Prey Size Selectivity in Utricularia vulgaris

Bladderwort are a genus of carnivorous plants that traps zooplankton of submillimeter size by active suction. They are among the smallest suction feeders, with gapes ranging from 0.2 to 0.9 mm in the species Utriculria vulgaris. Other suction feeders of similar size (larval fish) are relatively ineffective feeders, in contrast to adult fish. Even though small suction feeders such as larval fish are ineffective suction feeders, we wanted to explore if that principle applies to other small suction feeders such as aquatic carnivorous plants. The aim of this study is to explore the prey size selectivity and feeding morphology of bladderwort. Despite the hydrodynamic theory, suction feeding is ineffective in smaller predators, bladderworts were found to be effective suction feeders even due to their microscopic size. Furthermore, previous hydrodynamic theory studies suggest that there is a minimum gape length (lower limit) required to be able to suction feed prey successfully. We found that the traps of U. vulgaris range over one order of magnitude from 0.3 to 3 mm and with gape shape closely isometric with trap size (scaling coefficient 0.95). To explore prey size selectivity, we conducted laboratory feeding trials with ostracods (size range 0.07 to 0.7 mm). We found that larger bladderwort traps catch larger and a wider size range of prey, consistent with findings in fish. Despite their microscopic size, bladderworts are able to overcome the hydrodynamic theory prediction and suction feed prey near the lower size limit.