Bladderwort (Utricularia australis) is an aquatic carnivorous plant that uses underwater suction traps to capture zooplankton prey. These traps can fire multiple times and reset by osmotically expelling water. This resetting mechanism has been hypothesized to not only be energetically expensive, but to cause the formation of radical oxygen species, which in turn might lead to DNA damage, and therefor might contribute to the small genome and the rapid genome turnover of Utricularia. Such high costs of an active trapping mechanism should be balanced by benefits, such as high prey capture. To assess costs and benefits, we developed a new method to determine the total volume of water turned over by individual traps during the firing and resetting process. Plants were put in water with a UV fluorescent dye and were fed microcrustaceans. The traps took up dyed water when they fired, which accumulated in the trap lumen during the osmotic resetting process. We used UV imaging to estimate the amount of water volume consumed by each trap as a proxy for the energy expenditure and firing rate of each trap.
Results/Conclusions
We predicted that larger traps turn over more water because they have a larger volume (higher cost) and therefore they should be more successful at capturing prey (higher benefits). We expected that senescing traps are more likely to misfire (traps pump more water). We found that the pumped water volume and prey capture increased with trap size. ‘Pumped volume’ ranged from 0.3 to 3.9 mm3. Trap size distribution was strongly skewed toward smaller traps, yet small traps contributed little to total amount of water pumped and prey captured. We conclude that larger traps incur higher costs and benefits. Successful fires (leading to prey capture), misfires, and spontaneous fires lead to the same amount of water intake. We conclude from this that traps are energetically expensive due to firing and resetting even in the absence of prey, leading us to hypothesize that bladderworts should modulate trap number not only in response to dissolved nutrient availability but also prey availability. Contrary to expectations, we found that trap age had no strong effect on the amount of water pumped. We will use this new method in the future to explore the relationship between firing frequency and metabolic activity.