Globally, the widespread application of neonicotinoids insecticides in agriculture has led to their increased detection in both surface and ground water. The potential long-term effects on non-target aquatic organism are not well understood. Because of the limited number of studies investigating the effects of neonicotinoids on non-target organisms at acute or sub-chronic concentrations, we investigated the effects of growth and metamorphosis on an aquatic model organism, Xenopus laevis: the African clawed frog.
Xenopus laevis embryos were grown in neonicotinoid (thiamethoxam (THX) and clothianidin (CLO)) or control (pesticide -free) aquatic media in four replicate vivaria per treatment. This established a developmental study for X. laevis at sub-chronic levels of the insecticide, (20 ppm and 100 ppm). Measurements were taken in two phases: 44 days of exposure, during which length was measured and mortality recorded (n=1121). On day 44, total length, wet weight and developmental stage measurements were made on the randomly assorted (~400) tadpoles to determine growth. Eighty tadpoles were selected to observe progress further through metamorphosis, by taking daily developmental stage measurements. The 44 day time period spans the growth of the tadpoles from embryo until pre-metamorphic development, during which time the juveniles are very susceptible to contaminants. The species selected is an entirely aquatic filter feeder that uses gills for respiration, and has solute-permeable skin.
Both insecticide treatments resulted in statistically significant retarded development and growth when compared to controls. Tadpoles were on average 0.16g ± 0.05 less in wet weight, 10mm ± 1.36 shorter in length, and 2.15 ± 0.32 Nieuwkoop-Faber units less in stage development (all P values <0.03). Mortality was statistically highest in the THX 100 ppm treatment: 22% ± 4 versus control 9% ± 1 (P=0.0053). Development rate was slowest in the CLO 20 ppm treatment (33% less well developed as compared with control P<0.001). Broadly, this effect could disrupt tadpole ability to compete for resources and to avoid predators in the natural aquatic environment. Further investigations are needed to explore possible cell and molecular mechanisms underlying these affects.