Environmentally-relevant concentrations of atrazine induce non-monotonic acceleration of developmental rate and increased size at metamorphosis in Rhinella arenarum tadpoles

Despite of the various studies reporting on the subject, anticipating the impacts of the widely-used herbicide atrazine on anuran tadpoles metamorphosis remains complex as increases or decreases of larval period duration are almost as frequently reported as an absence of effect. The aim of the present study was to examine the effects of environmentally-relevant concentrations of atrazine (0.1, 1, 10, 100, and 1000 μg/L) on the timings of metamorphosis and body size at metamorphosis in the common South American toad, Rhinella arenarum (Anura: bufonidae). None of the atrazine concentrations tested significantly altered survival. Low atrazine concentrations in the range of 1–100 μg/L were found to accelerate developmental rate in a non-monotonic U-shaped concentration–response relationship. This observed acceleration of the metamorphic process occurred entirely between stages 25 and 39; treated tadpoles proceeding through metamorphosis as control animals beyond this point. Together with proceeding through metamorphosis at a faster rate, tadpoles exposed to atrazine concentrations in the range of 1–100 μg/L furthermore transformed into significantly larger metamorphs than controls, the concentration–response curve taking the form of an inverted U in this case. The no observed effect concentration (NOEC) was 0.1 μg atrazine/L for both size at metamorphosis and timings of metamorphosis. Tadpoles exposed to 100 μg/L 17β-estradiol presented the exact same alterations of developmental rate and body size as those treated with 1, 10 and 100 μg/L of atrazine. Elements of the experimental design that facilitated the detection of alterations of metamorphosis at low concentrations of atrazine are discussed, together with the ecological significance of those findings.

► Atrazine accelerates tadpole development in a U-shaped non-monotonous manner. ► Atrazine cause tadpoles to transform into significantly longer and heavier metamorphs. ► 17β-estradiol and atrazine cause similar alterations of developmental rate and body size.