| Summary: | The Fen Raft Spider, Dolomedes plantarius, is a semi-aquatic spider that inhabits lowland wetlands. It is one of the largest spider species in the UK, and one of the most striking, with white or yellow lateral bands. It is rare throughout Europe and is known from only three separate wetland sites in the UK. It was first identified at Redgrave and Lopham Fen in 1956, and subsequently on the Pevensey Levels in 1989, and the Tennant Canal in 2003. Water is required by all life-stages, for hunting, courtship and breeding, which makes the species very vulnerable to drought. The most vulnerable UK population is at Redgrave and Lopham Fen where the species has been lost from c. 80% of its historic range as a result of water loss and degradation. Despite improved water levels across the site, the spider has not re-colonised its former range at this site. Consequently, this study investigates the ecological requirements of D. plantarius in relation to its highly restricted distribution, small population size and requirements for recovery. This species has a two year life cycle with 2% of spiderlings surviving to adult. All life-stages are poor dispersers, despite spiderlings showing the ability to utilise aerial dispersal. D. plantarius is tolerant of a wide range of environmental conditions, but maintenance of a year-round water supply is crucial, with drought resulting in reductions in egg-sac survival and spiderling growth rates. Vegetation structure also appears to be important for nursery web construction. Experimental manipulation of temperature regimes and food supplies shows that growth and maturation are more rapid at higher temperatures and higher feeding frequencies. Feeding frequency also influences adult body size, which may impact on fecundity. Changing climatic conditions are therefore very likely to impact on these spiders, but any beneficial effects of rising temperatureso n growth rates may be offset by drought effects on productivity and immature development. Conservation strategies for D. plantarius are likely to require restoration of wetland habitats and, given the poor dispersal capacity of the species, translocation of spiders to new locations. The small numbers of spiders present in the wild in the UK means that captive breeding or rearing is likely to be necessary. This study has established the conditions necessary for successful captive rearing of D. plantarius, but captive breeding was much less successful. Captive rearing of spiders hatched from wild-caught egg-sacs and translocation into sites with reliable year-round water supplies is likely to be the best strategy for improving the status of D. plantarius in the UK.
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