According to a team of biologists headed by Dr Morito Hayashi of the Natural History Museum in London, UK, seafaring spiders use their legs as sails and their silk as an anchor.
Tetragnathid spider is using silk as anchor. Image credit: Alex Hyde.
Common spiders are frequently observed to fly using a technique called ballooning.
Ballooning spiders are estimated to move up to 30 km per day when wind conditions are suitable. This strategy, however, involves a significant risk.
The ballooning spider has little control over where it travels and could end up landing on water, which has been thought to be unsuitable for its survival.
“We have now found that spiders actively adopt postures that allow them to use the wind direction to control their journey on water,” said Dr Hayashi, lead author of the paper in the journal BMC Evolutionary Biology.
“They even drop silk and stop on the water surface when they want. This ability compensates for the risks of landing on water after the uncontrolled spider flights.”
Dr Hayashi and his colleagues from the University of Nottingham in UK, Miyagi University of Education in Japan, and the University of Granada in Spain, collected 325 adult spiders belonging to 20 linyphiid species and a single tetragnathid species. Their behavior was observed on trays of water in reaction to pump-generated air.
All the specimens tested had water repellent legs and the scientists observed six single behaviors and six behavioral combinations as follows:
“Sailing: once on the water surface, spiders react to the wind by raising their legs as sails. Sailing spiders smoothly and stealthily slide on the water surface without leaving any turbulence.”
“Upsidedown sailing: when on water, the spider reacts to the wind and raises its abdomen as a sail, in a handstand-like posture, and slides on water.”
“Anchoring: the spider releases silk on water surface and slows down its movement, or stops, against the prevailing wind. When the silken thread touches a floating object, the spider starts to walk on the silk until it reaches the floating object.”
“Death mimicry: the spider stays still and seems to mimic death on the water surface. Some individuals stopped moving for a few seconds then started to move again. Death mimicry behavior is likely to be a predator avoidance strategy, as is common to many animals.”
“Walking or moving legs: the spider attempts to walk on the water surface by rapidly propelling its legs and in the majority, it moves in a downwind direction.”
The researchers found that the spiders that adopted ballooning behavior for airborne dispersal were also the most eager and able sailors.
“Being able to cope with water effectively joins the dots as far as the spider is concerned. 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 away from where they started,” explained study co-author Dr Sara Goodacre from the University of Nottingham, UK.
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Morito Hayashi et al. 2015. Sail or sink: novel behavioural adaptations on water in aerially dispersing species. BMC Evolutionary Biology 15: 118; doi: 10.1186/s12862-015-0402-5
