The great raft spider, also named fen raft spider, is a large European species of spider in the family Dolomedidae. It is semiaquatic, hunting its prey on the surface of water. It occurs mainly in neutral to alkaline, unpolluted water of marshes.
Adult great raft spiders exist in two different phenotypes. The first has white or yellow longitudinal bands on a brown background, each located on the sides. The other is completely brown, with no white longitudinal bands at all. The latter is the more common of the two.
The diet of the fen raft spider is mostly made up of other invertebrates such as small spiders, dragonfly larvae and pond skaters. However, the spiders have been known to catch small vertebrates such as fish and tadpoles.
The great raft spider is generally shy and not aggressive. It may bite if it feels threatened or is accidentally disturbed. The venom is not dangerous to humans and bites typically cause only mild discomfort.