Raft Spiders

Raft Spiders are some of the largest spiders found in the UK. They are closely associated with still, freshwater and get their name from their common habit of floating on the water's surface. There are two very similar species of Raft Spider found in the UK, the Fen Raft Spider, Dolomedes plantarius, and the Bog Raft Spider, Dolomedes fimbriatus, which is often just referred to as the Raft Spider. Raft Spiders are often found sitting at the water's edge with their front legs resting on the water's surface. The sensory hairs on their legs can detect the smallest of vibrations on the water and quickly alert the spider to the presence of a potential meal. When prey is detected the Raft Spider glides across the water's surface at great speed and pounces on any invertebrates that are unlucky enough to have fallen into the water. As well as hunting on the water's surface Raft Spiders can also submerge themselves to catch prey beneath the surface, or to evade predators themselves. The hairs on the legs of the spider trap an air-bubble that provides the submerged spider with an oxygen supply. Neil Phillips has captured images of a Bog Raft Spider submerging itself in water whilst in captivity. - LINK.  The BBC has produced a great video of these amazing spiders in action. Watch the video here.

The adults of both species have a dark brown / black abdomen and carapace, both with yellow or white submarginal stripes. The legs are usually a slightly lighter brown than the body, but can be greenish in colour. Juveniles are a lighter brown with pale brown or green legs. The yellow or white submarginal stripes are typically wider on the Bog Raft Spider than those found on the Fen Raft Spider, but this isn't always the case and microscopic examination of the genitalia is needed for confirmation of species where both species are known to occur. Adult specimens are usually found in spring and early summer, with June being the best month to find adult females carrying their large egg-sac.

Raft Spiders build nursery webs in marginal vegetation around freshwater, similar to those of the Nursery-web Spider, Pisaura mirabilis. The nursery webs are not built to catch prey but are constructed solely to protect their young. The egg-sac is carried by the adult female for up to 4 weeks. When ready the adult female bites open the egg-sac and releases the tiny spiderlings into the nursery web. Here they will stay for around 5 days before they are ready to disperse. By this time the life of the adult is coming to an end and its not uncommon to find adult females dead on their nursery webs. 

Both Raft Spiders and Nursery-web Spiders are of the family Pisauridae, and have several similarities. Like the Nursery-web Spider Raft spiders can also often be seen at rest, perched on vegetation, with their legs held stretched out in pairs. Adult females of both types of spider also carry around their large spherical egg-sac, held underneath their body, held in position using their jaws and their pedipalps. See example here.

Juvenile Raft spiders can sometimes be confused with Pirata Wolf Spiders, which are slightly similar in appearance and can also be seen running on the water's surface. Due to the huge difference in size adult Raft Spiders are not likely to be confused with any species of Wolf Spider in the UK. 

A question that is asked quite regularly is what is the largest spider found in the UK? Well the answer depends on how you define the term "largest". Raft Spiders of the Dolomedes genus are one of the possible favourites for this title. Adult female Raft Spiders can have a body-length of around 22mm, and a leg-span of around 70mm. This makes them the longest bodied spider found in the UK.

The female Cardinal Spider, Tegenaria parietina, can have a body-length of up to 20mm so they are not far behind the Raft Spiders in body-length. The leg-span of the male Cardinal Spider is the longest in Britain has been known to reach an incredible 140mm! 

Females of the Giant House Spiders, Eratigena species, can have a body-length of up to 18mm, so slightly smaller than both the Raft Spiders and the Cardinal Spider. The leg-span can be considerably bigger than Raft Spiders though, but not quite as long as the Cardinal Spider. Males regularly have a leg-span of 75-85mm but some specimens have reached 100mm.

When it comes to the heaviest spider in Britain that title goes to the female Four Spotted Orb-Weaver, Araneus quadratus, with some gravid specimens weighing over 2.5 grams. Females can have a body length of up to 20mm and a leg-span in excess of 35mm. Males are around half that size. Gravid female Garden Spiders / Cross Spiders, Araneus diadematus, can reach a very similar size.

