Cybaeidae: water spiders

Credit: Norbert Schuller CC BY-SA 3.0 via Wikimedia Commons

The only British species in this family, the Water Spider Argyroneta aquatica, is the only known spider that lives almost permanently under the water surface. It inhabits clear ponds, ditches or slow moving streams. Despite their ability to dive and swim freely, their adaptations for aquatic life are subtle: the fine hairs of their abdomen and front of cephalothorax trap a silvery lining of air when the spider is under water, and the two pairs of rear legs are covered with rows of long fine hairs, which help with swimming and collecting air bubbles. They are grey and brownish spiders that can reach large sizes (sometimes over 2 cm).

Inside diving bells
The water spider builds an underwater refuge made of a dome of silk attached to submerged plants which she fills with air. This takes several trips to the surface, trapping large air bubbles with the aid of her rear legs, and then releasing the air out when she is under the dome. The dome then acquires a bell shape and effectively acts as an external gill. The spider spends much of its time in the bell, as oxygen diffuses from the water into the air bubble as the spider uses it up. Moulting either takes place inside the bell or on the water surface.

Reverse size dimorphism
Unlike most spiders, water spider males are on average larger than females. This could be due to the different hunting strategies of males and females: males are more active hunters and spend more time diving outside their bells, and large males are better divers than small ones in moving inside water as they are better counteracting air buoyancy. Females build larger bells and, in contrast to males, tend to practice ambush hunting from their bells, sitting head down with her front legs by the entrance, where they can detect vibrations of passing prey. Both males and females take their prey to their bells to eat.

Reproduction

Mature males (top shot) leave their bells in search for females. Mating takes place in the female's bell, although it involves both male and female swimming outside the bell. When mating, females prefer larger males, but they are also more wary of them, as - also uniquely to them - male water spiders are known to occasionally cannibalise females (although the reverse also happens). Females sometimes lose their bells after rejecting a male.
The female lays her eggs in her bell, in a thick white egg sac at the top of the bell. She partitions this area out with silk and remains in the bottom part of the bell herself. She stays there on guard until the spiderlings hatch. After a few moults they disperse and they quickly build their own little bells under water. Females can lay several egg clutches in a season.

Empty snail shells
At the end of autumn, the spider reinforces the bell with more silk, closes it and stays in it through the winter months. An alternative winter home are empty pond or ramshorn snails, lined with silk and filled with an air bubble.

More information
Schütz, D., & Taborsky, M. (2003). Adaptations to an aquatic life may be responsible for the reversed sexual size dimorphism in the water spider, Argyroneta aquatica. Evolutionary ecology research, 5, 105-117.

Schütz, D., & Taborsky, M. (2005). Mate choice and sexual conflict in the size dimorphic water spider Argyroneta aquatica (Araneae, Argyronetidae). Journal of Arachnology, 33, 767-775.