Water spider  (Argyroneta aquatica)

This species is the only spider in the world that spends its entire life under water (3). It has a number of adaptations for this aquatic life-style. The abdomen and legs are densely covered in short hairs that trap air when the spider is submerged (2). Although the spider is velvet-grey out of the water, when it is in the water the air trapped around its body gives it a silvery appearance, which has been likened to quick-silver (mercury) (1). This is one of the very few spiders in which the males are larger than the females (4). Although this species has been placed in a separate family, the Argyronetidae, recent scientific studies examining fossil spiders suggest that it should be placed in the family Cybaeidae (5).

The water spider is found throughout Britain (2). It occurs throughout northern and central Europe, in Siberia and northern Asia (6).

Inhabits ponds, slow-moving streams, ditches, and shallow lakes where there is plenty of aquatic vegetation (2).

Water spiders spin underwater retreats amongst the weeds which they fill with air by travelling up to the surface and returning to the retreat, carrying air bubbles around the body (2). As they fill with air, these retreats become bell-shaped and take on a silvery sheen. The scientific name of this species Argyoneta derives from the Latin for silvery net, and refers to this unique air-bell that the species creates. Amazingly, the spider does not have to replenish the air-supply in the bell very often, as oxygen diffuses in from the surrounding water and carbon dioxide diffuses out (7).

This species is largely solitary, holds an exclusive territory and is mainly active at night. Males tend to be more active then females and actively hunt their prey. In contrast, females spend most of the time inside the air-bell and catch prey that strays too close to the bell (3). Prey species include small aquatic invertebrates such as water boatmen and tadpoles (1).

Males will mate with females after building an air-bell next to that of a female. He then bites through and mates with the female. The female spins a cocoon around the eggs at the top of her air-bell. The young spiders hatch after a few weeks and disperse (1). Before hibernating, water spiders seal up their air-bell or occupy an empty shell, which they line with silk (1).

This spider is not threatened.

Conservation action is not required for this common species.

For more on invertebrates and their conservation see Buglife, the invertebrate conservation trust:
http://www.buglife.org.uk/

For more on British spiders see The British Arachnological Society:
http://www.britishspiders.org.uk/

Information authenticated by Dr Peter Merrett of the British Arachnological Society:
http://www.britishspiders.org.uk/index.html