Spatial and temporal distribution of gerrid (Heteroptera) and predation on microcrustaceans from a tropical shallow lake.

Predator-prey interactions involving an aquatic insect and zooplanktonic prey of different sizes were investigated to quantify prey mortality exposed to predators. Laboratory experiments were undertaken with the young and adult gerrid Rheumatobates crassifemur to test predation and size selectivity on the cladocerans Daphnia gessneri, Ceriodaphnia richardi, and Bosmina tubicen. Population fluctuations and spatial distribution of the gerrid were also evaluated in a small and shallow Brazilian lake throughout 12 months in fortnightly samples. The insects were more abundant in the littoral (mean density 7.0 ± 1.2 ind.m-2) compared to the limnetic zone. The period with the highest densities was late January to June, in both zones. Predation by young instars on Daphnia and Ceriodaphnia was significant (mean ingestion rate of 1.3 ± 0.1 D. gessneri and 0.7 ± 0.1 C. richardi per predator per hour). Adult insect fed only the large-sized prey (mean ingestion rate of 1.0 ± 0.1 D. gessneri per predator per hour). Young gerrids have greater potential to prey on cladocerans than adults, and size selectivity occurred for both predators. Preference of adults by the larger prey is probably related to difficulties in manipulating smaller planktonic prey, such as Ceriodaphnia. Due to higher densities of insects in the littoral, higher predation on zooplankton in this zone is expected. This study does contribute to a better understanding of trophic interactions in tropical shallow lakes and is the first to investigate predation of a gerrid on cladocerans in laboratory experiments.