Predation by Buenoa macrotibialis (Insecta, Hemiptera) on zooplankton: effect of light on selection and consumption of prey

In this study we test the hypothesis that the notonectid Buenoamacrotibialis shows a strong periodicity in foraging on smallzooplankton because it relies on sight to detect the smalleritems of its diet. In contrast, B. macrotibialis may be ableto detect larger prey organisms by mechanoreceptors and thereforeconsume them independently of light availability. We conductedlaboratory and field experiments with light and dark treatmentsto analyze the effect of light on the efficacy of B. macrotibialis(1.2–7mm) in preying on the rotifer Brachionus calyciflorus (~0.25mm body length), the cyclopoid copepod Tropocyclops extensus(~0.5 mm) and the cladoceran Daphnia pulex (~1.8 mm). We studiedthe feeding of B. macrotibialis on Tropocyclops extensus duringthe daytime and night-time in light and dark treatments in thelaboratory, and in natural daylight and full moonlight in thefield. Our results show that B. macrotibialis depends on sightto capture small (<0.5 mm) prey, probably because these preyproduce little disturbance in the water, but that it appearsto rely almost exclusively on mechanoreception to capture largerprey (>1 mm). Buenoa macrotibialis consumed Daphnia pulex,the largest prey offered, independently of light availability,while the consumption of T. extensus and Brachionus calycifloruswas recorded only in lit treatments. Furthermore, Buenoa macrotibialispresented a strong periodicity in feeding on small prey, controlledexclusively by light. The inability of B. macrotibialis to feedon small organisms during the night was found to be due to inadequatelight for prey detection, rather than to an endogenously controlleddecrease in feeding activity during the night. Under naturallight conditions, B. macrotibialis preyed voraciously on T.extensus in daylight but not in the full-moon treatment or inthe dark treatments, regardless of the time of day, suggestingthat there is a threshold of light intensity below which smallprey cannot be detected by the backswimmer.