Chester Zoo: Record-breaking year for giant fen raft spiders as thousands released across UK

A decade ago, Chester Zoo launched a programme to reintroduce thousands of fen raft spiders, one of the UK’s rarest arachnids, after their numbers had plummeted due to habitat loss.

Today, Chester Zoo proudly reports that the species has flourished as part of their conservation efforts, with over 10,000 breeding females now thriving across the country.

Chester Zoo said: "Ten years ago we helped release thousands of giant spiders back into the UK!. The Fen Raft Spiders were bred right here at the zoo, and we're super happy to report there are now more than 10,000 breeding females and they've just had the biggest mating season on record!”

Though their size may seem intimidating, fen raft spiders are harmless to humans and are vital to healthy aquatic ecosystems. Named for their ability to "raft" across water, these semi-aquatic spiders are skilled hunters, able to run along the water’s surface to capture prey like other spiders, damselflies, dragonfly larvae, pond skaters, and even small fish and tadpoles. Their legs, covered in fine hairs, detect subtle vibrations on the water, allowing them to strike quickly.

Fifteen years ago, the fen raft spider population was critically low, and Chester Zoo, in collaboration with the RSPB, began raising spiderlings in bio-secure conditions. “Our team delicately hand-fed tiny flies to each of the hundreds of spiderlings using tweezers, day in, day out, for weeks on end,” Chester Zoo explained, highlighting the meticulous care involved in the species’ recovery.

Thanks to these efforts, the fen raft spider population has reached unprecedented numbers. The RSPB estimates that the current population represents the best year on record for the species, which now plays an essential role in the ecosystem of restored wetlands across the UK.

Tim Strudwick, site manager for the RSPB’s Mid Yare nature reserves, said: “The Fen Raft Spider is one of the UK’s rarest invertebrates, and we are proud of the role our reserves and teams have played in its recovery. These spiders have an important role in maintaining the rich aquatic diversity... The females are impressive in size, but also beautiful – they are truly special to see!”