critters are finely tuned to this constancy. If temperatures keep ascending at their current rate, some troglobites may not adjust rapidly enough.

For now, the frontier is still there. In late August last year, four amateur cavers were poking around a cliff face in the Sierra. They found a softball-size blowhole, enlarged it, climbed in, and discovered one of the most spectacular caves in the western U.S. Its cathedral-like spaces are up to a hundred feet wide and richly decorated with sparkling crystals and formations in every color of the rainbow. It has been named Ursa Minor, for the massive skeleton of a bear found lying at the foot of a curtain of stone. The first people to rappel in saw unidentified reddish worms clinging to the wall next to them. Farther back, one man thought he saw a flicker of movement in a pile of rocks.

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