The sinister secret of this free-floating pond plant lies beneath the water’s surface. Meet the bladderwort and its highly effective trap
One moment a water flea is pinging its way around its pond microcosm; the next it has seemingly vanished. The unlucky crustacean has fallen victim to the fastest movement in the plant world, the greater bladderwort.
Plants are not known for their speed. Nor are they the first thing that comes to mind when you consider what might eat a water flea in a pond (and there are many). But this particular flea had the misfortune of bumping into the greater bladderwort (Utricularia vulgaris), probably one of our most interesting, yet overlooked plants.
What is bladderwort?
Bladderwort is a carnivorous pond plant. None of the UK’s carnivorous plants make themselves particularly obvious. The bladderworts, of which there are multiple British species, are even more obscure. They are relatively localised and thrive only in nutrient-poor habitats. Even here, they live below the surface of ponds, lakes and ditches, out of sight and out of mind.
What does bladderwort look like?
Among the assorted waterweeds, bladderwort seems to melt away. It is almost translucent, a delicate mesh of pale yellow-green stems with hairy, unleaf-like leaves and no roots, which gives it little by way of surface area or substance.
However, come summer, this elusive plant finds its extrovert side. It throws flower stems straight up from the water surface, each decorated with large, bright golden flowers resembling those of a snapdragon. But for me, the bladderwort’s beauty isn’t in its blooms, rather in its game-changing design.
This is a plant of a thousand mouths. Beneath the water’s surface, each of its strands is adorned with hundreds of ‘bladders’ just 2-5mm long, which give the plant its name. The bladders function as both stomachs and highly complex traps – all the better for ensnaring, and then devouring, unsuspecting passers-by.
How does bladderwort capture and kill its prey?
To set a trap, the plant pumps water out of the bladder, causing its thin walls to buckle inwards, storing elastic energy. The trap door closes and the unit is primed.
Around the mouth of the trap are various bristles. Some are long and filamentous and serve as baffles, funnelling free-swimming animals towards the deadly trigger: a pair of shorter hairs attached to the spring-loaded trap door.
When an animal – in this case our doomed water flea – stumbles into these trigger hairs, the door buckles enough to break the seal, causing an immediate release of elastic energy and a swift, inward rush of water that sucks it helplessly into the trap, a fate that is sealed when the door swings shut. All this happens in 0.3-4 milliseconds.
Deprived of oxygen, the flea soon suffocates. Within a few hours it has become a nutritious soup, broken down by digestive enzymes released from the inside of its planty prison (as well as a community of bladder-dwelling microorganisms). The whole dastardly process can even be witnessed through the translucent walls.
As well as water fleas, copepods and nematodes, the traps can ensnare larger mosquito larvae and even tadpoles. These often get stuck in the door and are thus slowly digested alive, getting sucked in a little further each time the trap is triggered.
A bladderwort trap is a highly effective mechanism for catching insect prey. Around a dozen insects can be caught in one go and the plant can reset itself – pumping out any excess water and re-sealing itself – within 20-30 minutes, ready to capture additional prey. The animal diet of these diminutive aquatic triffids allows them to subsidise their diet from photosynthesis with nitrogen, potassium and phosphorus
Bladderwort may, on the surface, seem like an innocent yellow flower, but you never know what goes on behind closed (trap) doors.