Great raft spider
Encyclopedia
The great raft spider or fen raft spider (Dolomedes plantarius) is a European species of spider
in the Pisauridae family. Like other Dolomedes
spiders, it is semi-aquatic, hunting its prey on the surface of water. It occurs mainly in neutral to alkaline, unpolluted water of fen
s and grazing marsh
.
Carl Alexander Clerck
in 1757 as Araneus plantarius. Its genus was reclassified by Pierre André Latreille
in 1804 to Dolomedes
.
The species has also been described under the name Aranea viridata by Müller and as Dolomedes riparius by Hahn
.
The species is currently recognised as Dolomedes plantarius and has two widely recognised common names
; the great raft spider and the fen raft spider.
Dolomedes fimbriatus with which it is often misidentified.
and grazing marsh
areas and is dependent on the presence of standing or slow moving neutral to alkaline water. Within these areas it can be found on the margins of pools or ditches. Emergent vegetation is highly important for use as perches for hunting and basking and to support nursery webs. As a warmth loving species they avoid areas where water surfaces are shaded.
larvae and smaller aquatic spiders. They will also feed on drowning terrestrial invertebrates and have been known to catch small vertebrates such as sticklebacks and tadpoles
.
To hunt aquatic prey they have developed a sensory system of chaetae
, a covering of sensory hairs on its legs. These are used to detect the vibrations made as prey hits the surface or moves through the water. It will typically position itself with the back legs on a plant stem and the front legs on the water surface to be able to detect any prey. When prey is found the spider is able to run across the surface of the water to reach it by use of surface tension
. They are also known to hunt underwater by running down the stems of plants to reach prey, this can also be used to avoid capture by predators.
over the winter and will mature into adults during their final spring. In the UK, adults will usually have two breeding attempts between July and September.
The chaetae sensory system is used to help find a mate and courtship is carried out on the water. The male will slowly and carefully approach the female while tapping the water surface with its legs. When they are close they perform a slow bobbing of the body. If accepted the mating is brief and over in seconds.
The female will lay several hundred eggs in a silk
sac, about 10mm across, which they carry under their bodies for around three weeks. During this time she will periodically dip the sac into water to prevent the eggs from drying out. She will also locate a suitable nest site amongst the emergent vegetation, this will usually be between 10 and 100 cm above the water. Shortly prior to hatching she will construct a tent-like nursery web
within which she can guard the young until they disperse into the surroundings, usually five to nine days after hatching.
If a second brood is attempted later in the summer these are usually smaller and less likely to be successful. Courtship and mating usually takes place early in the season and adult males will die shortly after with most dead by late July. Females will survive until the end of the summer.
, Belarus
, Belgium
, Czech Republic
, Denmark
, England
, Estonia
, Finland
, France
, Georgia (country)
, Germany
, Hungary
, Italy
, Latvia
, Lithuania
, the Netherlands
, Poland
, Romania
, Russia
, Slovakia
, Sweden
, Switzerland
and the United Kingdom
.
In 1956 an outlying population was discovered for the first time in the United Kingdom
at Redgrave and Lopham Fen by arachnologist Eric Duffey. A further two populations have since been identified at the Pevensey Levels
in East Sussex
by Peter Kirby in 1988 and near Swansea
, South Wales
by Mike Clark in 2003. The species was also recorded in the 1960s at Sound Heath
in Cheshire
. The lack of historical record makes it very difficult to estimate the extent of its decline or explain the widely separated current distribution in the UK.
and the Baltic States
. These reductions in population are primarily due to the degradation and loss of habitat. The great raft spider is listed as a vulnerable species
on the IUCN Red List
.
The great raft spider has only been identified at three sites in the UK. Due to this extreme rarity it is listed as endangered, is protected under Schedule 5 of the Wildlife and Countryside Act 1981
and was provided a Species Action Plan
in 1999. Revised targets were published in 2006 with reports on targets published in 2002, 2005 and 2008. Principal threats to the species in the UK are identified as water abstraction, inappropriate ditch management, deterioration in water quality and the loss of suitable wetland habitat.
