The Waterworld of the Ubiquitous Bladderworts, Utricularia: from the Arctic Tundra to the Southern Tasmania
Bladderworts Exist Worldwide
The species of the genus Utricularia, commonly known as bladderworts, comprise the biggest genus of all carnivorous plants with about 42% of all species classified as carnivorous plants; between 215 and 233 species depending on the source. Bladderworts, Utricularia, and its closely sister genera, Genlisea, the corkscrew plants, and Pinguicula, the butterworts, comprise the family Lentibulariaceae, exclusively carnivorous. Utricularia has the widest geographical distribution of any carnivorous plants genus, with many annual and perennial species found worldwide from the arid and humid tropics to arctic regions in sunny, moist, and nutrient poor environments. Thus bladderworts can live in various habitats including: hydrophytes, free-floating and affixed aquatics (i.e. attached to the ground below the water) (27%); lithophytes, fixed in or on rocks, usually in streams under running water, and epiphytes, fixed on other bigger plants (13%); and terrestrial species that grow in seasonally wet or moist soils (60%).
Common to all is their moist and water environment in which they evolved their unusual yet most efficient method of preying. All bladderworts are rootless and have specialized leaves similar to small bladders that evolved to capture microscopic life forms. The scientific name Utricularia comes from the Latin word utriculus, which can have many meanings but generally refers to wine flask or those leather bottles used to store water or wine. The bottle or bladder shape of the modified leaves of Utricularia species was thus the inspiration for its scientific name and also for their popular name. One of the difficulties in accurately distinguishing different species is that many are found growing in terrestrial conditions (on wet soil) and also in aquatic (submerged) environments.
Bladderworts are very unusual and highly specialized plants with their body morphology very different from the common idea that we have of plant. Only terrestrial bladderworts have photosynthesizing leaves. Usually, they are very small, hard to notice and mixed up with other weeds around them. Bladderworts, like its sister genus Genlisea has no roots, but stolons (stems at ground level or bellow water or soil) spreading radially, from which rhizoids (root-like filaments, normally only present at the base of shoots) spread laterally with many vesicles (the modified leaves specialized on trapping microscopic life). All together stollons and rhizoids can reach several meters in length.
In land plants normal photosynthesizing leaves grow from the stolons. Usually, occupying a central position on this body morphology there is a stalk that can reach up to 1 m, where single flowers or inflorescences with several flowers develop. Their long stalks with flowers make bladderworts easily noticed as they stands well above the soil, substrate or water where they normally grow. The special arrangement of stolons and rhizoids is what gives stability and keeps bladderworts attached to the soil, rock or substrate, in case of epiphytes. In wintertime, in the northern hemisphere, plants growing in more harsh climates stay dormant in the form tuber-like bodies enabling them to withstand the cold, by losing all their leaves. The same happens to may Australian species that will grow only during the wet season, reducing themselves to tubers of only 10 mm long to wait out the dry season.
Unlike to more common flowers, those of bladderworts have bilateral symmetry, what scientists called as being zygomorphic. The calyx has only two sepals; the corolla is formed of two lips (of fused petals) and a horn-shaped spur (an extension of the lower lip). The spur has the function to store the nectar to attract pollinators (insects) which are not the prey of bladderworts. With this strange appearance bladderworts have assured and ingenious way of getting their flower pollinated. Pollinators need to separate the two lips usually using their body weight on the lower lip and lowering it in order to have access to the plants nectar by sticking their heads or tongues inside the spur and suck the nectar at the bottom. This takes some time and while being occupied doing so they get pollen stuck on their back or head as present that they will leave on their next visit to another flower. This pollination syndrome, as biologists call it, and flower morphology also happen in the related genera Genlisea, the corkscrew plants, Pinguicula, the butterworts, and in the very different snapdragons, Antirrhinum, and orchids. In fact, the beauty of their flowers is the main reason why bladderworts are cultivated unlike most of carnivorous plants that are cultivated for their traps.
The Rapid and Efficient Traps
Unlike most genera of carnivorous plants, bladderworts, and Genlisea species, have their traps hidden underwater or underground (in the case of terrestrial species). Perhaps this is one of the reasons for the popular interest in these plants not being as big as, for example, in plants of the genus Nepenthes, Drosera and Dionea (Venus flytrap) with their beautiful and notorious traps. Basically, bladderworts traps are very small vesicles (pouches), from 0.2 to 1 cm of oval, translucent shape similar to bladders with a lid, the door, that that flexes inward and trigger hairs on the outside. Terrestrial have smaller bladders than aquatic species. Internal glands in the bladders pump water out, so that it sets bladders under tension with the door lodged against a lip of cells. That is, the pressure inside the bladders is less than outside.
