Massive spider seeks des-res - must have standing water and insects
9 October 2012
The fen raft spider was, until recently, found in only three sites in Britain - in Norfolk, East Sussex and South Wales - and in danger of being lost altogether from our countryside. Natural England has been helping to find new homes for the species and improve its fortunes.
In 2010, Natural England started a captive rearing project to bolster the wild population as part of its Species Recovery Programme. In that year, 3,000 baby spiders (or 'spiderlings') were artificially reared in the kitchen of Dr Helen Smith - an ecologist working with Natural England - who had looked after them continuously from spring onwards. They were kept in separate test tubes, so they didn't eat each other, and each was fed by hand with fruit flies. Hours of intensive care produced a bumper batch of these rare spiderlings, which were released into their wetland habitat at Suffolk Wildlife Trust's (SWT) Castle
Marshes reserve.
The fen raft spider is one of our largest and rarest spiders and, being semi-aquatic, needs a watery home. Its decline has probably been due to the historic loss of its favoured habitats - fen and grazing marsh - and a drop in quality of the habitat remaining. They are ambush predators, lurking on the water surface to
pounce on their prey, which they detect through vibration-sensitive hairs on their feet. Unfussy and opportunistic, they will eat anything they can overpower, from pond skaters to sticklebacks. To move across the water surface, they use their legs to 'row', 'gallop' or 'sail' (by raising some of their legs as sails). The spiderlings usually take two years to reach breeding maturity, and this year there have been over 40 nursery webs at Castle Marshes.
3,000 spiderlings is a lot of work for one person, and last year a partnership of zoos from the British and Irish Association of Zoos and Aquariums (BIAZA) stepped into help. Four zoos accepted up to 400 spiderlings each from Helen to rear for re-introduction. Almost 10,000 spiders were released into the wild in 2011 and it's hoped enough of them will have survived to establish a new breeding population next year.
Helen devised the test tube rearing techniques with the John Innes Centre in Norwich and has reared 5,000 spiderlings in her own kitchen over the last three years. She said "I think everyone who does captive rearing gets very attached to them. The baby spiders each have their own test-tube to avoid them eating each other so you have to devote yourself to feeding them for three months. We achieve survival rates of around 90% over this period, when survival in the wild would be very
low.
“Of course it is the fantastic work by many conservation bodies to restore some of our best wetland areas that makes this project possible. The fen raft spiders now need a helping hand to colonise these areas."
Assisting Helen in 2012 were Dudley Zoo (co-ordinating), Bristol Zoo, Beale Park, Chessington World of Adventures, Chester Zoo, The Deep, Lakeland Wildlife Oasis, ZSL London Zoo, Reaseheath Agricultural College and Tilgate Nature Centre. In October, a total of 2,500spiderlings were released at the RSPB's Mid-Yare reserve in Norfolk and at two other East Anglian sites. The programme aims to increase the number of populations from 3 to 12 by 2020. Although it is still very early days for the project, the increase to four populations is an encouraging start.
About the fen raft spider
- Scientific name: Dolomedes plantarius
- First discovered in Britain: 1956 by Dr. Eric Duffey This species was first discovered in the UK at Redgrave and Lopham Fen National Nature Reserve at the source of the River Waveney, in East Anglia.
- Habitat: fens and wetlands with sedges or water soldier on which to build its nursery web.
- Lifespan: The spiders usually take two years to mature: once adult, the female can produce two egg sacs during the summer but dies before winter.
- Breeding habits: after mating in early summer the female builds a silk sac in which she lays up to 700 eggs. She carries the egg sac in her jaws for around 3 weeks and then guards the young until they are old enough to face the outside world. Although they don’t spin webs to catch their prey, they care for their young in a large tent-like nursery webs built in vegetation above the water.
- Size: males up to 18mm body length. Females up to 23mm body length.
- Food: any animal small enough to tackle. The FRS is one of the few spiders big enough to catch vertebrates such as fishes and newts. The spiders can literally walk on water and hunt for prey both at the water surface and underwater.
- Conservation Status: UK Red Data Book 1. It is a BAP/S41 species and one of only two UK spiders to be fully protected by law under the Wildlife and the Countryside Act 1981.
- The fen raft spider is listed on the England Biodiversity Strategy requiring special help if they are to recover and thrive again in England.
