The genus Utricularia is among those carnivorous plants which prey on small aquatic organisms to cover their nutrient needs. The Common Bladderwort may be occasionally found in certain backwaters of the Lobau.
The Common Bladderwort is a rootless aquatic plant which floats just under the water's surface. Its leaves are feathered, highly branched and thread-like. Particularly notable are the numerous minute sacs – or bladders – which function as trap-doors and which are attached to its thread-like leaves. From June to August, flowers are borne erectly out of the water on long stalks. The yellowish-gold flowers are arranged in loose bunches (similar to snapdragons). The outwards-turned lower lip of the flower has a marked spur projecting below it and the flower stalk is 2-3 times as long as the bract. The quite similar Australes section of the genus features a flat lower lip and a flower stalk 3-5 times as long as the bract!
The species is widely found across the northern hemisphere, from North America to nearly all of Europe to northern Asia. In Austria, five different bladderwort types can be differentiated, whereby some are specialised in nutrient-poor moor waters. The Common Bladderwort – sometimes called Greater Bladderwort – may be found in all Austrian provinces except for Carinthia. It prefers lower elevations but has also been sighted in montane zones up to 1000m above sea level. Preferred habitats include stagnant and slow-moving waters, swamps and open reedy areas such as those found in the Lobau.
Endangerment and Conservation Status
Common Bladderwort sites are highly local and rare. The species is classified as vulnerable and in the so-called "Bohemian formation", a geological zone comprising parts of Czech Republic, Upper Bavaria and Lower Austria, as endangered. Those members of the species specialised in moor locations are even rarer and in fact critically endangered.
The Common Bladderwort over-winters on the bottom of waters in the form of winter buds which are more like turions, or strong young shoots. In spring, these buds develop and the plant rises to the surface. Thick patches of bladderwort and shallow waters are especially conducive to flowering; the yellow snapdragon-like flowers are pollinated by hoverflies. Propagation sometimes takes place by way of the rounded seeds. More often however, the winter buds formed each autumn at the end of each stem are responsible for reproduction. These buds separate from the dying parent plant in the autumn and are thus dispersed. Because bladderwort does not put down roots, nutrients must be obtained from the surface areas of the plant and from the crustaceans and other organisms trapped in its bladder-like sacs. The feathery threads of leaves enlarge the plant's surface, which in turn facilitates nutrient uptake and gas exchange. The feathered leaf form is typical of various submerged aquatic plants.
Bladderwort is the only native plant which can capture aquatic organisms. While the sundew hunts its prey with long glandular tentacles and the butterwort preys on insects and small spiders with its sticky rosette, the Common Bladderwort hunts small aquatic organisms like water fleas and mosquito larvae with the bladder-like traps at the end of its thread-like leaves. These traps use a vacuum-effect to capture their prey: when a potential victim approaches the valve of the trap, it opens quickly and the trap walls curve outward. Water flowing into the trap pulls the victim inside and the valve shuts again. Inside the bladder, various types of hairs are eagerly waiting to secrete protein-digesting fluids in order to digest the victim rapidly. Then the bladder is pumped "dry" and the plant can return to the hunt – several times a day. Those bladders currently at work digesting food can be differentiated from empty bladders on the basis of their faint blue colouring.