Marsilea is a genus of approximately 65 species of aquatic ferns of the family Marsileaceae. The name honours Italian naturalist Luigi Ferdinando Marsili (1656–1730).[3]

These small plants are of unusual appearance and do not resemble common ferns. Common names include water clover and four-leaf clover because of the long-stalked leaves have four clover-like lobes and are either present above water or submerged.

The sporocarps of some Australian species are very drought-resistant, surviving up to 100 years in dry conditions. On wetting, the gelatinous interior of the sporocarp swells, splitting it and releasing a worm-like mass that carries sori, eventually leading to germination of spores and fertilization.

Sporocarps of some Australian species such as Marsilea drummondii are edible and have been eaten by Aborigines and early white settlers, who knew it under the name ngardu or nardoo. Parts of Marsilea drummondii contain an enzyme which destroys thiamine (vitamin B1), leading to brain damage in sheep and horses. During floods in the Gwydir River basin 2,200 sheep died after eating nardoo. Three-quarters of the sheep that were affected did however respond to thiamine injections.[5] Thiamine deficiency from incorrectly prepared nardoo likely resulted in the starvation and death of Burke and Wills.[6][7]

The leaves of Marsilea crenata are part of the East Javanese cuisine of Indonesia, especially in the city of Surabaya. It is called Pecel Semanggi and is served with spicy peanut and sweet potato sauce.

A few species in the genus, such as Marsilea crenata, Marsilea exarata, Marsilea hirsuta, and Marsilea quadrifolia, are grown in aquaria.

Molecular phylogenetic analysis of the genus Marsilea shows the following tree.[9] This tree indicates that M. crenata is the same species (or a subspecies) of M. minuta, and possibly M. fadeniana also. Additionally, this analysis contradicts reports[10] that M. polycarpa is a synonym for M. minuta