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Water Spider Spins Its Own "Scuba Tank"

Sara B. McPherson
for National Geographic News
August 28, 2007
 
Water spiders survive below the surface thanks to a rudimentary "scuba tank" they spin from their own silk, a new study says.

Scientists at the University of Bern, Switzerland, determined that spiders use these scuba tanks, called air bells, as reservoirs, monitoring and replenishing oxygen levels to enable the animals to live underwater.

"The water spider's air bell is in some ways working like an external lung," said study co-author Michael Taborsky.

Found in ponds throughout northern and central Europe, the water spider is the only spider that spends its entire life underwater.

Since the small brown arachnids are air breathers, they have adapted the air bell system to gather oxygen from the atmosphere. (See spiders spinning their deadly silk.)

Living in a Bubble

The air bell serves multiple purposes, said Paul Selden, a professor of invertebrate paleontology at the University of Kansas who was not involved in the study.

"[The water spider] uses this air bell as a place to live away from terrestrial predators and as a safe nest in which to keep her eggs and tend the young spiderlings," Selden said.

It is also used as a safe harbor for consuming prey and breeding.

Using short hairs on their abdomens and legs, water spiders trap air bubbles from the water's surface, which they then carry back to specially designed underwater reservoirs spun from silk, the recent study found. (Related: "Gene for Key Spider-Silk Protein Found" [August 2, 2005].)

As the spider fills the web structure with air, the structure takes on a bell shape and a silvery sheen. The silk membrane allows oxygen to diffuse in from the water and carbon dioxide to diffuse out, so the spiders do not have to replenish the air supply often.

But until this recent study, scientists did not know that the water spiders also used the air bells to breathe.

The research will appear in the October issue of the Journal of Experimental Zoology Part A: Ecological Genetics and Physiology.

Monitoring Air Quality

For the study, scientists examined eight female water spiders, because females spend considerably more time in the air bells than males.

The team replaced the gas volume in each spider's air bell with pure oxygen, pure carbon dioxide, or a control of ambient air to test whether the spiders keep tabs on the air quality in the bells.

The team speculated that if the arachnids rely on air bells as a source of oxygen, their ability to detect elevated amounts of carbon dioxide and restore a proper balance is critical.

The test spiders only reacted to the carbon dioxide treatment, surfacing more frequently and increasing bell-building behavior until oxygen levels had been sufficiently replenished, co-author Taborksy said. (Related: "'New' Spider Species Weaves Uncommonly Regular Webs" [June 24, 2004].)

"The fact that the spiders did not react differently to the oxygen and control situations may indicate that they measure carbon dioxide levels in their bells rather than oxygen," he said.

"An increased carbon dioxide concentration may mean to the spider that the silk structure is not holding the air reserve well."

This meant the water spiders not only actively monitor the quality of the bell's atmosphere, but that they also depend on the bubbles for underwater respiration.

The new discovery provides scientists with an incentive to continue studying water spiders, Taborsky said.

Further research of the oxygen and carbon dioxide exchange through the silk membrane could provide a better understanding of the spiders' biology.

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