British Wildlife of the Week
Generally speaking, spiders and water do not mix. If you’re an arachnophobe, ponds might therefore seem like a reasonably safe place to go to avoid eight-legged critters. There are, however, a small number of British spiders that are adapted to life on, and even in, the water.
One of these semi-aquatic arachnids is the fen raft spider, which hunts from perches by the water’s edge or on the surface of the water itself. This is one of Britain’s largest spiders, the females of which have leg spans of up to 70 mm . It feels for vibrations in the water using sensitive hairs on its long legs, and although it preys mainly on aquatic invertebrates such as mayflies, dragonfly larvae and pond skaters, its great size also allows it to catch tadpoles and small fish.
The fen raft spider can go underwater by running down the stems of aquatic plants, but it can’t stay submerged for very long. Only one spider, out of the 48,000 or so species in the world, can be considered truly aquatic. This is the appropriately named water spider, which can be found in wetland habitats across much of the UK.
The water spider lives almost exclusively underwater, spinning a bell-shaped web that it securely anchors to aquatic vegetation. But like all spiders, it must breathe air and it occasionally comes to the surface to do so. When it dives down again, it uses bubbles of air caught by the hairs on its legs and abdomen to fill its aquatic web. Eventually, a contained underwater pocket of air is created; a diving bell in which the spider can live, feed, mate and lay eggs while remaining totally submerged.
It was once thought that the water spider came to the surface to collect air bubbles to resupply its diving bell throughout the day, perhaps as often as every 20 minutes. But more recent findings have shown that dissolved oxygen naturally diffuses into the bell from the surrounding water, while carbon dioxide leaks out. This allows the spider to remain within its underwater home all day. However, there is a net diffusion of nitrogen out of the bell, which results in a gradually shrinking air bubble, necessitating occasional replenishment. The spider may sometimes seek out an empty water snail shell, fill it with air, and then tape it shut with silk; this acts as an emergency reserve of oxygen that the spider can later access should the surface of its pond freeze over in winter.
The water spider feeds mainly on aquatic insects and crustaceans, darting out from its home to catch prey that touches its bell or the silk threads that anchor it. Like all spiders, it feeds by releasing digestive enzymes into its prey while it is still in the arachnid’s jaw; this dissolves the prey and allows the spider to suck up the juices. But this can only be done in air because the fluid would be washed away in water, so the spider must take its prey back to its diving bell in order to eat it.
When a male water spider has mating on his mind, he scours the neighbourhood for the nearest female resident. Upon finding one, he starts building another diving bell right next door to hers and fills it with air as usual. He then spins a small silken tunnel that connects the two bells, eventually breaking through the wall and into the female’s chamber. If she is impressed by this intrusion, mating will occur. She will then lay up to 70 eggs. The tiny spiderlings that hatch will leave their mother’s bell, each silvered by their own tiny air bubble, and go out in search of a place to build their own home.
In the next British Wildlife of the Week, we’ll be looking at one of the greatest avian colonisers in the world; a species that only turned up here about 65 years ago, but which is now extremely common and widespread – the collared dove.
 The largest spider in Britain is usually accepted as being the cardinal spider Tegenaria parietina, which you can read about in this article about house spiders.