Molluscs are an important part of the biodiversity of Wicken Fen. This group includes the snails, slugs and bivalves, and 88 species have been recorded at Wicken over the years.

There have been a few detailed surveys of the molluscs of Wicken Fen such as the work of C. Oldham (1920s), C.R.C. Paul (1960s) and M.J. Bishop (1970s). In recent years the molluscs of the water bodies (ditches, ponds, Lodes) have been investigated several times, as they are good indicators of the quality of aquatic habitats.

Several nationally rare species have been found at Wicken Fen.

Pisidium pseudosphaerium (Pisidiidae) - A Red Data Book (Rare) pea mussel found in several of the brickpits and ditches on the Sedge Fen.

Oxyloma sarsi (Succineidae) - A Red Data Book (Vulnerable) wetland snail that was found at Wicken Fen in the 1950s.

Pseudanodonta complanata (Unionidae) - The Depressed River Mussel. This Red Data Book and UK BAP species lives happily in the bottom sediments of Wicken Lode and nearby Reach Lode.

Valvata macrostoma (Valvatidae) - A Red Data Book (Vulnerable) water snail that has been found in the ditches and brickpits of Little Breed Fen at Wicken.