Original targets aimed to: Increase the range of the population by ten-fold and to increase the population density to a mean maximum of around 15 individuals per pool at Redgrave and Lopham Fen. To maintain the density and range of the fen raft spider on the Pevensey Levels and to introduce populations to two suitable new sites by 2010. In 2006, these were revised to: Increase the range of the spider to 13 Ha of habitat occupied 3 years in 5 by 2010 and to 65 Ha by 2020 at Redgrave & Lopham Fen. To prevent an overall decrease in range at Pevensey Levels at any time from levels recorded in 1990 and to increase sites with sustainable populations by 6 by 2010 and 12 by 2020.
The spider was first found in South Wales in 2003 in the disused Tennant Canal at Pant-y-Sais Fen. Surveying found a stable population as well as identifying further occurrences at the nearby Crymlyn Bog
and in connecting wetlands but the full range of the spider is not fully known. This due to the difficulty of surveying work in wetland terrain and problems with identification. The habitat at the site is considered good enough for the population not to be declining.
A survey in 1992 at the Pevensey Levels estimated the population at over 3000 adult females. This population is considered stable and conservation efforts at the site have focused on maintaining this level. A population has been found at a small artificial pond site which derived from Pevensey material. A review of the Pevensy population since its last count has been recommended.
Redgrave and Lopham fen was the first site in the UK at which a population of the great raft spider was recorded. Following their discovery in 1956 a number of new pools were dug to encourage population expansion. However, water extraction from the nearby borehole and a series of droughts in the 1980’s reduced the population to only two isolated areas on the reserve. Throughout this period irrigation of the pools inhabited by the spider enabled the continuation of the population. The removal of the borehole in 1999 was expected to trigger an increase in population as water levels returned to normal. However, a study carried out in 2006 showed that no noticeable change had occurred. The population of the fen raft spider remains small and restricted in distribution but stable. Recommendations for future management of the population include increasing the depths of turf pool, creating more pool habitats and greater, more focused use of water management in the reserve.
and Suffolk Wildlife Trust
and supported by a grant from the BBC Wildlife Fund. The project saw around 3000 spiderlings bred and reared by Dr. Helen Smith and the John Innes Centre
, 1600 of which were released into suitable dykes at the Suffolk Wildlife Trust Castle Marshes nature reserve. The site is part of the Suffolk Broads and lies 50 kilometres (31.1 mi) downstream, from Redgrave and Lopham fen, between Lowestoft
and Beccles
. Work was carried out to improve the ditch network at the site to prepare for the reintroduction and provide optimal habitat for the new spider population.
The spiderlings were bred from adults from both the Redgrave and Lopham fen and Pevensey Levels sub-populations. This was to increase the genetic variability
of the new population and increase its chances of survival. Each spiderling was hand reared in separate test tubes and fed with fruit flies
. If the new population successfully establishes itself it will be one of only four great raft spider populations in the UK. Another population of spiders was also introduced at the same time to Redgrave and Lopham fen to support the small population already present.
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
in the Pisauridae family. Like other Dolomedes
Dolomedes
Dolomedes is a genus of large spiders of the family Pisauridae. They are also known as fishing spiders, raft spiders, dock spiders or wharf spiders. Almost all Dolomedes species are semi-aquatic, with the exception of the tree-dwelling D. albineus of the southwestern United States...
spiders, it is semi-aquatic, hunting its prey on the surface of water. It occurs mainly in neutral to alkaline, unpolluted water of fen
Fen
A fen is a type of wetland fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients...
s and grazing marsh
Grazing marsh
Grazing marsh is a British Isles term for flat, marshy grassland in polders. It consists of large grass fields separated by fresh or brackish ditches, and is often important for its wildlife.-History:...
