The Fen Spider
Classified as an Endangered in the UK
Facts
Kingdom Animalia
Phylum Arthropoda
Class Arachnia
Order Araneae
Family Pisauridae
Genus Dolomedes
Size
Body length (female) 17 - 22 mm
Body length (male) 13 -18mm
Description
As well as being one of Britain's largest spiders, the fen raft spider 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 endangered species was only discovered in the UK in 1956, and until recently there were only two sites where the fen raft spider was found in Britain:- Redgrave and Lopham Fen, on the border between Norfolk and Suffolk, and the Pevensey Levels in Sussex, but in 2003 they were found on the Tennant Canal at Pant-y-Sais, near Jersey Marine.
Habitat
Fen raft spiders are largely aquatic animals, dependant 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 floating rosettes of water soldier. Raft spiders are warmth-loving species and are lost from areas where water surfaces become shaded by common reed and invading scrub.
Biology
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 their 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 'raft' spider. They can also break the surface tension to run down stems under the water to catch prey or escape from predators. Raft spiders are voracious hunters. Adults eat drowning terrestrial insects and many aquatic species, including pond skaters, other species of aquatic spiders, dragon fly larvae and even sticklebacks.
In Britain the spiders are thought to live for just over two years, maturing into adults in their final spring. Adult 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 1cm 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 100cm 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.
Threats
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 surface 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.
Conservation
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 the summer include 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 become completely confined to these irrigated pools. The vegetation around the pools was managed to maintain a healthy growth of sedge and the removal of 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, partly 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 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 there 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.
Kingdom Animalia
Phylum Arthropoda
Class Arachnia
Order Araneae
Family Pisauridae
Genus Dolomedes
Size
Body length (female) 17 - 22 mm
Body length (male) 13 -18mm
Description
As well as being one of Britain's largest spiders, the fen raft spider 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 endangered species was only discovered in the UK in 1956, and until recently there were only two sites where the fen raft spider was found in Britain:- Redgrave and Lopham Fen, on the border between Norfolk and Suffolk, and the Pevensey Levels in Sussex, but in 2003 they were found on the Tennant Canal at Pant-y-Sais, near Jersey Marine.
Habitat
Fen raft spiders are largely aquatic animals, dependant 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 floating rosettes of water soldier. Raft spiders are warmth-loving species and are lost from areas where water surfaces become shaded by common reed and invading scrub.
Biology
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 their 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 'raft' spider. They can also break the surface tension to run down stems under the water to catch prey or escape from predators. Raft spiders are voracious hunters. Adults eat drowning terrestrial insects and many aquatic species, including pond skaters, other species of aquatic spiders, dragon fly larvae and even sticklebacks.
In Britain the spiders are thought to live for just over two years, maturing into adults in their final spring. Adult 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 1cm 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 100cm 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.
Threats
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 surface 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.
Conservation
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 the summer include 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 become completely confined to these irrigated pools. The vegetation around the pools was managed to maintain a healthy growth of sedge and the removal of 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, partly 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 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 there 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.
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