Their mechanism is quite simple, purely mechanical and works as follows: the prey is initially stimulated or guided by numerous branched hairs to swim towards the trigger hairs. It will pull one of them eventually. This makes the lid to open suddenly giving access to the interior of the vesicle. By having a much lower pressure inside compared to the outside, this propels water to flow rapidly into the vesicle dragging the prey and everything with it. This trapping happens at very fast rate and much faster than the speed of closure of the leaves of venus flytrap, Dionaea. In fact, this the fastest known movement that any plant species can perform. Once the vesicle is filled with water the lid is closed with the prey inside. Then, the digestion of prey begins, which can be also very fast, due to the secretions of special glands inside. Once it is done, the bladder is emptied and set up again so that it can re-capture another incautious prey. Once preys are entirely digested the bladder can be ready for its next capture in as little as 15 to 30 minutes. The digestion of preys can take some minutes, several hours or even days, depending on the type of prey mostly; e.g. some protozoa have been observed to resist well to the digestive ambient inside the trap and survive for days. For this reason there must be water surrounding the vesicles and that is why terrestrial and epiphytic species need moist soil or substrate literally drenched. The hairs surrounding the mouth trap also serve to fend the trap mouth away from larger bodies which might trigger the mechanism needlessly.
Bladders are known to trap small and microscopic aquatic animal prey, such as rotifers (wheel animals), copepods, ostracods (seed shrimp), cladocerans (water fleas), and chironomids larvae (mosquito-like insect s) from which they absorb nitrogen and phosphorus after diggesting these prey. Thus, in Utricularia species the benefit for investing in bladders is assumed to be nutrients derived from trapping and digesting aquatic organisms. There is also evidence of communities of microorganisms and associated detritus living as commensals in Utricularia bladders. These communities were derived from the external environment, as very young bladders lack them, and the number of inhabited bladders, as well as inhabitant density and diversity, increases over time. The ubiquitous presence of these communities supports the hypothesis that Utricularia plants also derive some benefit from by-products of this community along with carnivory. This suggests that mutualism can be as important as the predator–prey interaction in Utricularia bladders. The bladders in Utricularia, therefore, may provide benefit through a detrital food web together with a carnivorous interaction. Due to its efficient specialization, bladderworts are thus considered the most evolved of all carnivorous plants and thus closer to what we can call a true predatory plant.
Bladder trapping mechanism:
Not Carnivorous at First
Although small, bladderworts have long caught the attention of botanists, not only because of their unusual body and their beautiful flowers but also because of their strange way of living, very different from most common plants. The more visible bladders of aquatic species were first thought of flotation devices before their carnivorous nature was discovered, in 1875, by the American botanist Mary Treat. One of the more important studies about bladderworts was presented be the American botanist Francis Ernest Lloyd, in 1940s, who conducted many experiments that clarified many points which had previously been the subject of much conjecture and disagreement. He proved that the mechanism of the trap was purely mechanical and that bladderworts can in fact consume larger prey, if they have access, such as young tadpoles and larger mosquito larvae by catching them by the tail, and ingesting them bit by bit. These observations were already known to scientists but some doubted the capacity of bladderworts to really capture such large preys. Lloyd, however, demonstrated that bladderworts are quite capable of ingestion of larger preys by stages without the need of multiple stimuli; being only limited by the size of the mouth trap in the case of rigid parts of preys that could not be digested entirely.
Also as Popular Pet Plants
Unlike some of their more exotic carnivorous plants cousins, bladderworts are in fact quite easy to grow. If you look around they are in fact quite common in the wild, although small. Here are some tips if you feel venturous on growing, or even collect them:
- Protect from direct sunlight and keep the substrate wet, with regard to aquatic species, it is best to grow them in ponds or aquariums. Food is not a problem, since the conventional substrate (tree fern, sphagnum, etc.) are rich in microorganisms which provide food for bladderworts.
- Keep the tuberous species dry in the dormant period; most of them are epiphytes of cold climates. Also note that many species are annual plants, so, if possible, inform yourself so that you will not get disappointed when they die naturally.
- Propagation can be carried out by seeds, or simply by splitting the plants. With terrestrial plants make sure to let some of the old substrate/soil together with the “new” plants.
- One of the most common plagues that affect bladderworts is aphids; they look like small green fleas and usually locate on the underside of leaves and flower stalks. One way to fight them is to leave the infected plants immersed in water for some hours or even days and let the bugs drown. This is something that does not cause any damage to bladderworts, even for terrestrial species. Although, there are a lot of insecticides and some that target specifically aphids, their use is not recommended as bladderworts do not tolerate well and proved to be quite sensitive to those drugs.
More About Carnivorous Plants:
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