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Is it a Dolomedes?
As adults, both European Dolomedes species are usually striking and distinctive in appearance. As well as being very large (adult females ca 18-23 mm and males ca 13-18mm body length), they are characterised by a white, cream or yellow stripe along the sides of both the carapace and abdomen which contrasts with the dark body (click here for a picture).
However, unbanded adults, which occur at a low frequency in D. plantarius populations, can be more difficult to distinguish from other species, as can both banded and unbanded juveniles. At all growth stages there is considerable variation in body colour and well as the colour and width of the stripe. Body and stripe colour may be the same or differ between the carapace and abdomen. Stripe width and boldness is very variable and some individuals also have paired white dots along the abdomen. Colour variation, particularly of the body, tends to be greater in D. fimbriatus than in D. plantarius and the stripe width also tends to be greater.
Despite all of this variation, the European Dolomedes have their own distinctive 'jizz'. Their size range, patterning and colour variation all overlap with those of other European spider species but their overall appearance remains distinctive at all growth stages. Their bodies are essentially cigar-shaped and powerful in appearance, tapering towards both ends. The legs and body are shortly hairy: when the spiders are under water, air trapped in the body hairs makes them appear silver.
The sexes are indistinguishable until they are subadult. In common with other spider species, as well as clubbed palps, the mature males have a broader, slightly shiny carapace, narrower abdomen and longer legs than the females. Their overall body length is several millimeters less than that of the females. Adult females are most conspicuous and unmistakable when they are carrying their bulky egg sacs: these vary in size from ca 8mm to ca 12mm and are held in the chelicerae and attached to the spinners. The abdomen is curled around the sac and so often only the carapce is visible (click here for a picture).
Is it Dolomedes plantarius or D. fimbriatus?
Definitive separation of the two European Dolomedes species used to rely on expert examination of microscopic characters that are only developed in adults. Adult female Dolomedes plantarius lack light hairs that, in D. fimbriatus, obscure the epigynal structures. The palps of adult male D. plantarius have a bifid tibial apophosis whereas those of D. fimbriatus have a simple structure. DNA fingerprinting has provided an alternative method of separating the two species and can be used for all developmental stages.Although it cannot provide definitive identification, the two species tend to differ in their colouration. Colour variation, particularly of the body, tends to be greater in D. fimbriatus than in D. plantarius and the stripe width also tends to be greater. Unbanded morphs appear to be completely absent from D. fimbriatus populations. Habitat type is also an indicator of species identity but, again cannot be relied upon: the two species can occur in quite close proximity. In general D. fimbriatus is a species of acidic, wet heaths and upland mires. It is less dependent on open water than D. plantarius, which is a species of fens, grazing marsh ditches, canals and other unpolluted neutral/alkaline water bodies. Click here for pictures of both species.
Species that cause confusion
The main source of confusion in the UK is with the semi-aquatic Pirata species, several of which have more or less distinct white lateral lines on the carapace. Mature Piratas are a similar size to Dolomedes of a third to a half of their adult size. Their 'jizz' is very different with a high and squared-off front to the carapace and rather bulbous abdomen. In the breeding season female Piratas are often seen carrying egg sacs but these are small (ca 5mm) and white and carried at the back, in the spinners (click here for a picture).Because it shares a similar habitat, the water spider Argyroneta aquatica is another source of confusion with Dolomedes. This is a truely aquatic species, living underwater and constructing silk retreats that are filled with air bubbles, making them resemble diving bells. They often occur in the same water bodies as D. plantarius although they look very different.
Although it occupies a very different habitat - it is a common grassland species - the nursery web spider Pisaura mirabilis, is very closely related to the Dolomedes species and can look quite similar. It is the only other European member of the family Pisauridae, to which the genus Dolomedes belongs. Although smaller than Dolomedes (adult females are 12-15mm in length), P. mirabilis are a similar shape and very variable in colour, sometimes with bold abdominal markings. However, they resemble Dolomedes most closely when they are carrying their egg sacs and attending their nursery webs. As in Dolomedes, egg sacs are carried under the body, gripped by the chelicerae. The nursery webs are also very simliar to those of Dolomedes, although smaller, and constructed in grass and tall herb vegetation (click here for pictures).
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