Selenium nanoparticles (SeNPs) are increasingly used in aquaculture and related sectors, raising concerns about their release and potential impact on aquatic organisms, especially during early life stages. This study assessed the acute toxicity of chemically synthesized surfactant-stabilized, non-functionalized SeNPs with a hydrodynamic diameter of 52.7 ± 0.2 nm (DLS) and a primary size of 67 ± 11 nm (SEM) in three freshwater species representing different trophic levels and standardized ecotoxicological test systems: Daphnia magna neonates (48 h), Danio rerio embryos and larvae (96 and 120 h), and Xenopus laevis tadpoles (96 h). Survival and developmental endpoints were quantified, and a log-logistic benchmark dose (BMD) approach was applied to quantal concentration-response data, using the lower 95% confidence limit of the BMD at 10% extra risk (BMDL₁₀) as the point of departure. BMDL₁₀ values varied by more than three orders of magnitude, from 9.3 mg L^-1 total Se for D. magna to 0.007 mg L^-1 total Se for X. laevis, with D. rerio showing intermediate effect thresholds and toxicity increasing with exposure duration. X. laevis tadpoles showed the lowest effect thresholds, with BMDL₁₀ values approximately 1329-fold lower than those of D. magna. The EC₅₀ or LC₅₀ to BMDL₁₀ ratios (1.8-3.6) demonstrated that median effect metrics may underestimate hazard at low-effect levels relevant for environmental protection. Using the most sensitive BMDL₁₀ and applying a conservative assessment factor of 1000, a provisional freshwater predicted no-effect concentration of 7 ng L^-1 total Se was derived for screening-level risk assessment. Overall, these findings support the application of BMD-based approaches in nanomaterial ecotoxicology and identify amphibian larval test systems and growth-related endpoints as sensitive components for environmental hazard screening of SeNPs. However, the observed cross-test differences in effect thresholds should be interpreted cautiously because the assays were conducted under species-specific standardized exposure conditions that differed in developmental stage, exposure duration, feeding regime, and exposure pathways. Consequently, the present study provides a comparison of standardized acute toxicity test systems rather than a strict stage-equivalent assessment of intrinsic species sensitivity.