Diving bell spider
|Diving bell spider, Water spider or Airbubble spider|
|Female (on left) and male|
The diving bell spider or water spider, Argyroneta aquatica, is the only species of spider known to live entirely under water.
Argyroneta aquatica is found in northern and central Europe and northern Asia up to latitude 62°N. It is the only spider known to spend its whole life under water. As with other spiders, it breathes air, which it traps in a bubble held by hairs on its abdomen and legs. This gives it a silvery appearance, despite it being velvet-grey. The spider inhabits ponds in Europe and northern Asia, and lives for approximately two years. The appearance of the diving bell gave rise to the genus name Argyroneta, from the Greek "argyros" (ἄργυρος), meaning "silver", and "neta", a neologism (perhaps for *νητής) derived from the verb "neo" (νέω) "spin", intended to mean "spinner of silver".
Females build underwater "diving bell" webs which they fill with air and use for digesting prey, molting, mating and raising offspring. They live almost entirely within the bells, darting out to catch prey animals that touch the bell or the silk threads that anchor it. However, they have to surface occasionally to renew their personal air supplies and those of their webs. Males also build bells, but these are smaller and the males replenish their bells' oxygen supply less often. The males also have a more active hunting style. Prior to mating, the male constructs a diving bell adjacent to the female's, then spins a tunnel from his bell, breaking into hers to gain entrance. Mating then takes place in the female's bell. The female spider lays between 30 and 70 eggs in her bell.
Males are around 30% larger than females, which is unusual for spiders. This is possibly because their more active hunting style requires greater strength to overcome water resistance and counteract the buoyancy of their mobile air supplies. The larger body size is also associated with longer front legs, which has been shown to affect diving ability, giving the males superiority in diving over the more sessile females. The size of females may be limited as they put more energy into building and maintaining their larger bells. The spiders prey on aquatic insects and crustaceans. Their bite is quite painful as the fangs can pierce the skin, causing localised inflammation and feverishness. The spiders themselves fall prey to frogs and fish.
The replenishment of air is unnecessary in well-oxygenated water, because the bell permits gas exchange with the surrounding water; there is net diffusion of oxygen into the bell and net diffusion of carbon dioxide out. This process is driven by differences in partial pressure. The production of carbon dioxide and use of oxygen by the spider maintains the concentration gradient, required for diffusion. This system has been referred to as "the water spider's aqua-lung of air bubbles", though an aqua-lung lacks gas exchange with the surroundings; this system is more properly referred to as a form of physical gill.
Studies have considered gas diffusion between the diving bell and the spiders’ aquatic environment. Larger spiders are able to produce larger bubbles which have a consequently higher oxygen conductance, but all spiders of this species are able to enlarge their bells in response to increased oxygen demands in low aquatic P(O2) environments. These spiders voluntarily tolerate internal conditions of low oxygen, only renewing their bells with air when the P(O2) drops below 1 kPa; this replenishment process may not need to occur for several days, in some cases. 
- Norman I. Platnick (March 27, 2010). "Fam. Cybaeidae Banks, 1892d: 95". The World Spider Catalog, Version 11.0. American Museum of Natural History. Retrieved August 30, 2010.
- Schütz, D., and Taborsky, M. (2003). "Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica" (PDF). Evolutionary Ecology Research 5 (1): 105–117.
- Thorell, Tord (1869). On European Spiders. Uppsala, Sweden: Royal Society of Upsala. p. 137. Retrieved 2012.
- Chandramita Bora. "Water Spider".
- Ross Piper (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- Seymour, R. S.; Hetz, S. K. (2011). J. Exp. Biol.: 2175–2181.
- Flynn, M. R.; Bush, JOHN W. M.: "Underwater breathing: the mechanics of plastron respiration"; J. Fluid Mech. (2008), vol. 608, pp. 275–296. Cambridge University Press doi:10.1017/S0022112008002048
- Seymour, RS and Hetz, SK. 2011. The diving bell and the spider: the physical gill of Argyroneta aquatica. Journal of Experimental Biology; 214(13): pp2715-2181.
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