The role of phytoplankton in the diet of the bladderwort Utricularia australis R.Br. (Lentibulariaceae)

It is becoming more apparent that the aquatic carnivorous plants Utricularia (Lentibulariaceae) can capture and utilise a wide range of small aquatic organisms. Although they can take up nutrients directly from the water by rootless shoots, much of their overall nutrition comes from prey caught in their traps. Most of the early literature was focused mainly on animal prey, hence the term carnivorous to describe Utricularia spp., but more recently the contribution of other microorganisms and detritus to their diet is being realised. Specifically, the importance of the phytoplankton in traps as prey is proving interesting. However, most investigations have focused on the trap communities with few comparisons to phytoplankton as potential prey, which could reveal the occurrence of feeding selectivity. The eurytopic species Utricularia australis was surveyed across 12 sites in central Italy in lentic waterbodies ranging from oligotrophic to eutrophic. Phytoplankton communities outside the traps were composed of 135 taxa from 21 orders belonging to eight phyla and the algal community in the traps comprised 124 taxa from 17 orders belonging to eight phyla, recorded from all sampling sites. Among the 12 sites phytoplankton communities were highly diverse; however, communities inside and outside the traps at each site were very similar. This similarity indicated that feeding was not selective. Algal movements are not able to elicit a trap response, yet around 70% of the traps contained phytoplankton only (no animal prey). Therefore, regular spontaneous trap firing, known to occur in U. australis, is thought to be responsible for the similarities in trap and plankton algal communities. At two sites, a Euglenophyte (Lepocinclis sp.) and a Cryptophyte (Chilomonas sp.) proliferated, which were otherwise very rare in the phytoplankton community. This is indicative of favourable growth conditions in the traps and possibly of commensal relationships. It seems that the diet requirements of U. australis are met by having a highly varied prey spectrum.