.
Taxonomy
The species was first described by arachnologistArachnology
Arachnology is the scientific study of spiders and related animals such as scorpions, pseudoscorpions, harvestmen, collectively called arachnids. However, the study of ticks and mites is sometimes not included in arachnology, but is called Acarology...
Carl Alexander Clerck
Carl Alexander Clerck
Carl Alexander Clerck was a Swedish entomologist and arachnologist.Clerck came from a family in the petty nobility and entered the University of Uppsala in 1726. Little is known of his studies; although a contemporary of Linnaeus, it is unknown whether he had any contact with him during his time...
in 1757 as Araneus plantarius. Its genus was reclassified by Pierre André Latreille
Pierre André Latreille
Pierre André Latreille was a French zoologist, specialising in arthropods. Having trained as a Roman Catholic priest before the French Revolution, Latreille was imprisoned, and only regained his freedom after recognising a rare species he found in the prison, Necrobia ruficollis...
in 1804 to Dolomedes
Dolomedes
Dolomedes is a genus of large spiders of the family Pisauridae. They are also known as fishing spiders, raft spiders, dock spiders or wharf spiders. Almost all Dolomedes species are semi-aquatic, with the exception of the tree-dwelling D. albineus of the southwestern United States...
.
The species has also been described under the name Aranea viridata by Müller and as Dolomedes riparius by Hahn
Carl Wilhelm Hahn
Dr. Carl Wilhelm Hahn was a German zoologist and author of the first German monograph on spiders. C. W. Hahn was an all-round natural scientist – not at all unusual for his time. Surprisingly he seems to have been almost forgotten...
.
The species is currently recognised as Dolomedes plantarius and has two widely recognised common names
Common name
A common name of a taxon or organism is a name in general use within a community; it is often contrasted with the scientific name for the same organism...
; the great raft spider and the fen raft spider.
Morphology
D. plantarius 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 spiderRaft spider
The raft spider, Dolomedes fimbriatus, is a European spider of the family Pisauridae. The raft spider is one of the two largest spiders in the United Kingdom...
Dolomedes fimbriatus with which it is often misidentified.
Habitat
The great raft spider, as with most other Dolomedes species, is a semi-aquatic spider. It inhabits lowland fenFen
A fen is a type of wetland fed by mineral-rich surface water or groundwater. Fens are characterised by their water chemistry, which is neutral or alkaline, with relatively high dissolved mineral levels but few other plant nutrients...
and grazing marsh
Grazing marsh
Grazing marsh is a British Isles term for flat, marshy grassland in polders. It consists of large grass fields separated by fresh or brackish ditches, and is often important for its wildlife.-History:...
areas and is dependent on the presence of standing or slow moving neutral to alkaline water. Within these areas it can be found on the margins of pools or ditches. Emergent vegetation is highly important for use as perches for hunting and basking and to support nursery webs. As a warmth loving species they avoid areas where water surfaces are shaded.
Diet and feeding
Great raft spiders are predatory and hunt from perches at the waters edge. They primarily feed on aquatic invertebrates such as pond skaters, dragonflyDragonfly
A dragonfly is a winged insect belonging to the order Odonata, the suborder Epiprocta or, in the strict sense, the infraorder Anisoptera . It is characterized by large multifaceted eyes, two pairs of strong transparent wings, and an elongated body...
larvae and smaller aquatic spiders. They will also feed on drowning terrestrial invertebrates and have been known to catch small vertebrates such as sticklebacks and tadpoles
Tadpoles
Tadpoles are a psychedelic rock band formed in 1990 in New York City by Todd Parker , Michael Kite Audino and Josh Bracken In 1992, Nick Kramer , David Max and Andrew Jackson of the fledgling Manhattan group, Hit, joined the Tadpoles after putting Hit on hiatus.In 1993 Kite and Jackson left the...
.
