Argyroneta aquatica, is the only species of spider known to live entirely under water... found in northern and central Europe and northern Asia... 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.Awesome.
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.