A diving bell spider has captured a water flea and is consuming it inside the air-bubble chamber.
Credit: Stefan Hetz.
PORTLAND: An underwater spider can spend up to a day without surfacing for air, scientists have found.
According to Roger Seymour, a biologist at the University of Adelaide in South Australia, the diving bell spider (Argyroneta aquatic) creates silk nets in vegetation, called diving bells, beneath the water's surface and fills them with bulbs of air carried down on its abdomen.
'It is an iconic animal; I had read about the spider as a small boy in popular literature about ponds," said Seymour, who reported his finding in the current issue of The Journal of Experimental Biology.
"We were interested to see what it was like inside the diving bell, so we placed a tiny oxygen-sensitive fibre optical probe into it. This revealed that oxygen was coming into the bell from the surrounding water, which is the essence of a gill."
Living in a bubble
It was previously thought that the spiders had to surface every 20 minutes or so throughout the day, but spending longer amounts of time beneath the surface made predation less likely.
Seymour and another researcher in the study, Stefan Hetz from Humboldt University in Berlin, used oxygen-measuring devices called optodes to record the oxygen levels inside the bells. They also determined how the spiders extracted oxygen and how much they consumed.
What they found was that the spiders used the bells like gills to extract oxygen from water so that they didn't have to surface. "We expected [oxygen levels] would drop fast, urging the spider to bring in new air with the bubble every few minutes," said Hetz. "But as we waited for the oxygen to drop, we noticed that this took hours and hours ... with the spider sitting almost motionless within the bubble."
Scientists found that, even on a hot day, the diving bells could extract enough oxygen to last the spider for more than a day, and they would even lay their eggs in the diving bells.
The 'sit and wait' strategy
The researchers experimented, adding nitrogen or oxygen into the bubble, and found that the spiders had very low oxygen consumption rates and that oxygen could be supplied from the water, even though it was stagnant, said Hetz.
In addition, the spiders were able to tolerate very low oxygen levels. "This all adds up to help the spider with its 'sit and wait' strategy," said Hetz.
'It is advantageous for the spiders to stay still for so long without having to go to the surface to renew the bubble, not only to protect themselves from predation but also so they don't alert potential prey that come near," said Seymour.
Michael Greene, a biologist at the University of Colorado in Denver added, "Diving bell spiders possess the ability to trap bubbles of air in which they breathe while attempting to capture prey - an amazing adaption."
