Body-morphing spiders sail on water
From New Scientist by Andy Coghlan
Sailors and travellers, including Charles Darwin aboard HMS Beagle, have often reported seeing "ballooning" spiders flutter from the air into the sails of their ships, far away from any shore.
Dispersing spiders are known to use strands of silk to remain airborne in gusts of wind, but what happens if they are swept offshore and land in water?
We thought they would drown, but it turns out they are as adept at sailing as they are aeronautics.
"It was like an illusion," says Morito Hayashi of London's Natural History Museum, who first noticed common UK spider species sailing in the lab.
He was studying their flight, trying to figure out how they take off when he spotted the sailing behaviour.
"I was amazed that these common spiders, found in everyone's gardens, had such skilful sailing behaviour that no one had noticed before."
Water tolerance and tiptoeing.
The relationship between tiptoeing, sailing and the ability to float on water.
All tiptoeing individuals were also sailors, except for two individuals, suggesting that the sailing behaviour is almost completely associated with, and possibly a requirement for, the aeronautic behaviour
"One of the most amazing things is that no one had noticed this behaviour before," says his colleague Sara Goodacre of the University of Nottingham, UK.
Some species of spider form diving bells out of silk to enable them to breathe under water, while others are known to catch and eat fish.
But until now, no one realised that common spiders can sail, probably because the species that do it are small, typically just a couple of millimetres long.
"Water was always thought to be the ultimate barrier to dispersion," says Goodacre.
"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."
To find out how they do it, Goodacre, Hayashi and their colleagues observed the sailing skills of 325 spiders of 21 species caught at random on islands in ponds and lakes in various nature reserves around Nottingham in the UK.
Back in the lab, they placed individual spiders on small water trays and then used small air pumps to expose the spiders to breezes of between 3 and 80 centimetres per second.
All the spiders were able to stand on water thanks to their water-repellent legs.
And 201 of them, covering most species, showed off sailing skills.
Spider behaviour on water surface. Sailing behaviour: linyphiid (a, c) and tetragnathid (b, d) spiders moving on the water surface with their legs (a, b) or abdomen (c, d) used as sails. When the abdomen was used the behaviour was referred to as upside-down sailing. A spider can sail stably even on turbulent sea salt water. Anchoring behaviour: use of silk as anchor to slow down or stop movement on water surface by linyphiids (which dropped the anchoring silk) (e) and the tetragnathid (which dragged the anchoring silk after it caught a floating object) (f). Each scale bar represents 1 mm
source : BMC series blog
Most attempted to catch the wind and cruise forward by making "sails" from parts of their bodies. Some pointed two forelegs up in a V-shape, while others thrust their abdomen skyward – the equivalent of a handstand on the water.
When exposed to a breeze on solid ground, they showed none of the behaviours, which suggests these are used specifically for sailing.
The spiders sailed just as well on salt and fresh water, and were able to manoeuvre even in turbulent water.
Some also created the equivalent of an anchor by throwing out strands of silk for attachment to surfaces, such as the side of the water tray.
They may use these to haul themselves onto objects from water, or onto a suitable landing spot.
Goodacre thinks that size is a limiting factor – only those not too heavy can skim across the water – which means this behaviour is not common.
"I'd say the limit is probably around 5 millimetres long," she says.
Her team now hopes to show that spiders sail in natural conditions, too.
Also, they want to examine what this means for evolution and geographic dispersion, given that spiders may be able to travel much further than thought.
"This may help explain why spiders are among the first species to colonise new habitats like islands," says Stefan Hetz of Humboldt University in Berlin.
"Spiders were thought to colonise exclusively by air; maybe they are good sailors too."
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