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. If you thought humans were the first to invent bell diving, you might reconsider...... 3
Diving bell spiders only need to come up for air once a day, according to researchers.
The spiders are named for their sub-aqua webs which they fill with air in order to breathe underwater.
Scientists studying the European arachnids measured oxygen levels inside and around an air bubble web.
They found that the bubble acted like a gill, extracting dissolved oxygen from the water and dispersing carbon dioxide.
Argyroneta aquatica live in ponds, pools and slow-moving streams across Europe and northern Asia.
They are the only spiders that live their entire lives underwater, mating, laying eggs and catching prey from their webs.
The silk webs are constructed among vegetation beneath the surface of the water.
To fill the "diving-bell" webs with air so they can breathe, the spiders use fine hairs on their abdomen to transport bubbles from above the water surface.
Scientists previously debated whether the spiders had to return to the surface regularly to replenish their air supply.
To settle the argument, invertebrate experts Professor Roger Seymour and Dr Stefan Hetz collected specimens from Germany's Eider River.
In the lab they simulated a stagnant, weedy pond on a hot summer day and tested how the spiders fared in the challenging conditions with a device called an optode.
"The previous literature suggested they had to come to the surface as often as every 20 to 40 minutes throughout the day," said Prof Seymour.
"It required the tiny, oxygen-sensitive optodes to do what we did. These have only been available during the last five to 10 years," he said.
By measuring the differences in oxygen levels inside the bubble and in the surrounding water, the scientists identified a gas exchange similar to that performed by the gills of animals that breathe underwater.
The spiders even maintain cocoons underwater
"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 Seymour.
"The carbon dioxide that the spider produces is not a problem at all, because it is readily dissolved in the water and it never builds up."
Unlike animals that exchange oxygen and carbon dioxide across gills however, the spiders have to contend with the other gases in the air they transport.
"If you absorb one gas from a gas mixture in a collapsible bubble, the remaining gases must increase in concentration," explained Prof Seymour.
"Because oxygen is taken from the bubble air, and CO2 does not build up, it causes the nitrogen in the bubble to rise in concentration," he said.
As the nitrogen disperses from the bubble, the bubble collapses but it does so slowly, roughly over the course of a day according to the scientists' results.
"The spider is able to remain in the diving bell on very hot days, when its metabolic rate is higher than normal, if the water is well oxygenated," said Prof Seymour.
This means the spiders can return to the surface infrequently, avoiding the risk of being caught by predators such as birds.
The extended dive times also allow the spiders to wait undisturbed for prey to pass.
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
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 The diving bell spider’s characteristic silver bubble belly. It can actually breathe like this, because it pulls oxygen through its abdomen instead of its mouth, which would be a pretty neat party trick if you could have parties underwater.
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.
Diving bell spiders are decent swimmers, but being out in the open puts them at risk of predation by hungry fish and frogs, who are like, “Uh this is our turf. You’re a spider, for the love of Pete.” “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 ofconvergence—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.
Proponents of naturalism need a lot of faith to believe that this incredible spider is the result of evolution.
For the diving bell spider, home is wherever it can use its butt to stash air. 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.
Scuba Spiders: Diving Arachnids Can Breathe Underwater 1
|A diving bell spider has snagged a water flea and is consuming the prey inside the air-bubble chamber.|
Credit: copyright Stefan Hetz.
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While scientists knew diving bell spiders (Argyroneta aquatica) — spanning just 10 to 15 millimeters in length — used an air bubble to breathe underwater in lakes and ponds, this is the first study that measures exactly how that happens and calculates how long the spider could stay underwater before resurfacing to replenish its bubble with fresh air.
"We were surprised how low the oxygen in the bubble could get before the spiders venture to the surface," study researcher Roger Seymour, of the University of Adelaide, told LiveScience. [See images of the underwater spiders]
Seymour and Stefan Hetz from Humboldt University in Germany, brought diving spiders into the lab, placing them into tanks mimicking conditions of a stagnant pond on a hot summer's day — revealing how the animals fare in extreme, low-oxygen conditions.
Immediately, most of the spiders constructed webs between the pondweeds and aquarium sides. Then each spider came to the surface to collect a large air bubbleheld between the hydrophobic (water-repelling) hairs on its abdomen and its rear legs. Webbing was placed around the lower sides of this gas chamber, which the spiders entered from the bottom.
