The last place one would expect to see a spider crawling around is underwater. But that is the only place where one is to encounter Argyroneta aquatica, better known as the water, or diving bell, spider. The only spider known to spend its entire life underwater, it survives by breathing air in bubbles it traps on its body at the water's surface. Thus, as strange as it may seem, with its hairy legs grappling the stems of aquatic plants and its lines of silk waving in the water's gentle ebb and flow, the water spider is a master of underwater survival.
Water spiders occur across central and northern Europe and northern Asia, where they inhabit small, shallow sources of fresh water, such as ponds and streams. In many ways, they are like little fresh water scuba divers. The large air-filled bubbles that become trapped in the fine hairs on their abdomens, where their respiratory organs are located, act like oxygen tanks, providing much needed air while diving and moving about in their habitat. The bubbles have also been likened to diving bells (hence the spider's other common name, the diving bell spider).
In many cases, water spiders construct their dome-shaped silk houses on underwater vegetation, which serves as a sort of foundation and prevents the structure from floating away. Once the silk framework for the house is assembled, the spider sets to work filling it with air. The spider does this by transporting bubbles collected at the water's surface to the house, where it then releases them, thereby inflating the silk framework. Once its house is sufficiently inflated, the spider tunnels inside.
Exactly how often the spiders must return to the water’s surface to replenish the air supply in their underwater homes was a matter of much speculation until recently. Research conducted by a team of scientists based at the University of Adelaide, Australia, and Humboldt-Universität zu Berlin, Germany, revealed that the spiders need to replenish the air in their underwater homes only about once each day. This is because the inflated silk structure acts like a fish’s gills, absorbing dissolved oxygen from the water and thus allowing the spider to remain underwater for extended periods of time.
Within the oxygenated sanctity of their underwater homes, water spiders carry out a variety of life activities, including mating, feeding, and caring for offspring. Females, however—because they bear eggs and care for them, and because they prefer to hunt by capturing unsuspecting prey that comes within striking distance of their silk bubbles—spend much more time in their bubbles than do males. Hence, when it comes to courtship and breeding, to catch a female's attention, a male needs to build his house next door to that of a female's. When the opportunity arises, he tunnels into her bubble to mate.
Female water spiders lay several dozen eggs each. The eggs are bundled into a sort of cocoon that is secured within a sectioned off “room” in the female's house. After hatching, the young spiders take to the water, often setting up makeshift homes in abandoned snail shells that they fill with air. Once mature, they construct their own inflatable silk homes.
When it comes to food, water spiders have eclectic taste, at least when compared with their eight-legged land-lubber cousins. Nocturnal hunters, water spiders prey on aquatic invertebrates, such as phantom midge larvae and water mites, and have even been known to capture small fish.
Free from competition for food and habitat, and safe from typical spider predators such as birds, the water spider is the arachnid Neptune of its freshwater habitat. So, from the water spider's perspective, life in an underwater bubble is pretty good.
Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.
This post also appears on the Britannica Blog.