Several species of spiders can be found in, on and around water but only one habitually lives in water.
The Water Spider Argyroneta aquatica
If you are lucky enough, this lovely spider can be found in fresh water where there is little or no water flow and at any time of the year. Beneath the water surface it spins an inverted silken retreat. At the surface, it traps air in the fine hairs on its body, submerges, and scrapes the air off into the silken retreat. After a number of trips to the surface, it is able to fill the retreat with air and form a 'diving bell'. Once filled, the oxygen levels in the bell remain stable as oxygen produced by green plants diffuses into it from the surrounding water.
Water spiders are excellent swimmers. During the day they rest within the bell, facing down and looking out of the entrance. Should suitable prey pass by they rush out and catch it then return to the bell or to the surface to feed.
Females lay their eggs within the bell in late spring and early summer. These hatch and the spiderlings can be found at the surface walking amongst water plants. In late autumn, the spiders dive deep to avoid being trapped in ice. Here they build a stronger retreat that they fill with air and then seal themselves inside until the weather warms in early spring.
Coastal wolf spider Pardosa purbeckensis
The wolf spider Pardosa purbeckensis can be seen running about on salt marshes and mud flats when the tide is out. As the tide comes in, they either stay at the base of the plants or walk down the stem and deliberately submerge themselves, taking a bubble of air trapped on the dense hairs on the abdomen. Hanging on to the submerged plant, they are able to breath, using the layer of air trapped in the hairs.
Raft spiders Dolomedes species
Two species of Raft Spider occur in Britain, Dolomedes fimbriatus and the rare and endangered Dolomedes plantarius (for more information on this species visit www.dolomedes.org.uk).
Dolomedes fimbriatus is one of our largest and a most handsome spider, measuring up to 22mm. They are quite capable of taking large prey like damselflies and even small fish. When hunting, they stand on a piece of vegetation with their front legs resting on the surface of the water. When prey comes in range, they dash across the water and grasp it, or even dip down below the surface to catch it. If disturbed, Dolomedes will run down a water plant and dive beneath the surface, hiding there until the danger has passed (see Wikipedia).