Hundreds of giant spiders capable of eating fish have been put back in the wild.
The great raft spider is Britain's biggest species and can grow up to 8cm across.
Scientists had to boost numbers by raising them in test tubes and feeding them dead flies until they were big enough to be set free.
The spiders can live for up to two and a half years but they only survive in marshes.
Bristol Zoo Gardens keeper Carmen Solan raised 170 and used a special tube operated with her own mouth to feed them.
She said: "It does take a lot of time."
Natural England, the BBC Wildlife Fund and the Broads Authority are trying to reintroduce them to the Norfolk Broads.
Around 200 babies have been released at the RSPB's Strumpshaw Fen near Norwich, with 10 zoos each rearing their own broods from birth.
Ecologist Dr Helen Smith said it will boost survival rates by 90%.
She said: "Having invested their summer into feeding them, people do get quite attached.
"But when you return the next year and see 500 to 700 babies, it's worth the effort."