To hunt aquatic prey they have developed a sensory system of chaetae
Chaeta
A chaeta or cheta is a chitinous bristle or seta found on an insect, arthropod or annelid worms such as the earthworm, although the term is also frequently used to describe similar structures in other invertebrates. The plural form is chaetae or chetae.In the Polychaeta, they are located on the...
, a covering of sensory hairs on its legs. These are used to detect the vibrations made as prey hits the surface or moves through the water. It will typically position itself with the back legs on a plant stem and the front legs on the water surface to be able to detect any prey. When prey is found the spider is able to run across the surface of the water to reach it by use of surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...
. They are also known to hunt underwater by running down the stems of plants to reach prey, this can also be used to avoid capture by predators.
Life cycle and reproduction
Water is essential to the whole life cycle of the great raft spider. The spiders will live for two and a half years. As juveniles they will hibernateHibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...
over the winter and will mature into adults during their final spring. In the UK, adults will usually have two breeding attempts between July and September.
The chaetae sensory system is used to help find a mate and courtship is carried out on the water. The male will slowly and carefully approach the female while tapping the water surface with its legs. When they are close they perform a slow bobbing of the body. If accepted the mating is brief and over in seconds.
The female will lay several hundred eggs in a silk
Spider silk
Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring...
sac, about 10mm across, which they carry under their bodies for around three weeks. During this time she will periodically dip the sac into water to prevent the eggs from drying out. She will also locate a suitable nest site amongst the emergent vegetation, this will usually be between 10 and 100 cm above the water. Shortly prior to hatching she will construct a tent-like nursery web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....
within which she can guard the young until they disperse into the surroundings, usually five to nine days after hatching.
If a second brood is attempted later in the summer these are usually smaller and less likely to be successful. Courtship and mating usually takes place early in the season and adult males will die shortly after with most dead by late July. Females will survive until the end of the summer.
Distribution
Populations of the great raft spider are found throughout Europe in AustriaAustria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
, Belarus
Belarus
Belarus , officially the Republic of Belarus, is a landlocked country in Eastern Europe, bordered clockwise by Russia to the northeast, Ukraine to the south, Poland to the west, and Lithuania and Latvia to the northwest. Its capital is Minsk; other major cities include Brest, Grodno , Gomel ,...
, Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...
, Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
, Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, Estonia
Estonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
, Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
, Georgia (country)
Georgia (country)
Georgia is a sovereign state in the Caucasus region of Eurasia. Located at the crossroads of Western Asia and Eastern Europe, it is bounded to the west by the Black Sea, to the north by Russia, to the southwest by Turkey, to the south by Armenia, and to the southeast by Azerbaijan. The capital of...
, Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, Hungary
Hungary
Hungary , officially the Republic of Hungary , is a landlocked country in Central Europe. It is situated in the Carpathian Basin and is bordered by Slovakia to the north, Ukraine and Romania to the east, Serbia and Croatia to the south, Slovenia to the southwest and Austria to the west. The...
, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
, Latvia
Latvia
Latvia , officially the Republic of Latvia , is a country in the Baltic region of Northern Europe. It is bordered to the north by Estonia , to the south by Lithuania , to the east by the Russian Federation , to the southeast by Belarus and shares maritime borders to the west with Sweden...
, Lithuania
Lithuania
Lithuania , officially the Republic of Lithuania is a country in Northern Europe, the biggest of the three Baltic states. It is situated along the southeastern shore of the Baltic Sea, whereby to the west lie Sweden and Denmark...
, the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
, Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...
, Romania
Romania
Romania is a country located at the crossroads of Central and Southeastern Europe, on the Lower Danube, within and outside the Carpathian arch, bordering on the Black Sea...
, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, Slovakia
Slovakia
The Slovak Republic is a landlocked state in Central Europe. It has a population of over five million and an area of about . Slovakia is bordered by the Czech Republic and Austria to the west, Poland to the north, Ukraine to the east and Hungary to the south...
, Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....
and the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
.
