|A clump of Utricularia|
vulgaris in flower.
Below the water, and at a much smaller scale, is where things get interesting. Look closely at those fine feathery leaves. Find the little rounded bladders that cling to them all over. Those aren't some kind of parasite -- they're part of the plant. In fact, they're its means of trapping prey.
The bladders, when they first develop, have a near vacuum inside. Their trap-door openings stay firmly shut, maintaining that suction-cup strain. Just outside the door, poking into the water like delicate whiskers, are one or more tiny hairs. Those hairs are fine triggers, poised, connected to the door.
When a water flea or tiny insect larva trundles by, brushing against those hairs -- snap! The trap door pops open, sucking water into the bladder, and the prey is swept along with it. The door snaps shut a fraction of a second later. By now, you can probably predict the end result: digestive enzymes, liquid bug, and the plant gets a very necessary dinner. Without the traps, the plant's growth slows dramatically, as it can't get enough nutrition from the acidic, nutrient-poor water of its native bogs.
Bladderwort traps are powerful things, especially for their size. If the plant is removed from the water (say, by a curious human), it often comes out with a spatter of audible crack! sounds: its traps springing shut on empty air.
Image source: Hillewaert, Hans. Utricularia vulgaris. Retrieved 17 Apr 2012, from Wikimedia Commons: <http://commons.wikimedia.org/wiki/File:Utricularia_vulgaris.jpg>