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Zebrafish larvae (Danio rerio) are among the most used model species to test biological effects of different substances in biomedical research, neuroscience and ecotoxicology. Most tests are based on changes in swimming activity of zebrafish larvae by using commercially available high-throughput screening systems. These systems record and analyse behaviour patterns using visible (VIS) and near-infrared (NIR) light sources, to simulate day (VIS) and night (NIR) phases, which allow continuous recording of the behaviour using a NIR sensitive camera. So far, however, the sensitivity of zebrafish larvae to NIR has never been tested experimentally, although being a critical piece of information for interpreting their behaviour under experimental conditions. Here, we investigated the swimming activity of 96 hpf (hours post fertilization) and 120 hpf zebrafish larvae under light sources of NIR at 860 nm and at 960 nm wavelength and under VIS light. A thermal source was simultaneously presented opposite to one of the light sources as control. We found that zebrafish larvae of both larval stages showed a clear negative phototactic response towards 860 nm NIR light and to VIS light, but not to 960 nm NIR light. Our results demonstrated that zebrafish larvae are able to perceive NIR at 860 nm, which is almost identical to the most commonly used light source in commercial screening systems (NIR at 850 nm) to create a dark environment. These tests, however, are not performed in the dark from the zebrafish´s point of view. We recommend testing sensitivity of the used test organism before assuming no interaction with the applied light source of commonly used biosensor test systems. Previous studies on biological effects of substances to zebrafish larvae should be interpreted with caution. [ABSTRACT FROM AUTHOR]

• The effects of ASTM-dispersed and wastewater-borne AgNPs (NM-300 K) and TiO 2 NPs (NM-105) were assessed in Daphnia magna. • The behavioural and biochemical responses are distinct after a 96-h exposure to wastewater-borne and ASTM-dispersed NPs. • At the behaviour level, the most pronounced effects on the allocation time are obtained with ASTM-dispersed AgNPs. • At the subcellular level, the wastewater-borne AgNPs are more toxic than wastewater-borne TiO2NPs. Due to their widespread use, silver (Ag) and titanium dioxide (TiO 2) nanoparticles (NPs) are commonly discharged into aquatic environments via wastewater treatment plants. The study was aimed to assess the effects of wastewater-borne AgNPs (NM-300 K; 15.5 ± 2.4 nm; 25−125 μg L−1) and TiO 2 NPs (NM-105; 23.1 ± 6.2 nm; 12.5−100 μg L−1), from a laboratory-scale wastewater treatment plant, on Daphnia magna , at individual and subcellular level. For effect comparison, animals were also exposed to ASTM-dispersed NPs at the same nominal concentrations. The behaviour of D. magna was evaluated through monitoring of swimming height and allocation time for preferred zones after 0 h and 96 h of exposure. Biochemical markers of neurotransmission, anaerobic metabolism, biotransformation, and oxidative stress were subsequently determined. No 96-h EC 50 (immobilization ≤ 4 %) could be obtained with wastewater-borne NPs and ASTM-dispersed TiO 2 NPs, whereas the ASTM-dispersed AgNPs resulted in an immobilization 96-h EC 50 of 113.8 μg L−1. However, both wastewater-borne and ASTM-dispersed TiO 2 NPs, at 12.5 μg L−1, caused immediate (0 h) alterations on the swimming height. Allocation time analyses showed that animals exposed to ASTM-dispersed AgNPs spent more time on the surface and bottom at 0 h, and in the middle and bottom at 96 h. This pattern was not observed with ASTM-dispersed TiO 2 NPs nor with wastewater-borne AgNPs and wastewater-borne TiO 2 NPs. At the biochemical level, the more pronounced effects were observed with wastewater-borne AgNPs (e.g. induction of lactate dehydrogenase and glutathione S -transferase activities, and inhibition of catalase activity). This integrative approach showed that: (i) the behavioural and biochemical response-patterns were distinct in D. magna exposed to environmentally relevant concentrations of wastewater-borne and ASTM-dispersed NPs; (ii) the most pronounced effects on allocation time were induced by ASTM-dispersed AgNPs; and (iii) at the subcellular level, wastewater-borne AgNPs were more toxic than wastewater-borne TiO 2 NPs. This study highlights the need for the assessment of the effects of wastewater-borne NPs under realistic exposure scenarios, since processes in wastewater treatment plants may influence their toxicity. [ABSTRACT FROM AUTHOR]