Diving bell spiders use bubble webs 'like gills'

ORIGINAL: BBC
By Ella Davies Reporter, BBC Nature
9 June 2011

Dinner time in the diving bell

Diving bell spiders only need to come up for air once a day, according to researchers. The spiders are named for their sub-aqua webs which they fill with air in order to breathe underwater. Scientists studying the European arachnids measured oxygen levels inside and around an air bubble web. They found that the bubble acted like a gill, extracting dissolved oxygen from the water and dispersing carbon dioxide. The study is published in the Journal of Experimental Biology.

Argyroneta aquatica live in ponds, pools and slow-moving streams across Europe and northern Asia.

They are the only spiders that live their entire lives underwater, mating, laying eggs and catching prey from their webs.

The silk webs are constructed among vegetation beneath the surface of the water.

WATER SPIDER FACTS 

The Latin genus name Argyroneta means "silvery net" because of the shining appearance of the spiders' bubble webs underwater.

It is one of few spider species where the male is larger than the female In Germany, A. aquatica are rare and require a permit for collection

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To fill the "diving-bell" webs with air so they can breathe, the spiders use fine hairs on their abdomen to transport bubbles from above the water surface.

Scientists previously debated whether the spiders had to return to the surface regularly to replenish their air supply.

To settle the argument, invertebrate experts Professor Roger Seymour and Dr Stefan Hetz collected specimens from Germany's Eider River.

In the lab they simulated a stagnant, weedy pond on a hot summer day and tested how the spiders fared in the challenging conditions with a device called an optode.

"The previous literature suggested they had to come to the surface as often as every 20 to 40 minutes throughout the day," said Prof Seymour.

"It required the tiny, oxygen-sensitive optodes to do what we did. These have only been available during the last five to 10 years," he said.

By measuring the differences in oxygen levels inside the bubble and in the surrounding water, the scientists identified a gas exchange similar to that performed by the gills of animals that breathe underwater.

The spiders even maintain cocoons underwater

"As the spider consumes oxygen from the air in the bell, it lowers the oxygen concentration inside. The oxygen can decrease below the level of dissolved oxygen in the water, and when this happens, oxygen can be driven into the bubble from the water," said Prof Seymour.

"The carbon dioxide that the spider produces is not a problem at all, because it is readily dissolved in the water and it never builds up."

Unlike animals that exchange oxygen and carbon dioxide across gills however, the spiders have to contend with the other gases in the air they transport.

"If you absorb one gas from a gas mixture in a collapsible bubble, the remaining gases must increase in concentration," explained Prof Seymour.

"Because oxygen is taken from the bubble air, and CO2 does not build up, it causes the nitrogen in the bubble to rise in concentration," he said.

As the nitrogen disperses from the bubble, the bubble collapses but it does so slowly, roughly over the course of a day according to the scientists' results.

"The spider is able to remain in the diving bell on very hot days, when its metabolic rate is higher than normal, if the water is well oxygenated," said Prof Seymour.

This means the spiders can return to the surface infrequently, avoiding the risk of being caught by predators such as birds.

The extended dive times also allow the spiders to wait undisturbed for prey to pass.