Some spiders created chambers just large enough to enclose their abdomens, leaving their rears and rear legs hanging out; others had larger bubbles that enclosed their entire bodies. For instance, the spiders would enlarge the bubble by laying down more web and adding air before pulling just-snagged prey into the chamber.
Tiny sensors measured oxygen levels inside the bubbles and in the surrounding water, finding that the spiders extracted oxygen from the water as if it were a gill; the sensors also showed that the spiders could survive on very low oxygen levels.
Even so, the bubble shrinks over time and forces the spider to resurface for a fresh one. Like the atmosphere, the bubble contains primarily both oxygen and nitrogen, and as the spider takes oxygen from the bell, the nitrogen must increase. That increase pushes nitrogen out of the bubble by diffusion. Eventually, the lab spiders had to resurface.
The tiny spiders still were able to sit tight for more than a day, much longer than previous estimates suggesting a 20-minute underwater stint.
"It is advantageous for the spiders to stay still for so long without having to go to the surface to renew the bubble, not only to protect themselves from predation, but also so they don't alert potential prey that come near," Seymour said.
The research is detailed in the current issue of the Journal of Experimental Biology.
The diving bell and the spider: the physical gill of Argyroneta aquatica
Argyroneta aquatica is a unique air-breathing spider that lives virtually its entire life under freshwater. 2 It creates a dome-shaped web between aquatic plants and fills the diving bell with air carried from the surface. The bell can take up dissolved O2 from the water, acting as a ‘physical gill’. By measuring bell volume and O2 partial pressure (PO2) with tiny O2-sensitive optodes, this study showed that the spiders produce physical gills capable of satisfying at least their resting requirements for O2 under the most extreme conditions of warm stagnant water. Larger spiders produced larger bells of higher O2 conductance (GO2). GO2 depended on surface area only; effective boundary layer thickness was constant. Bells, with and without spiders, were used as respirometers by measuring GO2 and the rate of change in PO2. Metabolic rates were also measured with flow-through respirometry. The water–air PO2 difference was generally less than 10 kPa, and spiders voluntarily tolerated low internal PO2 approximately 1–4 kPa before renewal with air from the surface. The low PO2 in the bell enhanced N2 loss from the bell, but spiders could remain inside for more than a day without renewal. Spiders appeared to enlarge the bells in response to higher O2 demands and lower aquatic PO2.
INTRODUCTIONThe Eurasian diving bell spider Argyroneta aquatica (Clerck 1757) (Cybaeidae) is a unique spider that is said to live its entire life underwater. Their book lungs and tracheal system utilize air that is trapped at the surface of the water by hydrophobic hairs on the abdomen (opisthosoma) and the ventral side of the cephalothorax (prosoma) (Fig. 1A) (Levi, 1967; Messner and Adis, 1995). Moreover, they construct silk webs on underwater vegetation and create an air-filled bubble, the ‘diving bell’ (Fig. 1B). The air is held by surface tension between the silk fibres, and the chamber is open on the bottom. Smaller diving bells accommodate the abdomen only, but larger ones allow the whole spider to move in and out through the bottom. They fill the bell by sequentially carrying large bubbles from the surface, held with the abdomen and fine hairs on the rear legs (Fig. 1C). When moving free of the diving bell, however, the airspace on the body surface is thin (Fig. 1A). The spiders consume aquatic invertebrates and small fish, progress through many moults and, when adults reach approximately 50–100 mg in mass, lay eggs in a cocoon inside the diving bell (Fig. 1D) (Schütz and Taborsky, 2003). The hatched spiders (0.5 mg) are independent and, after spending a few days near the cocoon, emerge and construct tiny diving bells of their own. Surprisingly little is known about the gaseous conditions inside the bell, and in particular how much gas exchange occurs across the walls.
Argyroneta aquatica with its physical gill.
(A) Air clinging to the hydrophobic hairs on the abdomen of a spider away from the diving bell.
(B) A small diving bell, supported by invisible web, large enough to admit the abdomen only.
(C) A large bubble, captured at the surface and held on the abdomen and rear legs, is carried down to the diving bell.
(D) A female in her diving bell, below the cocoon, showing the lateral extension of volume and surface area.