Fen raft spider

Fen raft spider by Martin SmithFen raft spider by Martin Smith

Dolomedes plantarius

The fen raft spider breaks many of our spider stereotypes. It is large, beautiful and found not in dry dusty corners, but on sparkling water surfaces in our richest wetlands. One of our rarest species, fen raft spiders were first discovered in the UK at the Trust’s Redgrave and Lopham Fen reserve in 1956. They have since been found at only two more sites – in southern England and South Wales.

With dark, cigar-shaped bodies highlighted by white or cream go-faster stripes, and reaching lengths of 23mm, the fen raft spider is a striking animal. It lives around the margins of water bodies such as fen pools and grazing marsh ditches where it sits and waits for prey. It is a formidable hunter above and below water, taking prey much larger than itself, including large dragonfly larvae and even sticklebacks!

Water is essential for breeding as well as for hunting. An elaborate courtship ritual of bobbing and leg tapping is translated through vibrations in the water surface. The eggs are laid into silk sacs that are immersed in water in hot weather. When they hatch, the hundreds of tiny spiderlings are guarded by their mother in a huge silk ‘nursery’ web built in stiff-leaved vegetation above the water’s edge.

For a species so dependent on a permanent supply of clean, still or slow moving water, the drainage and pollution of many of our wetlands spelled near-disaster. At Redgrave and Lopham Fen, former abstraction of water from the chalk aquifer reduced the population to perilously low levels. The spiders have poor powers of dispersal and so recovery has been very slow.

To overcome this problem, and help the fen raft spider recolonise some of our restored wetlands, the Trust began an exciting new programme in 2010, working with Dr Helen Smith from Natural England and the BBC Wildlife Fund.

Thousands of baby spiderlings were reared in captivity, hugely increasing their survival. They were released in 2011 at the Trust’s Castle Marshes reserve, creating the UK’s first new population, and at Redgrave and Lopham Fen to help recovery of the population there. Future releases will secure these populations and establish other new ones in East Anglia. Fen raft spiders will never be common but, over the next decade, we hope that they will become less rare – no less a special species but one with a much more secure future and which many more people can enjoy.