Fen raft spider (Dolomedes plantarius)

KingdomAnimalia
PhylumArthropoda
ClassArachnida
OrderAraneae
FamilyPisauridae
GenusDolomedes
SizeBody length (female): 17 - 22 mm
Body length (male): 13 -18 mm

The fen raft spider is classified as Vulnerable (VU) on the IUCN Red List (1).

As well as being one of Britain's largest spiders, the fen raft spider (Dolomedes plantarius) is also one of the rarest. It is a handsome dark brown species with a characteristic white or cream stripe along the sides of its body.

This spider 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.

Fen raft spiders are largely aquatic animals, 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.

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.

Due to their relatively recent discovery in the UK, we know nothing 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.

The fen raft spider was one of the first species to be included in English Nature's Species Recovery Programme. Since 1991 concerted efforts, including annual monitoring of population size, have been made to prevent extinction of the residual population at Redgrave and Lopham Fen. Efforts to increase the water supply in summer included the excavation of new pools, the deepening of existing pools and, most critically, the artificial irrigation of pools in the core of the spider's range. In dry years the spiders became completely confined to these irrigated pools. The vegetation around the pools was managed to maintain healthy growth of sedge and remove shading scrub. These measures helped to sustain the spider population until 1999 when the bore-hole that had drained the Fen was re-located and the natural hydrology restored. This was the culmination of a four-year programme of work, part-funded by the European LIFE fund, to restore conditions likely to favour recovery of the fen's internationally endangered plant communities. Although problems remain, particularly in re-establishing suitable vegetation over much of the fen and in controlling water quality, it is hoped that the spider population will be able to increase substantially over the next decade.

On the Pevensey Levels the fen raft spider population is much larger than at Redgrave and Lopham Fen. Although it is not regarded as endangered, regular monitoring will be undertaken to ensure that water quality and management at the site continue to favour the maintenance of a large population. Any species confined to just two localities is especially vulnerable to extinction. To reduce this risk, and help to ensure the future of the fen raft spider in the UK, English Nature aim to introduce the spiders to at least two other UK sites in the next ten years. DNA signatures will be used to examine the relationships between the UK populations and determine the most suitable material for these introductions.

For more on this species see the Dolomedes home page:
http://www.dolomedes.org.uk/

Information supplied by English Nature.

http://www.english-nature.org.uk

  1. IUCN Red List (April, 2011)
    http://www.iucnredlist.org/