The fen raft spider (Dolomedes plantarius) is one of Britain's rarest, largest and most spectacular spiders. Adult females have bodies over 2cm long and a leg span of about 7cm. Their bodies are usually dark brown or black with a striking white or cream stripe along their sides. They are semi-aquatic and spend their lives in and around pools and water courses. They appear to prefer unpolluted, often neutral to alkaline water. A second very similar of species of raft spider (Dolomedes fimbriatus), also occurs in Britain but tends to be restricted to acidic bogs. Although it is very localised in its distribution, it is much commoner than the fen raft spider.

The life cycle of the fen raft spider

Habitat for the spider - Peat Diggings

At Redgrave & Lopham Fen, the spider currently only lives on and immediately around selected pools (former peat diggings) within the deep peat fen, in areas dominated by saw sedge.

Water is essential for many aspects of the fen raft spider's life history. They hunt at the water surface, typically lying in wait for their prey with their back legs on emergent vegetation and their front legs on the water surface. Sensory hairs on their legs enable them to detect vibrations in the water set up by predators or by their prey, which they dart out to grab either from the water surface or from underwater. Their prey species include other smaller species of aquatic spiders, pond skaters, tadpoles, and even sticklebacks and dragonfly larvae much larger than themselves. The prey are immobilised by biting and injecting poison. Initial digestion is external. The spider injects digestive enzymes into the body and then sucks out the resulting fluids. Whilst spider poisons are very effective against their prey they are harmless to man: only about six of the world's 50,000 spider species are poisonous to humans and none of these occurs in Britain.

Breeding

Water is also essential for successful breeding. Most females probably breed when they are two years old. They lay several hundred eggs in a silk sac, about a centimetre across, which they carry under their bodies for about three weeks. During this time the female dips the sac under water every few hours to keep the eggs moist. She selects a site in vegetation emerging from the water, where she spends increasing amounts of time until her eggs are ready to hatch. She then constructs a tent-like web in the vegetation, in which she guards the young spiderlings until they disperse into the surrounding damp vegetation, up to nine days later. This 'nursery web' can be anywhere between 10 and 100 cm above the water. At Redgrave and Lopham Fen most nursery webs are built in saw sedge (Cladium mariscus), the first usually appearing in late June or early July. Some females produce smaller, second broods later in the summer although these are less likely to be successful: the females have often lost one or more legs by that stage and weather in early autumn may become unfavourable. Although females have been recorded with egg sacs up to the end of September, courtship and mating takes place relatively early in the season and adult males all die by late July. Courtship appears to be a risky business. The male spends many hours in his painstaking approach to the female, carefully tapping the water surface with his legs. If the approach is successful, mating lasts only a few seconds.

Overwintering

Raft spiders are rarely seen after the first frosts, usually in early-October. They overwinter as first or second-year juveniles, hibernating until the first warm days of spring. In mild seasons this can be as early as February but reliable sightings are unlikely until mid-March. Their hibernation sites are not known although it seems likely that they are in air-filled cavities amongst the bases of sedge leaves. Spiders grow by shedding their skins. Shed skins, with their white stripes still clearly visible, can often be seen floating on the water or suspended by silk threads from the marginal vegetation.

Discovery and Current Status

Despite its size, the fen raft spider was not discovered in Britain until 1956, when the Redgrave and Lopham Fen population was first found. It has since been found at a second site, on the Pevensey Levels in Sussex.

This species is widely distributed on the continent but, like many wetland species, it is declining rapidly as a result of wetland destruction and degradation and is thought to be endangered over much of its range. The presence of only two populations in Britain means that it is very vulnerable to extinction. This is recognised in the full legal protection it is given under Schedule 5 of the Wildlife and Countryside Act 1981. It is also the subject of a Species Action Plan, drawn up as part of Britain's response to the Rio Earth Summit on Biodiversity.

As well as being an extreme national rarity, the fen raft spider population at Redgrave and Lopham Fen has been critically endangered by the drying out of the fen as a result of ground water extraction for public water supply since 1960. By 1990 the population probably occupied only about 15% of its former range and was confined to deep pools in two small, isolated areas, on Middle Fen and on Little Fen. Despite the excavation of new pools in the 1970's and '80s, droughts in the late 1980s left very little standing water, even in these pools in several summers. It is likely that virtually no breeding took place during these years.

By 1991, concern for the tiny remaining population prompted the establishment of a Species Recovery Programme, funded by English Nature with help from Suffolk Wildlife Trust. Conservation action under the programme was designed to maintain the residual population until the borehole could be re-located. Management measures included the excavation of additional pools and deepening of existing ones, re-instatement of rotational management of the beds of saw sedge and, most critically, the artificial irrigation of pools in the two remaining centres of population. In the driest summers of 1990s, the spiders were entirely confined to irrigated pools. Changes in the size of the population are monitored by an annual census of spiders on selected pools. This has shown that, although the programme has been successful in ensuring the survival of the spiders in their two remaining locations on the Fen, there has been no sustained growth in the size of the population, which has remained extremely vulnerable to sudden extinction.

The bore-hole that drained the fen was re-located in July 1999 and the progressive restoration of water levels has introduced the possibility of increasing substantially the size of the spider population. Some obstacles remain, including controlling pollution levels in water entering the fen from surrounding farmland and restoring larger areas of the types of vegetation which the spider favours. If these can be overcome we should be able to look forward to seeing this magnificent animal recolonising large areas of the reserve during this first decade of the new Millennium.

More information
www.dolomedes.org.uk