Return of the UK's biggest spider

Chester Zoo keeper Karen Lambert explains what happened on release day.

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With a leg span of up to 8cm, the great raft spider (Dolomedes plantarius) is one of the UK's largest arachnids.

Before you jump on the coffee table in fright, you should know that these water-gliding giants only live in wetlands, and very few now survive there.

Fortunately for the spiders ecologist Dr Helen Smith has recruited an army of experts nationwide in the latest phase of a programme to revive the three remaining populations.

With help from 10 zoos from across the country, thousands of spiderlings have been reared for reintroduction to their former stronghold in the Norfolk Broads.

"Most invertebrate groups don't receive a lot of conservation effort," said Dr Smith.

A raft spider on water

"The ones that tend to receive the attention are the big and spectacular ones... this is very much the case with [great raft spiders]."

"They're big, they're beautiful and they have to fly the flag for other species."

With funding from Natural England, the Broads Authority, the BBC Wildlife Fund and volunteers, a lot has been invested in the 5mm babies released at the RSPB's Strumpshaw Fen reserve near Norwich.

Test-tube babies

Having collected eggs from Redgrave and Lopham Fen reserve, Suffolk, Dr Smith distributed them to willing volunteers from almost a dozen zoos and wildlife parks.

Instead of being raised in their usual nursery webs, suspended from wetland plants, the tiny animals were individually housed in test tubes away from the elements.

As well as helping carers to keep track of their tiny spider wards, the test tubes aided survival rates by keeping the young from their predatory siblings.

Keeper Carmen Solan cares for her spiderlings before their journey to Norfolk.

Rather than feeding on each other, the spiderlings were given a diet of flies, fed to them by their keepers through the summer months.

Bristol Zoo Gardens keeper Carmen Solan has raised 170 of the arachnids for the programme. She even fed them with a special tube she operated with her mouth.

"It does take a lot of time," she said, "You have to be careful to make sure they've got enough humidity to be able to moult."

The affection Carmen has for her batch was apparent as she described their voracious appetites and quicker-than-the-eye movements.

"I try not to have favourites... but they've definitely got a special place," she said as she prepared them for their journey to Norfolk.

Dr Smith told BBC Nature that many of the keepers would be sad to see the spiderlings go.

"Having invested their summer into feeding them individually, people do get quite attached," she said.

"But when you return the next year and see 500 to 700 babies, it's worth the effort."

Where spiderlings were reared

  • Dudley Zoo
  • Bristol Zoo
  • Beale Park, Berkshire
  • Chessington World of Adventures, Surrey
  • Chester Zoo
  • The Deep, Hull
  • Lakeland Wildlife Oasis, Cumbria
  • ZSL London Zoo
  • Reaseheath Agricultural College, Cheshire
  • Tilgate Nature Centre, West Sussex

The ecologist, who has been working with fen raft spiders for 20 years, explained that the conservation programme is largely concerned with giving the spiders the best chance in the wild.

By rearing the spiderlings in isolation, project staff can boost their survival rates by as much as 90% which in turn gives them a greater chance of success in the wild.

This is the third year of the translocation project, which began with Dr Smith raising spiderlings in her kitchen, before she was joined by staff from four zoos as a trial last year.

"We're all improving our captive-rearing expertise through this process," she said.

"It's early days. We're two years down the line now but the indications are that the spiders are settled in well."

Earlier this year, Dr Smith's team reached "a major milestone" when they reported breeding success for the previous batch of spiderlings.

They discovered four nursery webs, each containing around 200 spiderlings.

Dr Smith describes herself as "cautiously very optimistic" following the results, which suggest the translocation could be successful.

But she also explained to BBC Nature that the project's intention is not to flood the fens with giant spiders, merely to repair the damage already done.

Male great raft spider The spiders are named for their ability to glide on water

"With animals that are this rare you can make lots of arguments about food chains and food webs, and how important that is," she said.

"We simply don't know what pulling out one brick will do further down the heap."

For the same reason however, researchers on the project have had to carefully consider what adverse effects could arise from introducing a species known to eat prey larger than themselves - including sticklebacks.

"They are generalist predators," explained Dr Smith, "most of what they eat are common things."

With this in mind, the spiders have only been introduced to the most biodiverse fen habitats: those that have recovered from the agricultural drainage that dramatically altered the landscape in the 20th century.

"This is never going to be a common species, it's always going to be consigned to our best wetlands," said Dr Smith.

Fen raft spiders in test tubes at Bristol Zoo Gardens Test-tube spiderlings at Bristol Zoo Gardens

But, according to the passionate ecologist, the message that we can help such a "spectacular species" could have knock-on effects to raise awareness of wetland habitats and other vulnerable invertebrates.

"Translocation isn't the answer to all our species-loss problems," said Dr Smith.

"You can't put most things back - but for some of them, it's worth doing."

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