Argyroneta Aquatica, aka Diving bell spider, Water spider or Airbubble spider

 This is the Diving Bell Spider. It is the world’s only fully aquatic spider, a native of Europe that uses its silk to create not a web but a bubble to call home. First, a net is woven. Then the spider collects air from the surface in the hairs on its legs and deposits it inside the net. Once the bubble is large enough to live in, it will never need to return to the surface again; unlike human aqualungs, it will never need to be replenished. The silk that contains the bubble facilitates osmotic gas exchange, bringing in dissolve oxygen from the water and diffusing exhaled carbon dioxide. All molting, feeding, and mating is done inside the bubble; the spider will only leave it briefly to hunt. The only other animal that can create a bubble that allows it to breathe underwater is a rare species of hedgehog.

 In the water, this layer of air around the abdomen creates an image of "mercury" surrounding it.

 The spider is very agile underwater and will catch prey, then subdue it with its venomous jaws. The spider carries the prey back  to its retreat to consume it.

Pair of Diving bell spiders (♀ left, ♂ right), found at Wien Prater park in Vienna, Austria (Photo credit: Norbert Schuller Baupi) Wikipedia