In the ponds of northern Europe lives a tiny brown spider with a bubble on its back. The 10-millimeter-long Argyroneta aquatica is the only spider in the world that spends its entire life underwater. But just like land spiders, it needs oxygen to breathe. So every so often, it leaves its underwater web home to visit the surface and brings back a bubble of air that sticks to its hairy abdomen. It deposits the bubble into a little silk air tank spun for the purpose. This "diving bell," researchers have now found, is not just a repository. It's actually a gill that sucks oxygen from the water, allowing the spider to stay under for up to 24 hours.
Fascinated by the water spider ever since he read about it as a boy, physiologist Roger Seymour of the University of Adelaide in Australia decided to study it "out of sheer curiosity more than anything else." So he traveled to Berlin to meet with physiologist Stefan Hetz of Humboldt University in Germany. The pair spent a summer collecting spiders, putting them in tanks, and watching them spin their silvery nets and fill them with air bubbles.
The researchers decided to try out a new technology: a tiny fiber-optic oxygen sensor called an optode. Only 15 micrometers in diameter, the optode was small enough not to rupture the diving bell when the researchers poked it through the webby membrane and measured how gases move across the bell's surface. The bell, they found, functions like a gill: As the spider removes oxygen from the bell by breathing it in, more oxygen flows in to take its place. This gives the spider a constant oxygen supply without requiring it to venture to the surface often. But after about 24 hours, water pressure on the silk begins to collapse the bell, so the spider makes a break for the surface to grab another bubble. The optode also showed that the spider consumes surprisingly little oxygen over the course of its trips back to the bell to grab a breath of air.
The key to a gill, Seymour says, is keeping its volume proportional to an animal's body size and oxygen needs. So when the spider catches its dinner in its underwater web and needs to metabolize more oxygen while eating it, it adds more web to the bell to increase its surface area, adds more air bubbles to the bell, puts its food inside, and then crawls in after it. Females also enlarge their bells when they're about to lay their eggs, spinning a cocoon around the eggs and placing them inside the bell. When the baby spiders hatch, they crawl out with their own little diving bells, "as small as the head of a pin," Seymour says.
Previous researchers thought the spiders had to replenish their air every half-hour, but as they report online today in the Journal of Experimental Biology, Seymour and Hetz found that the spiders can hang out near their bells for up to a day, waiting for prey while keeping safe from birds. "I'm surprised there aren't more species like this," Seymour says. The spider, he adds, seems incredibly well-adapted for many environments; its bell can continue to function well even when the water is heated, suggesting A. aquatica may survive a warming climate.
Evolutionary physiologist Craig White of the University of Queensland in Australia calls the spider bell study an "elegant and technically demanding piece of work." He uses the tiny new optode in his own research and says it has opened many interesting questions for researchers who want to measure respiration in tiny animals—it can even measure the oxygen inside the trachea of a moth.
Seymour, who studies the physiology of many unusual animals, including tadpoles and Brachiosaurus (which he believes ate like a vacuum cleaner), hopes to return to Berlin someday and do more experiments on the spider. "We had such a fun time," he says.