Brasenia schreberi, commonly known as water shield or watershield, is a unique aquatic perennial plant belonging to the floating-leaved aquatic family Cabombaceae. Native to a remarkably wide geographic range, it is found in North America, parts of Central and South America, Asia, Australia, and Africa, typically inhabiting quiet, shallow ponds, lakes, slow-moving streams, and bogs. The genus name Brasenia honors the German physician and botanist Christoph Heinrich Braeser, while the species epithet schreberi commemorates the German naturalist Johann Christian Daniel von Schreber. Historically, water shield was valued as an important wild food source by indigenous peoples in North America, as well as a traditional vegetable delicacy in East Asia, particularly in China and Japan where it is known as juncai.

The plant grows from a creeping underwater rhizome anchored in the muddy substrate of ponds. From this rhizome, long, slender, flexible stems (petioles) rise to the water's surface, where they terminate in peltate (shield-shaped), floating leaves that measure about 5 to 10 centimeters in length. A defining biological feature of Brasenia schreberi is its thick, transparent, gelatinous mucilage (slime) that completely coats all submersed plant parts, including the stems, leaf undersides, and unopened buds. This protective polysaccharide coating serves as a defense mechanism against herbivores and pathogens. The plant produces small, inconspicuous, dark purple or reddish-brown flowers that emerge just above the water surface for pollination before submerging to mature into small, pod-like fruits containing a few seeds.

In culinary traditions, water shield is prized not for an intense flavor, but for its uniquely smooth, slippery, and gelatinous texture. The tender young leaf shoots and unopened buds, harvested while still coated in their thick natural mucilage, are considered a gourmet delicacy. In Japanese cuisine, water shield (juncai) is lightly boiled and served chilled as an appetizer, dressed with soy sauce and vinegar (su-mono), where its signature slithery mouthfeel is celebrated as the epitome of refreshing summer dining. In traditional Chinese cooking, it is added to delicate soups and broths for its thickening properties and smooth texture.

Nutritionally and biochemically, water shield is low in calories while containing valuable dietary minerals, amino acids, and plant compounds. The most notable chemical feature is its abundant extracellular polysaccharide matrix, composed of complex mucilaginous polysaccharides that exhibit soothing properties. In traditional herbal medicine in Asia, infusions of the plant were sometimes used to soothe gastrointestinal irritation, clear heat, and act as an anti-inflammatory tonic. Modern phytochemical studies have isolated various antioxidant flavonoids and polyphenols from the leaves and stems, validating its potential health-supporting attributes.