Legend has it that none other than Alexander the Great was among the first people to plumb the ocean depths in a glass diving bell, a simple submersible that traps surface air for the diver to enjoy. Still further legend claims it was one of Alexander's mistresses who lowered him down—but unfortunately for Alex another suitor was on board that boat. As he watched through his transparent submarine, the scoundrel wooed the woman, who eventually let go of the chain, thus leaving Alexander in a bit of an aquatic pickle.

There's a moral lesson somewhere in that legend, perhaps something like "hell hath no fury like a woman you encumber with a ridiculously heavy diving bell while you enjoy yourself among the fishes," but in the fresh waters of Europe and Asia swims a spider that mastered this kind of submersible millennia before Alex's ill-fated dive: the diving bell spider. It's the only spider on Earth that spends its entire life underwater, a lovely reminder that where life finds a niche, it fills it—oh, and that if you're afraid of spiders you aren't really safe anywhere at all. (Other species are flying through the air, in case you were wondering, using dangling silk threads to ride the wind in a process called ballooning.)

Just like humans and their submersibles, to become a master diver this spider must first become a master engineer. It begins by spinning a web among the underwater vegetation, according to biologist Roger Seymour of Australia's University of Adelaide, who has established populations in the lab to study the dynamics of their novel way of getting air. Instead of expanding the web by spinning laterally, the spider adds more and more silk to the bottom, which flares until the structure indeed resembles a bell.

In addition, "there are a series of silk lines that go up from the bell toward the surface," said Seymour. "And the spider moves along those web lines as if it's climbing up a ladder." At the surface, it doesn't gulp air as you might expect, but instead pokes its butt up into the air. When it submerges again, air gets trapped by hydrophobic hairs in a bubble that encompasses the spider's entire abdomen and turns it a beautiful shiny silver (its genus name, Argyroneta, derives from the Greek for "silver"). Holding this bubble in place with its rear legs, the spider—now far more buoyant, yet trying to dive like an antsy kid wearing floaties—climbs back down with the help of those convenient web lines and deposits the bubble in its bell.

It's worth pausing here to talk a bit about arachnid respiration. Diving bell spiders have two systems in place: slits in their abdomen that open into "book lungs," which look like they're made up of pages, as well as what are known as tracheae—holes in their exoskeleton that ferry oxygen directly into tissues and organs. Because of the positioning of these book lungs and tracheae, the diving bell spider need only place its bum into the bell in order to breathe, all the better for eying potential prey through the opening (more on that later).

Interestingly, insects have the same tracheae, which limit both groups in their potential size. The bigger they get, the longer these tracheae must be to reach their innards, which reduces their efficiency in supplying the creature with enough oxygen. When oxygen levels rise, though, this becomes less of an issue. This probably is why there were so many ridiculously enormous insects during the Carboniferous period that ended 300 million years ago, when oxygen levels were at their highest, comprising 35 percent of the air compared to 21 percent today. Thus the 6-foot-long millipede Arthropleura and the hawk-sized dragonfly Meganeura. So feel free to breathe as much oxygen as you can to keep that from happening again. Deep breaths, people. Deep breaths.

Anyway, it previously was believed that diving bell spiders had to incessantly return to the surface to replenish their supply of air, as often as once every 20 minutes. What Seymour and his colleagues found, though, is those trips are far less frequent. Thanks to a neat trick of chemistry, the diving bell spider gets so much free air that it only needs to return to the surface once a day if totally inactive (furious activity, of course, would force it to eat up more oxygen and therefore surface more frequently).

"Because the bubble is supported by the web, most of the area of the bubble is just air-water interface between the fibers of the web," said Seymour. "And that allows oxygen to be exchanged between the water and the bubble." You see, just as oxygen is always entering and leaving water at the surface, so too does it move between the water and the spider's air-filled chamber. As the spider consumes the oxygen it's brought down from the surface, more pours in from the water through the web.

Incredibly, the spider's web essentially mimics a fish's gill. It's a strange kind of convergence—two unrelated organisms arriving at the same adaptation, like birds and bats separately evolving flight—only the spider has engineered its solution, what is known as a physical gill, though technically all gills are physical but whatever. And it's incredibly efficient. "It can supply up to eight times the amount of oxygen through the wall as originally put in the gill from the surface to fill it up," said Seymour.

So, cozy and safe not only from the multitudinous predators at the surface (making the brave transition into the water in the first place could have been a strategy to avoid these scoundrels), the spider can hang tight in its bell all day to avoid hungry hunters. But this is no cowering arachnid. It's a prolific hunter itself, taking down everything from small fish to crustaceans to water-borne insect larvae.

"Interestingly, while they're waiting in the bubble," said Seymour, "and a little fish or some other aquatic insect larva comes by and touches the silk, the spider will run out and grab it and kill it. But before eating it, it goes back to the bubble and enlarges it," then stocks the bell with air from a few trips to the surface. With the table set, the spider then drags its victim in and chows down.

Males, it should be noted, are far larger and more voracious than females, and this is very strange indeed for spiders, as females usually tower over males. This could be due to diving bell males being the ones that seek out mates, which would make a larger body an advantage when trying to move through water, a liquid that can be surprisingly viscous for tiny creatures. Plus, females not only need to build a bell that's big enough for themselves, but also for their brooding young (the actual breeding isn't particularly remarkable, so I'll spare you the details). A smaller body means more room for eggs, which take several weeks to develop.

The youngsters will stick around with mom for a few more weeks before setting off on their own to—yes—immediately build teeny tiny bubble homes of their own. Thus has the incredibly specialized diving bell spider colonized the waters of Earth, invading what we long assumed to be our last safe refuge from arachnids.

Oh well. There's always the moon.

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