In 1956 an outlying population was discovered for the first time in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
at Redgrave and Lopham Fen by arachnologist Eric Duffey. A further two populations have since been identified at the Pevensey Levels
Pevensey Levels
The area known as the Pevensey Levels is a Site of Special Scientific Interest notified under Section 28 of theWildlife and Countryside Act 1981. It is an area of marshland situated between Bexhill in the east, Pevensey in the west and Hailsham in the north...
in East Sussex
East Sussex
East Sussex is a county in South East England. It is bordered by the counties of Kent, Surrey and West Sussex, and to the south by the English Channel.-History:...
by Peter Kirby in 1988 and near Swansea
Swansea
Swansea is a coastal city and county in Wales. Swansea is in the historic county boundaries of Glamorgan. Situated on the sandy South West Wales coast, the county area includes the Gower Peninsula and the Lliw uplands...
, South Wales
South Wales
South Wales is an area of Wales bordered by England and the Bristol Channel to the east and south, and Mid Wales and West Wales to the north and west. The most densely populated region in the south-west of the United Kingdom, it is home to around 2.1 million people and includes the capital city of...
by Mike Clark in 2003. The species was also recorded in the 1960s at Sound Heath
Sound Heath
Sound Heath, also known as Sound Common, is an area of common land in Sound, near Nantwich in Cheshire, England, which includes heathland, grassland, scrub, woodland and wetland habitats...
in Cheshire
Cheshire
Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...
. The lack of historical record makes it very difficult to estimate the extent of its decline or explain the widely separated current distribution in the UK.
Conservation
Although the species is widely distributed in Europe, under-recording and confusion over identification make assessment of its status difficult. Populations are known to have declined substantially throughout its range particularly in western and central areas but remaining populations are believed to be well established in ScandinaviaScandinavia
Scandinavia is a cultural, historical and ethno-linguistic region in northern Europe that includes the three kingdoms of Denmark, Norway and Sweden, characterized by their common ethno-cultural heritage and language. Modern Norway and Sweden proper are situated on the Scandinavian Peninsula,...
and the Baltic States
Baltic states
The term Baltic states refers to the Baltic territories which gained independence from the Russian Empire in the wake of World War I: primarily the contiguous trio of Estonia, Latvia, Lithuania ; Finland also fell within the scope of the term after initially gaining independence in the 1920s.The...
. These reductions in population are primarily due to the degradation and loss of habitat. The great raft spider is listed as a vulnerable species
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...
on the IUCN Red List
IUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
.
United Kingdom
Wildlife and Countryside Act 1981
The Wildlife and Countryside Act 1981 is an Act of Parliament in the United Kingdom and was implemented to comply with the Directive 2009/147/EC on the conservation of wild birds...
and was provided a Species Action Plan
United Kingdom Biodiversity Action Plan
The United Kingdom Biodiversity Action Plan is the governmental response to the Convention on Biological Diversity signed in 1992. When the Biodiversity Action Plans were first published in 1994, the conservation of 391 species and 45 habitats was covered. 1,150 species and 65 habitats are...
in 1999. Revised targets were published in 2006 with reports on targets published in 2002, 2005 and 2008. Principal threats to the species in the UK are identified as water abstraction, inappropriate ditch management, deterioration in water quality and the loss of suitable wetland habitat.
Original targets aimed to: Increase the range of the population by ten-fold and to increase the population density to a mean maximum of around 15 individuals per pool at Redgrave and Lopham Fen. To maintain the density and range of the fen raft spider on the Pevensey Levels and to introduce populations to two suitable new sites by 2010. In 2006, these were revised to: Increase the range of the spider to 13 Ha of habitat occupied 3 years in 5 by 2010 and to 65 Ha by 2020 at Redgrave & Lopham Fen. To prevent an overall decrease in range at Pevensey Levels at any time from levels recorded in 1990 and to increase sites with sustainable populations by 6 by 2010 and 12 by 2020.
