Arachnophobes might be shocked to learn that some spiders can fly hundreds of miles across the ocean.
Two million years ago, airborne arachnids colonized remote Pacific islands by ballooning, a technique in which spiders use their silk as a kind of kite that can carry them long distances. (Read about spiders that can fly without silk—and steer in midair.)
These so-called ghost spiders likely landed on Robinson Crusoe Island (map), roughly 400 miles off Chile, where they blossomed into several new species. And now scientists have identified at least three previously unknown to science, a new study says.
The rugged island is named in honor of a privateer marooned there in the early 1700s who may have inspired Daniel Defoe's 1719 novel Robinson Crusoe.
“Everything that lives there comes from somewhere else and evolved in a very short span of time,” says Martín Ramírez, a spider researcher with Argentina’s National Scientific and Technical Research Council.
Ghost spiders—whose moniker comes from their sometimes pale color and ability to run so fast that their legs blur—originated in mainland South America.
“It’s not infrequent [to] find flying spiders in the middle of the ocean,” Ramírez says, noting that a number of different species are known to use this strategy. (Related: "Millions of Spiders Coat an Australian Town With Silk.")
In February 2011, Ramírez and colleagues visited Robinson Crusoe Island to collect and study as many ghost spiders as they could. They hiked up hillsides examining fallen logs and knocking tree limbs and foliage to dislodge spiders.
Getting there isn’t easy—the weather is unpredictable, and if it takes a turn for the worst, the team's six-seater plane has to turn back for Santiago.
But the determined team has so far discovered four new ghost spider species, and three more are still unnamed. (How scientists debunked the myth of the real Robinson Crusoe.)
One particularly large species, Philisca ingens, is a whopping inch long; most other ghost spiders are a fraction of that size.
These castaways have another trait that differs from their continental relatives: Males have unusually small genitals.
“We don’t know what’s going on, but the genital organs are tiny [and] the animals are very large,” says Ramírez, senior author of a recent study published in Molecular Phylogenetics and Evolution.
But there is a clue in how the spiders court each other. On mainland Chile, foreplay is limited to a little mutual leg groping. But Ramírez says ghost spiders on the islands interact with their mouths—something like a spider kiss.
If the spiders do engage in more sophisticated courtship activities, these may be more useful on the islands than having the right size genitals, he says. (Read about a male spider that destroys female genitalia to prevent future mating.)
The ghost spiders seem to be doing relatively well on the island, but their populations are small, limited to less than 20 square miles.
Not much else is known about the species so far, he adds. The tiny, lightweight spiders likely feed on larger insects and live amid rotting logs and ferns or under tree bark.
When the ghost spiders first arrived, they likely lived on leaves—Ramírez’s molecular work indicates the species is descended from a species from mainland South America that lives on foliage.
The ability to shift habitats and rapidly colonize different environments is a special feature of animals that arrive on islands, says Darko Cotoras, a postdoctoral researcher at the California Academy of Sciences and the University of California, Santa Cruz, who recently published a study on other spiders on the island.
“They can start doing other things—things they couldn’t do on the continent because there was just too much competition,” he says. (See pictures of spiderwebs blanketing the Australian countryside.)
Cotoras says he has a lot of respect for the new study—and the scientists’ research efforts.
“It’s not just going there and picking up spiders,” says Cotoras, who has conducted other spider research on Robinson Crusoe. “It’s challenging because of the geography. Many of the places are incredibly steep and incredibly narrow.”
The human population on the island is understandably small as a result—less than a thousand in number—and this may be helping the wildlife.
But if invasive plants taking hold on the island change its ecosystems, making them less habitable for arachnids, Robinson Crusoe’s spiders could face a new fight for survival.
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