Development of swimming speed-based biomarker reveals interactive toxicity of TCEP and TCPP in amphibian larvae

Tris(2-chloroethyl) phosphate (TCEP) and tris(2-chloro-1-methylethyl) phosphate (TCPP) are increasingly co-detected in various aquatic environments, yet their combined toxicity remains poorly understood. In this study, we investigated the effects of chronic (20–23 days) exposure to TCEP and TCPP, both individually and in combination, at concentrations that included and exceeded environmentally relevant levels (3, 30, and 300 μg/L) on Polypedates megacephalus tadpole swimming performance. Using an information-theoretic approach, we revealed an interaction pattern: high TCEP concentrations significantly diminished TCPP's effects on swimming speed. A significant positive correlation between swimming speed and muscle acetylcholinesterase (AChE) activity established swimming speed as a reliable non-invasive indicator of neurotoxicity. We developed the swimming speed decline rate (SDR) as a novel biomarker, validated through its correlation with integrated biomarker response. The SDR assessment revealed that current environmental concentrations pose relatively low risks to tadpole swimming performance, except in wastewater treatment facilities where moderate risks were identified. Our findings established a validated non-invasive tool for neurotoxicity assessment.