The spider was first found in South Wales in 2003 in the disused Tennant Canal at Pant-y-Sais Fen. Surveying found a stable population as well as identifying further occurrences at the nearby Crymlyn Bog
Crymlyn Bog
Crymlyn Bog is a nature reserve and a designated Site of Special Scientific Interest of international significance, near Swansea, south Wales....
and in connecting wetlands but the full range of the spider is not fully known. This due to the difficulty of surveying work in wetland terrain and problems with identification. The habitat at the site is considered good enough for the population not to be declining.
A survey in 1992 at the Pevensey Levels estimated the population at over 3000 adult females. This population is considered stable and conservation efforts at the site have focused on maintaining this level. A population has been found at a small artificial pond site which derived from Pevensey material. A review of the Pevensy population since its last count has been recommended.
Redgrave and Lopham fen was the first site in the UK at which a population of the great raft spider was recorded. Following their discovery in 1956 a number of new pools were dug to encourage population expansion. However, water extraction from the nearby borehole and a series of droughts in the 1980’s reduced the population to only two isolated areas on the reserve. Throughout this period irrigation of the pools inhabited by the spider enabled the continuation of the population. The removal of the borehole in 1999 was expected to trigger an increase in population as water levels returned to normal. However, a study carried out in 2006 showed that no noticeable change had occurred. The population of the fen raft spider remains small and restricted in distribution but stable. Recommendations for future management of the population include increasing the depths of turf pool, creating more pool habitats and greater, more focused use of water management in the reserve.
Reintroduction
In October 2010 the first introduction of a great raft spider population into a new site in the UK was carried out in a joint project by Natural EnglandNatural England
Natural England is the non-departmental public body of the UK government responsible for ensuring that England's natural environment, including its land, flora and fauna, freshwater and marine environments, geology and soils, are protected and improved...
and Suffolk Wildlife Trust
Suffolk Wildlife Trust
The Suffolk Wildlife Trust is a wildlife trust covering the county of Suffolk, England.Suffolk Wildlife Trust is the county's leading conservation charity dedicated to all wildlife. Its aim is to create a Living Landscape where wildlife flourishes throughout the countryside, towns and villages...
and supported by a grant from the BBC Wildlife Fund. The project saw around 3000 spiderlings bred and reared by Dr. Helen Smith and the John Innes Centre
John Innes Centre
The John Innes Centre located in Norwich, Norfolk, England is an independent centre for research and training in plant and microbial science...
, 1600 of which were released into suitable dykes at the Suffolk Wildlife Trust Castle Marshes nature reserve. The site is part of the Suffolk Broads and lies 50 kilometres (31.1 mi) downstream, from Redgrave and Lopham fen, between Lowestoft
Lowestoft
Lowestoft is a town in the English county of Suffolk. The town is on the North Sea coast and is the most easterly point of the United Kingdom. It is north-east of London, north-east of Ipswich and south-east of Norwich...
and Beccles
Beccles
Beccles is a market town and civil parish in the Waveney District of the English county of Suffolk. The town is shown on the milestone as from London via the A145 Blythburgh and A12 road, northeast of London as the crow flies, southeast of Norwich, and north northeast of the county town of...
. Work was carried out to improve the ditch network at the site to prepare for the reintroduction and provide optimal habitat for the new spider population.
The spiderlings were bred from adults from both the Redgrave and Lopham fen and Pevensey Levels sub-populations. This was to increase the genetic variability
Genetic variability
Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. Variability is different from genetic diversity, which is the amount of variation seen in a particular population. The variability of a trait describes how much that trait tends to...
of the new population and increase its chances of survival. Each spiderling was hand reared in separate test tubes and fed with fruit flies
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
. If the new population successfully establishes itself it will be one of only four great raft spider populations in the UK. Another population of spiders was also introduced at the same time to Redgrave and Lopham fen to support the small population already present.