Suction feeding in the carnivorous bladderwort utricularia

Suction feeding is an important feeding mode in aquatic organisms and is used across a considerable size range, from tadpoles to whales. Our current understanding is based on how adult fish feed and suggests that suction feeding is not effective for organisms just a few millimeters in size. All suction feeders have to overcome the inertial and viscous forces exerted by the water when sucking in water plus prey, yet only the inertial forces contribute to prey capture, while viscous forces contribute just to the cost and reduce the effectiveness of prey capture. Large predators do not need to complete their suction strikes as quickly as small predators because the contribution of viscous forces is low. We therefore predicted that (1) small suction feeders complete feeding events more quickly than large suction feeders, and that (2) smaller suction feeders approach the lower size limit and hence cannot generate the same high flow speeds as larger suction feeders. We focused on two species of the aquatic carnivorous plant bladderwort, Utriculariagibba and U. vulgaris, that capture zooplankton in traps that are just 1-5 mm long. We quantified the movements of the bladders during feeding strikes and their peak flow speeds. We found that bladderwort feeding strikes are much briefer than those of adult fish, and that the smaller bladderwort species, U. gibba, generates slower flows than the larger U. vulgaris, suggesting that U. gibba feed near the lower size limit.