The watershield, scientifically known as Brasenia schreberi, is a perennial aquatic plant recognized for its distinctive floating foliage and widespread distribution. Its oval, green leaves rest on the water’s surface, attached by slender stems. The plant’s most notable feature is the thick, clear, gelatinous coating that covers all submerged parts, including young stems, buds, and the undersides of the leaves. This slick layer gives the plant its common name and has drawn the attention of biologists and culinary experts.
The Protective Mucilage
The unique, jelly-like coating of the watershield is a polysaccharide hydrogel that serves multiple biological functions. This mucilage is primarily composed of polysaccharides, which are long chains of monosaccharide units, including D-galactose, D-mannose, L-fucose, and D-glucuronic acid. Mannose is thought to form the polymer’s backbone, creating a highly branched, viscous substance.
The primary role of this thick, water-insoluble gel is defense against external threats. The slippery texture makes the plant unpalatable or difficult for herbivores to grasp and consume, functioning as an anti-herbivory mechanism. The mucilage also exhibits strong anti-fouling properties, preventing the attachment and growth of bacteria and algae. This anti-microbial action keeps submerged organs clean, allowing for efficient nutrient uptake and gas exchange.
The thickness of the mucilage layer is considered an economic attribute in regions where the plant is cultivated. Research suggests the mucilage has potential health benefits, including cholesterol-reducing properties, similar to other viscous polysaccharides. The mucilage layer gradually disappears as the plant matures, indicating a biological shift in the plant’s protective needs as it transitions from a young, vulnerable stage to a more established one.
Global Distribution and Growth Patterns
Watershield is an aquatic macrophyte with a cosmopolitan distribution spanning North America, South America, Australia, parts of Africa, and Eastern Asia. Despite this wide geographical spread, the plant is particular about its habitat, thriving only in specific, often undisturbed, freshwater environments. It prefers the quiet, clear waters of lakes, ponds, and slow-moving streams, often in areas with nutrient-poor or slightly acidic conditions.
The plant propagates using two methods: vegetative growth and wind pollination. Watershield spreads beneath the water surface using underwater runners known as rhizomes, which anchor the plant and stabilize the sediment. The reproductive cycle involves small, inconspicuous flowers that emerge briefly above the water surface.
Watershield utilizes wind for pollination, a less common strategy for aquatic plants. The flowers typically open for a two-day period, first presenting as female and then as male, ensuring cross-pollination. This combination of rhizomatous growth and specialized wind pollination allows it to colonize and form dense mats across suitable bodies of water.
Ecological Role and Culinary History
In its natural habitat, watershield serves several functions within the aquatic ecosystem. The dense coverage formed by its floating leaves offers shelter and shade for various small aquatic organisms, including amphibians, fish, and macroinvertebrates. Its submerged structures and rhizomes stabilize bottom sediments, helping to maintain water clarity.
The plant’s ecological significance is balanced by its culinary history, which is most pronounced in East Asia, particularly in China and Japan. In these regions, watershield is actively cultivated and harvested for human consumption, rather than being viewed as a weed. It is known as junsai in Japan and chún cài in China.
The young leaves and stems are considered a delicacy for their unique, mucilaginous texture, which results directly from the protective gel. Watershield is commonly used in soups, such as the West Lake Watershield Soup in Hangzhou, China. This history of consumption has also led to its use in traditional medicine, where its astringent leaves have been employed to treat conditions like boils.