Bog Raft Spider  -  (Dolomedes fimbriatus)

The Bog Raft Spider is classed as Nationally Scarce in the UK.  It has a widespread distribution on wet heathland sites in southern England but has a very scattered distribution elsewhere in Britain. Adult females typically have a body-length of 13-22mm. The smaller males usually reach 9-16mm. The Bog Raft Spider is semi-aquatic and occurs on wet, lowland heaths, bogs and in upland mires, with small shallow pools. Whilst adults hunt on the water's surface juvenile specimens can be found hunting in riparian vegetation, often 2-4ft above the water level. This species is also less closely associated with open water and it's not uncommon to find juvenile specimens on shrubs and other vegetation some distance from the nearest body of water. They have even been found on the lower branches of trees.

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Despite being a nationally scarce species where juvenile Dolomedes fimbriatus are found they can be quite numerous. On two consecutive visits to Thursley Nature Reserve, in Surrey, I was able to find 9 Bog Raft Spiders on my first visit and then on my second visit a few days later I was able to find 16 specimens. Most specimens were found in riparian vegetation, particularly long grass, within a 5 metre stretch along a narrow body of still water. The remaining specimens were found in shrubs and tall grass 10--50 metres from the water. A word of warning to anyone looking to photograph Bog Raft Spiders at Thursley Nature Reserve. Whilst walking through the undergrowth I spotted a several specimens perched on the grass that hung over the water. As I knelt down to photograph one of the Bog Raft Spiders I nearly knelt on a juvenile Adder! So wear appropriate footwear and watch where you step.

When handling Bog Raft Spiders I have seen no defensive behaviour from the juveniles at all. They tend to sit quite still which makes photographing them quite easy. When they do decide to make a dash for freedom they can move at considerable speed though.

This 9mm juvenile Bog Raft Spider was found 1 metre above the ground on a shrub growing about 50 metres from the nearest body of water at Thursley Nature Reserve in Surrey.

The Bog Raft Spider builds itself a small retreat, often in long grass. The seed heads of several grass stems are merged and tied together with silk. When looking for juvenile Raft Spiders it is easier to look for their retreats than for the spiders themselves. The retreats are easy to spot and once located the spiders are not usually far away either. 

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Fen Raft Spider  -  (Dolomedes plantarius)

The Fen Raft Spider, sometimes referred to as the Great Raft Spider, is a very rare species in the UK and is fully protected by UK Law against any disturbance. Until recently this species was confined to just three sites in Britain: in East Anglia, East Sussex and south Wales. Recent conservation efforts have seen this species re-introduced at other sites, particularly in East Anglia. Adult females typically have a body-length of 13-22mm. The slightly smaller males usually reach 10-16mm. The Fen Raft Spider occurs solely on lowland sites that have a permeant supply of neutral to alkaline fresh water. The ideal habitat being on fens or coastal grazing marsh ditch systems. The nursery webs of the Fen Raft Spider are constructed in stiff-leaved marginal emergent vegetation.

One site where the Fen Raft Spider is doing well is the Redgrave and Lopham Fen Nature Reserve, in Suffolk. Another site for the Fen Raft Spider is found at Pevensey Levels, a Site of Special Scientific Interest, located between Bexhill-on-Sea and Hailsham, in East Sussex. 

In Wales the Fen Raft Spider can be found at Crymlyn Bog Nature reserve, in Bon-y-maen. Here they are found in turf ponds. They are also found along near-by sections of the disused Tennant Canal, on the edge of Pant-y-Sais National Nature Reserve, where the vegetation structure for their nursery webs is provided mainly by Tussock Sedge, Carex paniculata.

Since 2010 the captive breeding and translocation project, started by Dr Helen Smith, means the Fen Raft Spider can now be found at the Suffolk Wildlife Trust reserves at both Carlton and Castle Marshes.

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