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The fen raft spider, Dolomedes plantarius, is one of two species of the genus Dolomedes (Family Pisauridae) that live in Europe. It is a large, semi-aquatic species, with adult females reaching body lengths of up to 23mm, spanning up to 7cm including their legs. Most individuals are strikingly marked, with black or brown bodies and white or cream stripes along the sides of the cephalothorax and abdomen (Fig. 1).
Fig.1 Adult female D. plantarius
Throught its range in central and western Europe, D. plantarius is a species of lowland wetlands. In the UK, it is found around the margins of grazing marsh ditches, canals and fen turf ponds where the water supply is base-rich but nutrient poor. It is typically associated with a rich invertebrate assemblage and with emergent and marginal vegetation with a high level of structural complexity. Stiff-leaved plant species that provide a ridgid framework on which nursery webs can be anchored are particularly favored. In fen sites these include Cladium mariscus and Carex paniculata and in ditches marginal Carex pseudocyperus and floating Stratiotes aloides. Stands of Phragmites australis are generally avoided because they are densely shading and offer a less suitable structure for supporting nursery webs. Follow these links for pictures of its habitat at Redgrave and Lopham Fen, the Pevensey Levels and the Tennant Canal, its three UK sites. In continental Europe it is also found in areas of monastic fish ponds and in extensive wetlands such as river deltas and areas of ancient fen peat diggings (e.g. Fig. 2). In Scandinavia it is associated mainly with the margins of natural lakes (Fig. 3). In contrast to the UK, it has been found in tall stands of Phragmites australis in Italy.
|Fig. 2 Habitat in the Brière wetland, France||Fig. 3 Lake-side habitat in southern Sweden|
Anecdotal evidence suggests that populations of D. plantarius decline and may be lost where its habitat becomes enriched by agricultural run-off.
In the UK, a population decline at Redgrave & Lopham Fen was associated with loss of a reliable year-round water supply. Drying out of the fen resulted from abstration of water compounded by summer droughts.
The habitat of D. plantarius contrasts with that of the congeneric and very similar D. fimbratus, which is typically a species of lowland wet heaths and upland acid mires. D. fimbriatus appears to be rather less dependent on proximity to standing water, often constructing its nursery webs in shrubby species such as Myrica gale and Calluna vulgaris. Conditions suitable for both Dolomedes species may exist in close proximity in some sites. Populations of both species have been recorded sympatrically from sites in France.
Hunting and prey
D. plantarius is a sit-and-wait predator. This species neither builds webs in which to capture its prey nor actively pursues its prey until it is within easy reach. In captivity the spiders produce diffuse webs but these are simply the accumulation of silk drag lines left as they move around in a confined space. The spiders appear to find it more difficult to capture prey in the presence of this web than in its absence.
The adults and most immature stages of D. plantarius are usually found amongst marginal and emergent vegetation, basking or lying in wait for their prey. Their typical hunting posture is to sit with their back legs on marginal or emergent riparian vegetation and their front legs resting on the water surface. Long, fine hairs (trichobothria) on the legs equip the spider with an excellent vibratory sensory system, allowing them to detect the movements in the air and on the water surface caused by both prey and potential predators. Water repellent (hydrophobic) hairs on the legs enable them to utilise the surface tension to rest on the water surface or to row or gallop across it to grab their prey. They can also break the surface tension and run down stems underwater to hunt for prey. Swimming between underwater stems clearly requires effort to overcome the buoyancy provided by air trapped in the hydrophobic body hairs - underwater, the spiders appear to shine in a silver sheath of trapped air (the plastron).
D. plantarius has a catholic diet, probably taking prey roughly in proportion to its abundance. The primary prey species are aquatic invertebrates, including smaller species of aquatic spiders, pond skaters, water beetles and dragonfly larvae. Terrestrial invertebrates that fall into the water are also taken, as are small vertebrates, such are fish and tadpoles, although these probably comprise only small proportion of the diet.
The front legs are used to capture small prey items. For larger prey the back legs may be used as well and the spider may turn over several times in the water in the struggle until the prey is immobilised by injection of venom from the fangs (see Stefan Sollfors video footage). Prey capture is so rapid that it is difficult to analyse the movements involved.
Little information is available on predators of D. plantarius. When small, the spiderlings are likely to be prey to a wide range of species. In the nursery they are vulnerable to avian insectivores, particularly when water levels are low and birds concentrate around the water margin to drink. However, their efficient escape mechanism from the nursery (see below) makes it unlikely that a high proportion of a brood would be consumed in this way. Direct records of predation of large and adult D. plantarius in the UK are confined to two observations of them being eaten by frogs Rana temporaria. An observation of a set of adult legs, with the body completely missing, on a ditch surface was most likely to be attributable to avian predation. Two observations of predators taking large specimens of D. fimbriatus are also likely to relevant to D. plantarius. One is of a hornet Vespa crabo capturing and taking a specimen in the New Forest, England, and the other is of a Fan-tailed warbler Cisticola juncidis photographed with a Dolomedes in its beak in Italy (Fig. 4). In the USA, Carico (1973) reports a collection in Florida containing 32 adult Dolomdes trition taken from the gut of an immature Little Blue Heron (Egretta caerulea).
D. plantarius use their ability to hide underwater to escape predators as well as a hunting tactic. As an escape response, this is often accompanied by excretion - the white cloud in the water obscuring their presence. Adult females carrying egg sacs are particularly vulnerable to predation by visual predators when they sit in emergent vegetation up to a metre above the water in the days before their eggs hatch. If near-by vegetation is disturbed they often escape by dropping vertically into the water.
Cannibalism is thought to be relatively rare in the wild although it occurs amongst small juveniles in artificially crowded conditions one they are past the nursery web stage. In the wild D. plantarius often bask or adopt and maintain hunting postures within a few centimeters of one another. Amongst observations of cannibalism in the field, most, but not all, are thought to have been triggered by one individual reacting to a rapid escape movement by another, often underwater, in response to human presence. At Redgrave & Lopham adult females have twice been recorded eating other adult females. Cannibalism of males by females during courtship is also very rare. It has never been observed in the field and only twice in the laboratory out of many pairings in a small experimental arena.
Spiders of the genus Dolomedes have two primary means of locomotion at the water surface: a rapid galloping used in the pursuit of prey or escape from predators, and a slower rowing action. Underwater they usually move along submerged stems but can also swim between then using a rowing action. They are also able to move across the water surface by sailing. This can involve elevating the whole body or raising between one and all eight legs to catch light breezes. This behaviour has been observed in both D. plantarius and other Dolomedes species. Arial dispersal of tiny spiderlings by 'ballooning' on silk lines is discussed under life-history.