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article imageOcean-going spiders can sail like tiny boats to reach land

By Megan Hamilton     Jul 4, 2015 in Science
When Charles Darwin made his famous voyage on the HMS Beagle, he found spiders "ballooning" across the ocean, fluttering through the air onto the ship's sails.
Since then, many a sailor has noticed this as well.
Scientists have recently discovered that spiders have another skill that helps them travel the world's oceans without drowning, New Scientist reports.
These tiny and elegant navigators have long been known to use strands of silk to become airborne in gusts of wind, but scientists had assumed they would drown if they were swept offshore into water.
Working in the "SpiderLab" at the University of Notthingham, Japanese research fellow Morito Hayashi and colleagues decided to find out what happens when spiders find themselves in water, The Guardian reports. Studying hundreds of spiders from more than 20 different species, the researchers placed them in a shallow tray filled with water to a depth of about 1cm. Then the scientists provided artificial wind, of a sort, by using "pump-generated air," and the spiders adopted positions that allowed them to use the wind to their advantage.
Spiders, the researchers discovered, are competent sailors even in salty water and turbulent conditions.
This tetragnathid spider demonstrates how it uses silk as an anchor.
This tetragnathid spider demonstrates how it uses silk as an anchor.
Alex Hyde
"It was like an illusion," Hayashi says. He first noticed common UK spider species sailing in the the lab, NewScientist reports. He was studying spider's flight, hoping to figure out how they take off when he noted their sailing behavior. "I was amazed that these common spiders, found in everyone's gardens, had such skillful sailing behavior that no one had noticed before."
Some species of spider form silken diving bells that enable them to breathe under water. Other species are known to catch and eat fish, NewScientist reports.
However, until now, no one knew that common spiders can set sail, probably because the species that do this are so tiny, usually only a couple of millimeters long.
"Water was always thought to be the ultimate barrier to dispersion," says Hayashi's colleague, Sara Goodacre, also of the University of Nottingham. "Now, we know they can survive in water, so with this get-out-of jail card, they can move far greater distances than we thought."
All of the spiders they tested had water resistant legs, and all of them had excellent sailing skills. The results of their study appeared in BMC Evolutionary Biology.
The scientists found that the spiders exhibited these water-related behaviors, The Guardian reports:
1. Sailing. To do this, spiders raise their legs, thus creating a kind of sail.
“Sailing spiders smoothly and stealthily slide on the water surface without leaving any turbulence, the scientists noted.
2. Upside-down sailing. In this instance, the spider does an eight-legged “handstand” using its abdomen rather than its legs to catch the wind.
3. Anchoring. The spider shoots a bit of silk onto the water surface. This slows down the tiny creature’s movement and may help it tether itself to the relative safety of a floating object.
4. Walking. The spider rapidly propels its legs in order to walk on the surface.
5. Death mimicry. This is likely a strategy to avoid predators, and as such, this behavior is common to many animals. In this case, the spider touches the water and freezes. Since its feet are water-repellent, the spider still floats.
In a related investigation, the researchers collected data on spider's ballooning habits. Unsurprisingly, the individuals and species that showed a readiness to balloon from a dry surface were also the ones who were the best sailors.
"The sailing behavior is almost completely associated with, and possibly a requirement for, the aeronautic behaviour," the scientists concluded.
"Being able to cope with water effectively 'joins the dots' as far as the spider is concerned," Goodacre, a co-author of the paper, said. "It can move from one land mass to another, and potentially across huge spatial scales through the air. If landing on water poses no problem then in a week or two they could be a long way from where they started."
As noted earlier, size is likely a limiting factor. Only spiders who aren't too heavy can skim across the water, New Scientist reports. So that means this behavior isn't common.
"I'd say the limit is probably around five millimeters long," Goodacre says.
Her team also hopes to show that the little arachnids sail in natural conditions as well.
They also want to examine what this means for evolution and geographic dispersion, since it's now obvious that spiders may be able to travel much further than previously thought.
"This may help explain why spiders are among the first species to colonize new habitats like islands," Stefan Hetz of Humboldt University in Berlin told NewScientist. "Spiders were thought to colonize exclusively by air; maybe they are good sailors too."
Darwin would have loved this, no doubt. A tidy set of observations that helps how spiders have come to colonize new habitats worldwide, The Guardian reports.
Not only have they been flying. They've been sailing, too.
More about Spiders, oceangoing spiders, Charles Darwin, HMS Beagle, world's oceans
 
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