The diving bell spider is the only member of its group to spend its entire life underwater. But it still needs to breathe air, and it does so by building its own diving bell. First, it spins a dome-shaped web between underwater plants. Next, it rises to the surface and traps bubbles using the fine hairs on its legs and belly. It carries them down to its web and releases them, gradually filling the dome with air. After a few trips, the spider has amassed a bubble so large that it can fit inside.
The bubble acts as a home, a staging ground for hunting trips, and a nursery for its eggs. It's also a gill. Human engineers eventually worked out ways of sending fresh air into their diving bells, either via hoses or sunken barrels. But the spider's bubble does this automatically. There's typically more oxygen in the surrounding water than in the air within the bell, so the gas naturally diffuses into the bubble. For similar reasons, carbon dioxide diffuses out and the air inside stays fresh and habitable. The bubble acts as a detachable gill that the spider can breathe with and leave behind. It's like one of the spider's own organs.
The spider's diving bell is extraordinarily efficient at exchanging gases. Roger Seymour from the University of Adelaide and Stefan Hetz from Berlin's Humboldt University have found that it can extract oxygen from even the more stagnant of water. As such, the spiders can stay inside their sanctuaries for more than a day without having to replenish them.
The duo collected diving bell spiders from the Eider River in Germany, and housed them in tanks that were designed to mimic a hot, stagnant, weed-filled pond. They stuck small fibre-optic probes called optodes into the bubble, without popping it, to measure the oxygen levels inside. The probes revealed that scientists have seriously underestimated the diving bell's abilities.
Previous studies suggested that the spider must replenish its bubble every half an hour or so. But Seymour and Hetz calculated that it only needs to do so once a day because the diving bell absorbs oxygen from the water as fast as the spider inhales it. Over a day, this absorbed oxygen accounts for 70% of what the spider breathes (the rest came with the air originally used to construct the bell). And that's in stagnant water; in ponds with a better supply of oxygen, the diving bells perform even better.
The spider could live in its bubble indefinitely, were it not for the fact that nitrogen tends to diffuse out of it. This means that the bell eventually shrinks. It's why the spider still has to travel to the surface periodically to top up its home, and prevent it from collapsing.
The diving bell also serves to replenish the spider's second trick for breathing underwater. Water-repellent hairs on its body trap a thin layer of air whenever it returns to its bubble. The trapped air acts as a scuba skin. It stays with the spider as it moves about to hunt, allowing it to breathe while swimming. It also produces a silvery sheen that gives the animal its scientific name - Argyroneta aquatica, Latin for "silver net in the water".