There is only one species of spider in the world that spends its entire life underwater. It is called the diving bell spider or the water spider. A spider that lives underwater is amazing enough but to make matters mind-blowing, the spider uses a "diving bell" or bubble of water that actually acts like a lung!
Found in ponds in Europe and Asia, these small spiders have adapted to hunt for insects and crustaceans under the surface, living safe from land-dwelling predators though they aren't safe from frogs and fish. They have made this transition from living on land to underwater by devising a SCUBA tank of sorts. Using silk to form a "bell", the air-breathing spiders trap air in hairs on their abdomen and legs at the surface of the water and fill the bell with the trapped air. They can then live inside the bell, and in fact females live almost their entire lives inside the bell only coming out to snatch prey or to refill their air supply.
But refilling happens rarely and here's the mind-blowing part: unlike our own SCUBA tanks that have to be refilled when we use up all the air, these diving bells can replenish the air supply on their own.
"As the spider consumes oxygen from the air in the bell, it lowers the oxygen concentration inside. The oxygen can decrease below the level of dissolved oxygen in the water, and when this happens, oxygen can be driven into the bubble from the water," said Prof Roger Seymour in a BBC Nature article.
[F]requent replenishment at the surface is unnecessary in well-oxygenated water, because the structure of the bell permits gas exchange with the surrounding water: oxygen is replenished and carbon dioxide expelled by diffusion...as oxygen in the bubble is used up, more can diffuse in, whereas as carbon dioxide accumulates, it dissolves in the water and is lost. This system has been referred to as "the water spider's aqua-lung of air bubbles," but it is actually more advanced than the real Aqualung, which needs to be refilled frequently with compressed air, not having the option of continuous exchange of oxygen and carbon dioxide with the gases dissolved in the water.
Discovery News states, "Indeed, the diving bell functions as a very effective physical gill as opposed to an anatomical gill. And, because the diving bell spider lives a quiet sedentary life, its oxygen requirements are easily met -even in extreme conditions of warm stagnant water."
So diving bell spiders only need to come up for air maybe once a day, thanks to their amazing air bubble webs.
Here is a video of the diving bell spider at work filling up its air bubble and then pulling its prey inside: