Using growth measures in the freshwater shrimp Caridina nilotica as biomarkers of Roundup® pollution of South African freshwater systems

There has been global concern about the effect of toxic chemicals on aquatic biota due to the upsurge in contamination of aquatic ecosystems by these chemicals, which includes pesticides. Roundup® and other glyphosate-based herbicides are frequently used in the chemical control of weeds and invading alien plant species in South Africa. These bio-active chemicals ultimately get into water courses directly or indirectly through processes such as drifting, leaching, surface runoff and foliar spray of aquatic nuisance plants. However, there is no South African water quality guideline to protect indigenous freshwater non-target organisms from the toxic effects of glyphosate-based herbicides. This study evaluated the possible use of growth measures in Caridina nilotica as biomarkers of Roundup® pollution as part of developing glyphosate water quality guideline for the protection of aquatic life in South Africa. Using static-renewal methods in a 25-day growth toxicity test, 40 days post hatch shrimps were exposed to different sub-lethal Roundup® concentrations of 0.0 (control), 2.2, 2.8, 3.4, 4.3 and 5.4 mg/L. Shrimps were fed daily with TetraMin® flake food and test solutions changed every third day. Shrimp total lengths and wet weights were measured every fifth day. These data were used to determine the shrimp’s growth performance and feed utilization in terms of percent weight gain (PWG), percent length gain (PLG), specific growth rate (SGR), condition factor (CF), feed intake (FI), feed conversion ratio (FCR) and feed conversion efficiency (FCE). Moulting was observed for 14 days and the data used to determine the daily moult rate for each concentration. Results of growth performance and food utilization indices showed that growth was significantly impaired in all exposed groups compared to control (p < 0.05). Moulting frequency was also higher in all exposed groups than in control (p < 0.05). Although all the tested growth measures proved to be possible biomarkers of Roundup® pollution, moulting frequency gives a clearer indication of the sub-lethal effects of Roundup® toxicity.