Ontogenetic Variation in the Sensitivity of the Gladiator Frog, Hypsiboas faber, to Inorganic Nitrogen

In a biosphere transformed by human activities, understanding the relationship between age, developmental stage, or body mass and toxicological sensitivity is a relevant endeavor because the environmental release of some quantitatively important contaminants follows predictable seasonal patterns, as do wildlife demographic events. We tested the hypothesis that the sensitivity of the gladiator frog, Hypsiboas faber, to components and/or dissolution products of fertilizers varies predictably through ontogeny by sequentially exposing eggs, embryos, and larvae to ammonium nitrate, sodium nitrate, sodium nitrite, and ammonium sulfate. Nitrate was the least toxic nitrogen ion regardless of stage; the most deadly compound was ammonium or nitrite, depending on stage. Overall, embryos were more sensitive to inorganic nitrogen than more advanced developmental stages, indicating that the jelly capsule envelopes that surround the egg do not provide effective protection against contaminants. However, for most compounds tested, the relationship between lethal concentrations (LCx), no-observed effect concentrations (NOEC), and lowest observed effect concentrations (LOEC) and developmental stage or mass was not monotonic. Whereas for sodium nitrate sensitivity declined asymptotically with developmental stage, for ammonium nitrate, sodium nitrite, and ammonium sulfate, sensitivity declined through early development and then increased again at Gosner stage 25. Together with previously published literature, this finding indicates no single and predictable trend in sensitivity through development, even if we restrict our analysis to a relatively narrow range of organisms and contaminants. Such a relationship creates complex scenarios of risk as a function of the phenology of application of fertilizers and of the temporal variation in environmental factors that modulate nitrogen speciation and toxicity in and near agricultural fields.