Bubble helps spider live a life aquatic

Branwen Morgan
ABC

Diving bell spider with air bubble (Humboldt University, Germany: Stefan Hetz)

An enduring bubble of air inside an underwater silk sack allows one species of spider to remain underwater for hours at a time, according to a new study.

In fact, the bubble is so efficient it allows spider to live virtually its whole life under water.

Professor Roger Seymour of the University of Adelaidesays that the diving bell spider (Argyroneta aquatica) creates a submerged oxygen store, five to ten centimetres below the surface and can "stay down for more than a day while resting."

The study, which is published in The Journal of Experimental Biology, overturns previous research that suggests the spiders need to return to the surface every 20 to 40 minutes.

Air-breathing aquatic insects, including other species of spider, carry a bubble of air from the surface down with them. But this usually contains enough oxygen to last for several minutes. But, by virtue of its 'diving bell', the diving bell spider is capable of remaining under water for most of the winter.

A. aquatica makes its diving bell by spinning an open-bottomed dome-shaped silk cocoon between the fronds of pond plants. It then fills it with a single air bubble that, according to Seymour, can be "as big as your ring fingernail."

The size of the bell is highly variable, sometimes only admitting entrance of the spider's abdomen. Female diving bell spiders tend to make larger diving bells that can be further enlarged according to need, such as to accommodate eggs or prey. They also enlarge the bell when the oxygen levels in the water drops.

The never-bursting bubble

Seymour, along with German colleague Dr Stefan Hetz of Humboldt University, set out to determine the effectiveness of the spider's diving bell by measuring oxygen levels within the bell and surrounding water.

"The bubble inside actually protrudes out between the fibres of the web, so it's a naked air-water interface," says Seymour. "This also gives the spider its name (Argyroneta), which means silvery net. The bubbles look like silver balls that are held under the water."

The scientists used oxygen-sensitive fibre optic probes to calculate the gas volume of the diving bell and the level of gas exchange occurring between the bell and the surrounding water. They also measured the spider's oxygen consumption.

"[We found] up to eight times the amount of oxygen can go from the water into the bubble from what was initially present," says Seymour.

Indeed, the diving bell functions as a very effective physical gill as opposed to an anatomical gill. And, because the diving bell spider lives a quiet sedentary life, its oxygen requirements are easily met -even in extreme conditions of warm stagnant water.

But, without supplements of fresh air, the bubble eventually shrinks as the nitrogen gas diffuses back into the water. Thus, the spiders need to make a daily dash to the surface to collect a replenishing bubble.

"They need to come up to the surface to get this little bubble to reintroduce into the bell," says Seymour.

It's carried down on the spider's abdomen and rear legs. Once inside the bell, the new bubble fuses with the underwater air supply.

Staying still and safe

"Being able to stay still for so long, without having to go to the surface to renew the air bubble, protects the spiders from predators and also keeps them hidden from potential prey that comes near," says Seymour.

He speculates the spiders may build their diving bells at night for this reason.

But Seymour adds that the Eurasian-dwelling spiders are become increasingly scarce.

"In Germany, they are popular with aquaticists because they are so interesting in their behaviour. This may be one of the reasons they are becoming harder to find."