Thiara granifera and Melanoides tuberculata are invasive parthenogenetic snails that affect the food webs in tropical freshwater systems. Both species dominate the community of herbivores during episodes of high nutrient availability and subsequent high standing crop of benthos. Thus, they are widely considered bio-indicators of water pollution and eutrophication. Both species should impact the trophic status through their differential grazing and control over the abundance and bio-diversity of primary producers. However, previous studies show gaps in knowledge about the dominance of T. granifera over M. tuberculata and how the nutrient releases and food habits will affect the primary productivity on benthos, affecting the food webs and local macro-fauna on freshwater systems. We wanted to determine the effects of grazing and nutrient excretion (via TKN, TP and TNO3) by these snails on periphyton biomass (via chlorophyll-a) and diversity (taxa analysis). Four treatments (1: control without snails, 2: with T. granifera, 3: with M. tuberculata, and 4: both species interacting) were set in artificial conditions. Each treatment was set in triplicate, and each replicate (except control) consisted of 14 adult snails, as an initial population density. Nutrient release and grazing were evaluated at 5 different periods, during one month, after the introduction of the snails in each system. Other parameters such as oxygen, temperature, pH, and turbidity were measured in-situ.
T. granifera reduced the standing crop of periphyton (lower chl-a), either alone or in interaction with M. tuberculata, promoting the dominance of toxic and indigestible cyanobacteria (Anabaena, Chroococcus and species from family Synechococcaceae), which in natural systems is a major problem for public health, welfare, recreation, wildlife and aquatic life. Interestingly, M. tuberculata alone promoted the largest chl-a values, suggesting that it exerts a different grazing pressure on algae, as compared to T. granifera.