Species: Dolomedes plantarius
Size: Body length (female): 17 - 22 mm
Body length (male): 13 -18 mm
Description: The Fen Raft Spider is a large species within its range. Adult females can have bodies of slightly over 20mm in length with a span of 70mm including their legs. It is typically black or brown in colouration with white or cream stripes along the sides of the body. It is very similar in look to the closely related raft spider Dolomedes fimbriatus with which it is often misidentified.
Habitat and Distribution: Fen Raft Spiders are largely an aquatic species, dependent on the presence of standing or slow-moving water. They frequent fens and grazing marshes in lowland areas and appear to require an unpolluted water supply. They inhabit the margins of pools or ditches where they hunt over open water surfaces. Plant stems which emerge from the water are used as perches for hunting or basking, and to support the large ‘nursery’ webs in which the spiders rear their young. The type of emergent vegetation is important; stiff-leaved species are vital to support the nursery webs. At Redgrave and Lopham Fen this support is usually provided by great fen-sedge, but on the Pevensey Levels it is provided by other sedges, and by the floating rosettes of water soldier. Raft spiders are a warmth-loving species and are lost from areas where water surfaces become shaded by common reed or invading scrub. This species is widely distributed in Europe, but is known to have declined substantially, particularly in the western and central parts of its range. It is thought to be endangered in most European countries. It was only discovered in the UK in 1956 and these populations form an important outlying community. There are still only two sites where the fen raft spider is found in Britain, Redgrave and Lopham Fen, on the border between Norfolk and Suffolk, and the Pevensey Levels in Sussex.
Biology and Ecology: Fen Raft Spiders are predatory and do not build webs to catch their prey. They hunt from perches at the water’s edge, typically sitting with their back legs on a stem and their front legs resting on the water surface in order to detect vibrations set up by potential prey. They can rush across the water to seize prey items, using the surface tension to support their weight. It is this ability to sit on the water surface that has given rise to the name of ‘raft’ spider. They can also break the surface tension to run down stems under the water to catch prey or escape from predators. Fen Raft Spiders are voracious hunters. Adults eat drowning terrestrial insects and many aquatic species, including pond skaters, other species of aquatic spiders, dragonfly larvae and even sticklebacks.
In Britain Fen Raft Spiders are thought to live for just over two years, maturing into adults in their final spring. Adults females die at the end of the summer but most males die by mid-July. Courtship takes place on the water surface and is elaborate and protracted, involving slow and careful approaches by the male, accompanied by tapping the front feet on the water surface and a slow bobbing of the body. Females lay several hundred eggs into a silk sac, about one centimetres in diameter, which they carry around under their bodies for around three weeks. During this period they select a site in vegetation above the water surface where they build their nursery web once the young are ready to hatch. Particularly in hot weather, females descend at frequent intervals to dip their egg sac under the water to keep it moist. Nursery webs comprise a large tent of webbing built between 10 and 100 centimetres above the water. The females guard their young in the web until they disperse into the surrounding damp vegetation, usually after five to nine days. The breeding season lasts from late June to late September, with most females making two breeding attempts. Fen Raft Spiders hibernate during the winter, from the first frosts until warm weather returns in February or March. Little is known about their hibernation although they are thought to hide amongst leaves in the dense bases of sedge tussocks.
Status and Threats: The Fen Raft Spider is classified as Vulnerable under the IUCN Redlist. Due to the species relatively recent discovery in the UK, little is known about the pattern of their decline. It is clear, however, that their extreme rarity is likely to have resulted from the massive loss of lowland wetlands. As well as being drained for agriculture and development, many wetlands have been degraded by pollution, or simply become too dry to sustain a species which is so dependent on a year-round supply of water. At Redgrave and Lopham Fen, water was abstracted from the aquifer underlying the site for public supply from 1960 onwards. This dried up the chalk springs that fed the fen and led to the progressive loss of many of the site’s rare species. By the end of the 1980s little standing water was left on the fen in dry summers and the raft spider population was close to extinction.