The water caltrop or water chestnut is either of two species of the genus Trapa--Trapa natans and Trapa bicornis. Both species are floating annual aquatic plants, growing in slow-moving water up to 5 meters deep, native to warm temperate parts of Eurasia and Africa. They bear ornately shaped fruits that resemble the head of a bull, each containing a single very large starchy seed. It has been cultivated in China for at least 3,000 years for these seeds, which are boiled and sold as an occasional streetside snack in the south of that country.
The plant's Chinese name is língjiǎo (菱角), líng meaning "water caltrop" and jiǎo meaning "horn."
This plant should not be confused with the unrelated Eleocharis dulcis, also called water chestnut, an aquatic plant raised for food since ancient times in China. Eleocharis dulcis is a sedge whose round, crisp-fleshed corms are common in Western-style Chinese food.
The water caltrop's submerged stem reaches 12 to 15 ft (3.6 to 4.5 m) in length, anchored into the mud by very fine roots. It has two types of leaves, finely divided feather-like submerged leaves born along the length of the stem, and undivided floating leaves born in a rosette at the water's surface. The floating leaves have saw-tooth edges and are ovoid or triangular in shape, 2–3 cm long, on inflated petioles 5–9 cm long which provide added buoyancy for the leafy portion. Four-petaled white flowers form in early summer and are insect-pollinated. The fruit is a nut with four 0.5 in (1 cm), barbed spines. Seeds can remain viable for up to 12 years, although most will germinate within the first two years.
The plant spreads by the rosette and fruits detaching from the stem and floating to another area on currents or by fruits clinging to objects, birds and other animals.
Investigations of archaeological material from southern Germany indicate that the prehistoric population of that region may well have relied significantly upon wild water chestnuts to supplement their normal diet and, in times of cultivated cereal crop failure, water chestnuts may even have been the main dietary component.
In the Chinese Zhou Dynasty, water caltrop was an important food for worship as prayer offerings. The Rites of Zhou (2nd century BC) mentioned that a worshipper "should use a bamboo basket containing dried water caltrops, the seeds of Gorgon euryale and chestnuts" (加籩之實，菱芡栗脯). The Chinese Herbal Medicine Summary (本草備要 published in 1694, written by Wang Ang 汪昂) indicates that water caltrop can help fever and drunkenness.
It was possible to buy water chestnuts in markets all over Europe until 1880. In northern Italy the nuts were offered roasted, much as sweet chestnuts (Castanea sativa Mill.) are still sold today. At many places in Europe water chestnuts were known and used for human food until the beginning of the 20th century. Today, however, it is a rare plant. There may be several reasons for its near extinction, such as climate fluctuations, changes in the nutrient content of water bodies, and the drainage of many wetlands, ponds and oxbow lakes.
Fasciolopsiasis can be transmitted by the surface of the plants.