The curious world of the spider in the bubble
Image Credit: © Stefan Hetz 2011.
In a neat experiment requiring a large measure of deftness - and the careful collection of some the pond's rarest denizens - two scientists from opposite corners of the globe have raised the veil on mystery of the diving bell spider. They have published a paper on how these amazing creatures make use of their bubble 'diving bells', in today's Journal of Experimental Biology.
Diving bell spiders are the only spiders that are able to survive under water, thanks to some rather clever bubble engineering. But the inner workings of how Argyroneta aquatica manages its air supply have eluded researchers, until now.
The diving bell spider spends its entire life under water, with the female making use of a large woven bell - filled with air - which entirely encloses her. The male also makes use of a big bubble of air, but this only wraps round his abdomen.
What has enabled the pair - Roger Seymour from the University of Adelaide in Australia, and Stefan Hetz from Humboldt University, Germany - to gain a fresh insight into the diving bell spider, is a curious measuring instrument called an optode. These devices consists of a tiny optical wire, coated in a polymer that fluoresces in the presence of oxygen.
Seymour had previously used his optode to track how several other species of underwater insects were able to get oxygen from the water that they swam in. But he was looking for a new challenge. Hetz suggested diving spiders, and offered up the facilities he had, to test different pond environments in his lab - so the two teamed up.
In order to conduct their experiments, the duo had to gain a permit to collect the spiders, which are rare in Europe. Eventually they found some specimens in the Elder River, near Denmark. By piercing the female's bubble, very gently, with the optode, they were then able to track the oxygen levels inside it. Combined with measurements just outside the bubble, the rate of the spider's oxygen use could be calculated.
That confirmed that these aquatic spiders have very low rates of metabolism, which makes sense if they are banking on a limited supply of oxygen. Diving bell spiders do return to the surface to recharge their bells - but most scientists had estimated this happened at 20 minute intervals.
But in fact, the pair found the spiders could in fact stay submerged for up to a day - even when the water was warm and stagnant. What drives them back to the surface is the slow leak of nitrogen out of the bubble, which shrinks it.
Keeping still for as long as possible is critical for the spider's 'wait-and-surprise strategy' to pay dividends. ''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.