The fen raft spider is one of the rarest spiders in the UK, only known to occur naturally at three sites, including at our own Redgrave and Lopham Fen, the Pevensey Levels and a canal in South Wales. However, since Dr Helen Smith started a translocation project in 2010, the species can now be found at Suffolk Wildlife Trust reserves Carlton and Castle Marshes. 

Helen raised young spiders, taken from the natural populations, in her kitchen and then released them onto the marshes which, with their network of dykes with good water quality, diverse structure of aquatic plants and biodiversity of invertebrates, are the perfect habitat for this semi-aquatic spider.

Due to their rarity and recent translocation to Suffolk Wildlife Trust nature reserves, it is essential that this species is carefully monitored to ensure that the translocation has been a success and also to inform any future translocations.

Monitoring the spiders

Volunteers head out on to the marshes every week from June to October to monitor the spiders, searching for their easy-to-find nursery webs which are built within plants on the dyke edge or on the dyke surface. Their favourite plants for web-building are the spiky ones, favouring rare plants such as saw sedge at Redgrave and Lopham fen, aptly named for its razor-sharp leaves and 'water soldier' at Carlton Marshes, which sticks out of the water's surface like a thorny crown.

The nursery webs are counted every week and we record whether they have young spiderlings inside to try and estimate the population size as well as understand how they are spreading across the reserves. Surveys at Castle Marshes can take a whole day, often in the hot summer sun without any shade available on the open marshland. Luckily, the volunteers persist because of the intriguing behaviour of these large and charismatic spiders, which make them eternally fascinating to watch and study.

Unlike many other spiders that we are used to in our homes and gardens, which create webs to catch flies and other unsuspecting insects, the fen raft spider only uses its web to raise its young.

To catch prey they wait on the water's surface, the hairs on their front legs able to sense vibrations on the water or in the air, pouncing when prey stray to close. They can pursue their prey underwater as well, breaking the surface tension to continue the chase. Prey species commonly include smaller invertebrates such as pond skaters and dragonfly larvae, however, they have also been recorded taking small sticklebacks and adult dragonflies.

A group of young wardens and I were enthralled to see a large fen raft spider pounce from under a water soldier leaf taking down a female southern hawker dragonfly as she attempted to deposit her eggs into the water. The dragonfly was mercilessly held down by the spider until being dragged off into the shadowy privacy of her water soldier den.

Fen Raft Spiders build their nursery webs purely to protect their young. They are very diligent mothers, first carrying their young inside an egg sac for up to four weeks then biting open the sac to release the young spiderlings into their nursery web, which the mother then guards for about five days until the spiderlings disperse. This arduous parenting takes a huge toll on the mother, with female spiders often seen dead on the nursery webs at the end of the season.

The surveys at Carlton and Castle Marshes have shown the translocations to be a huge success so far, with the populations expanding well from their initial release dykes.

Castle Marshes has been the most successful site, with Fen Raft Spiders now found on nearly every dyke and some dykes being so densely packed with webs they are hard to count. At Carlton Marshes the population has shown a slower but steady spread, for reasons we are yet unable to identify, but the last two years seem to have shown a breakthrough with many dykes being colonised for the first time.

The recent land purchases and habitat creation at Carlton Marshes nature reserve will continue to improve the spider's habitat here, as we are able to clean out choked dykes and eliminate any sources of pollution that could endanger water quality. To maintain the perfect conditions for the spiders to thrive into the future, the marsh dykes need cleaning out every seven years or more, just enough to keep them open and stop them silting over but also not causing too much disturbance and allowing a diverse structure of aquatic plants to develop across the dyke network.

To see these fascinating spiders come to Carlton Marshes in July and August and spend some time gazing into the dykes. If you spot a nursery web with spiderlings, the mother will be nearby, sometimes on top of the web and sometimes hidden nearby. Be patient and you will spot one of the rarest and most carefully protected spiders in the UK.

Find out more about Carlton Marshes

Find out more about Castle Marshes