From near-extinction in 2010, fen raft spiders – one of the largest spiders in the UK – are set to have their best year on record at RSPB nature reserves, say experts from the conservation charity.
Population numbers on the Norfolk and Suffolk Broads are steadily increasing thanks to conservation efforts, with the most recent survey estimating the total number of female spiders to now be up to 3,750 individuals.
The bolstering of numbers is largely thanks to a successful translocation partnership project, which has helped increased the number of sustainable populations from three to twelve. This was done by creating suitable habitat for the fen raft spiders on the Norfolk and Suffolk Broads, before moving individuals to the sites.
Sensitive restoration of grazing marshes by reserve teams across the RSPB Mid Yare reserves, which includes Strumpshaw Fen, Buckenham Marshes and Cantley Marshes, have created the perfect vegetation mix for nursery webs and provide plenty of prey for the spiders, explains Tim Strudwick, the RSPB Mid Yare nature reserves site manager.
These spiders have an important role in maintaining the rich aquatic diversity found in the grazing ditches in Norfolk and Suffolk.
"The females are impressive in size, but also beautiful – they are truly special to see,” says Strudwick, pointing out that arachnophobes will be pleased to know, fen raft spiders are harmless to humans.
Whilst the species remains Vulnerable and listed as a Priority Species, the annual spider surveys take place in September at these reserves and hope to be another successful breeding year.
“The spiders are only seen in the grazing marsh ditches and are fairly shy but are easier to see from June to September," explains Strudwick.
"Walking along the meadow trail gives visitors at Strumpshaw Fen the best chance of sightings of the mature females and their glistening nursery webs, so now is the perfect time to make a trip to the reserve."
What does a fen raft spider look like?
The fen raft spider has a striking appearance, with distinctive white or cream stripes running along the side of the abdomen.
How big are fen raft spiders?
Adult female fen raft spiders grow up to the size of a human palm.
What do fen raft spiders eat?
The spider is semi-aquatic and an ambush predator where they actively hunt their prey instead of building webs to catch their next meal.
Their legs are covered in tiny hairs that detect the slightest of vibrations on the water’s surface which they can run across to capture their prey.
Their diet includes other spiders, damselflies, dragonfly larvae and pond skaters, and they will even catch fish and tadpoles.
Fen raft spider webs
Female spiders create webs as ‘nurseries’ for their young – these webs are around the size of a regular pizza (25cm).
When and where to see fen raft spiders?
Late summer and early autumn is the perfect time to see these spiders – in the UK, head to the RSPB’s Mid Yare reserves on the Norfolk Broads.
Are fen raft spiders rare?
Previously threatened with extinction, population numbers on the Norfolk Broads are steadily increasing thanks to conservation efforts. The most recent survey estimated the total number of female spiders to be up to 3,750 individuals.
Fen raft spider females
The females demonstrate incredible parenting skills by building an egg sac to carry their babies around for three weeks, carefully dipping it into water every few hours to prevent the sac from drying out.
She does not eat during this period and eventually spins a silk nursery web just before her young are ready to hatch.
The nursery web is then suspended above water and fiercely guarded by the protective mother.
Are fen raft spiders dangerous?
No, fen raft spiders are harmless to humans.
Main image: Fen raft spider Dolomedes plantarius, adult resting in web above ditch, RSPB Cantley Marsh, Norfolk, September. Credit: Matt Wilkinson (rspb-images.com)
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