The rare fen raft spider (Dolomedes plantarius), also known as the great raft spider, is making a comeback in the UK. This semi-aquatic arachnid, that hunts its prey on the surface of water, can be as big as the palm of a human hand. It is the UK’s largest spider.
Fortunately, for people with arachnophobia (fear of spiders), the fen raft spider makes its home in marsh ditches and ferns, unlike its smaller eight-legged cousins that lurk under sofas in our homes during Autumn.
According to the RSPB (Royal Society for the Protection of Birds), thanks to ideal weather conditions and an extremely successful relocation project, the rare spiders are now thriving on its land close to Strumpshaw Fen in the Norfolk Broads.
A fen raft spider. They perch at the waters edge when hunting aquatic invertebrates such as dragonfly larvae, small fish, smaller aquatic spiders and pond skaters. (Image: RSPB. Credit: Helen Smith)
Nursery web number rising rapidly
From July to October, more than 480 nursery webs have been counted, compared to 184 in 2014, which puts the spider well on the way to a significantly more secure position as a British species.
Strumpshaw Fen nature reserve RSPB Site Manager, Tim Strudwick, said:
“It is fantastic to see the spiders now thriving on the reserve, having been first released in 2012.”
“The spiders are doing so well due to the excellent condition of the habitat and our management of the grazing marshes is maintaining ideal conditions for them. They have exactly the right vegetation mix along the ditches to support their nursery webs and the richness of invertebrate prey that the spiders need.”
“It is great to see the spiders are responding by extending their range into new ditches.”
The RSPB describes the fen raft spider as a striking creature with a dark body and creamy-coloured stripe down the side. They are very large air-breathing arthropods. Adult females are slightly over 20mm in length, with a span of 70mm including their legs.
Some adult fen raft spiders span the palm of a human hand.
Brought back from the risk of extinction
Before 2010, there were just three known populations in Britain, leaving the species at a real risk of extinction.
A ground-breaking translocation project between conservation partners and funders – including Natural England, the RSPB, the British Arachnological Society, and Suffolk and Sussex Wildlife Trusts – has already significantly reduced this risk by establishing new fen raft spider populations in the Broads.
Unlike most other invertebrates, their crystal-like nursery webs are easy to monitor. A much greater number of webs have been spotted this year at all of the sites where the spiders were released.
More than one thousand nurseries have been counted at a Suffolk Wildlife Trust nature reserve, where spiderlings (juvenile spiders) were first released five years ago.
They have also spread from the 500 metre stretch of ditch where they were released to occupy more than 3 kilometres of ditches within the reserve.
A ladybird spider. Once thought to be extinct in Britain, but then rediscovered.
Thriving in the Broadland grazing marshes
Coordinator of the project, Dr. Helen Smith, an ecologist contracted by Natural England, said:
“These large and beautiful spiders have really made these new sites home. This is a species that is clearly able to thrive in the Broadland grazing marshes that have been so carefully restored by conservation organisations in recent years. It’s already made a great start on colonising the area’s extensive ditch networks and, in the process, has taken a big step back from the brink.”
Other fascinating rare spider species can be seen at several RSPB reserves across Britain. Clubiona genevensis, which can be found at Ramsey Island, 1 km off the coast of the St David’s peninsula in Pembrokeshire, is one of the country’s smallest and rarest critters. This tiny island resident is just 3mm long.
Ladybird spiders (Eresus sandaliatus) were thought to be extinct until they were rediscovered at one site in Dorset at the end of the 20th century. Mature males have distinctive red and black markings, hence their name.
Video – Fen Raft Spider at Suffolk Wildlife Trust Castle Marshes
This is a video footage of fen raft spiders at Castle marshes. Sometimes they appear to be walking on water.