How the diving bell spider uses physics to breathe underwater

Diving bell spiders spin 'bubbles' of web that not only let them live under water, but serve as gills that take in oxygen from the water. Now two biologists have measured what makes these bubbles fail, and how often the spiders need to surface.

Just when you thought it was safe to go back in the water, you find out that you have a little friend down there. And it can breathe underwater longer than you can. Diving bell spiders live in fresh water ponds, and use bubbles made of web to dive down beneath the surface of the water. They use their little bubbles as a store of oxygen, monitoring the carbon dioxide levels in the bubbles carefully. When scientists filled some bubbles with carbon dioxide and some with oxygen, and only the spiders short on oxygen returned to the surface.

Later it was discovered that not only do the spiders breathe in their diving bells, but the bells themselves harvest oxygen from the water through diffusion. The atmosphere is composed of 78 percent nitrogen and 21 percent oxygen. Once under the water's surface, things change. Oxygen dissolves much more readily in water than nitrogen does. Air dissolved in water contains 35 percent oxygen. Substances diffuse from higher to lower concentration, so the high concentration of oxygen in the water crosses the membrane of the spider's silk and diffuses into the bubble.

How the diving bell spider uses physics to breathe underwater


If it weren't for nitrogen, the spider might be able to stay in the bubble indefinitely. Sadly, since the spider gathers surface air, with 78 percent nitrogen, to fill its bubble, and then goes to a place with a much lower concentration of nitrogen, the nitrogen leaks out of the bubble the way the oxygen leaks in. This causes the bubble to shrink, and the spider has to make runs to the surface of the water to re-fill.

Two biologists recently studied how often the spider has to get to the surface of the water. They found diving bell spiders in the wild, and poked sensors into their bells. To their surprise, the spiders were untperturbed by this and just kept going. Over time, they measured the oxygen levels in the bells. The bubbles could harvest oxygen from even hot, stagnant water, and the spiders dropped their metabolic rate until it resembled the rate that other spiders have when waiting for prey. Between the two, the researchers calculated that the spider would only have to make a run to the surface once a day. The rest of its life can be spent underwater. Looking for a way into your swimsuit.

Via National Geographic, Chemistry Toolbox, and The Journal of Experimental Biology.

Top photograph by Heiko Bellmann.