Did you know that there is a species of spider that spends its entire life underwater? Argyroneta aquatica, or the diving bell spider, is an intriguing air-breathing spider found in shallow bodies of fresh water across Europe and northern Asia. Its unique scuba diving skills come from its ability to weave silk webs between underwater vegetation that they use to create an air-filled bubble, or “diving bell,” much like the devices used by humans to transport divers between the seafloor and the surface. Some of the bells constructed are small, covering only the abdomen, whereas others can accommodate the whole spider.
The spiders fill these chambers by repeatedly transporting air bubbles from the surface that are trapped by fine hairs on the spiders’ body. The spiders live their entire lives in their shimmery bubble houses and even hunt, mate and lay eggs whilst inside them. Of course, the spiders need to replenish the oxygen inside the bubble and so make regular trips to the surface to renew the air, sometimes as often as every 20-40 minutes. However, it is also evident that some aquatic insects can use these bubbles as a “physical gill” that can exchange gases with the water, but it is unclear whether this gas transfer is sufficient to meet their oxygen needs.
To find out just how effective these diving bells are as a physical gill, scientists Roger Seymour and Stefan Hetz collected spiders from the Eider River, northern Germany, and housed them in aquariums complete with water weeds. To challenge the spiders, the researchers stopped replacing the water and allowed it to become stagnant, simulating a pond on a hot summer's day.
They then poked the bubbles with tiny, oxygen-sensitive optodes to measure oxygen levels inside the bubble and in the surrounding water. This allowed the team to calculate the amount of oxygen flowing into the bubble and subsequently oxygen consumption rates.
They found that absorbed oxygen accounts for as much as 70% of what the spider breathes; the rest comes from the air gathered from the surface during bubble construction. Furthermore, this gas exchange was similar to that performed by anatomical gills of aquatic organisms.
In contrast to previous studies, the team found that the spiders could live inside a bubble for more than a day without needing to nip to the surface to replenish the air. They would be able to stay longer, but nitrogen steadily diffuses from the bubble, causing it to gradually shrink. Because oxygen is continually being consumed by the spider and the carbon dioxide produced rapidly dissolves into the surrounding water, nitrogen must increase in concentration to compensate for the loss of gas from the bubble. As this disperses, the bubble slowly collapses, forcing the spider to surface and refill.
Being able to remain underwater for such a long time has its advantages. It means that the spiders reveal themselves less frequently to predators lurking at the surface and also allows them to stealthily wait for prey without scaring them off by moving.