Assessing the ecotoxicity of ofloxacin: Effects on swimming behaviour and morphophysiological endpoints in Daphnia magna

Background: Ofloxacin (OFL) is a widely used fluoroquinolone antibiotic that is frequently detected in aquatic environments due to its persistence and limited removal in wastewater treatment plants [1]. Its environmental presence raises concerns about potential adverse effects on non-target organisms. Objective: This study aimed to evaluate the ecological risk of OFL in Daphnia magna as a freshwater invertebrate model, by examining swimming behaviour and morphophysiological endpoints. Methods: D. magna neonates (<24 h) were exposed to an environmentally relevant (1 µg·L⁻¹) and a 100-fold higher (100 µg·L⁻¹) concentration of OFL for 9 days, with 5 replicates per treatment. After exposure, swimming behaviour (swimming speed, swimming activity, and total distance) was determined by analysing 1-min video recordings. Additionally, morphophysiological parameters (body size, heart size, and area) were also evaluated. Results: Results showed a significant reduction in both swimming speed and activity at the highest concentration (100 µg L⁻¹), indicating an impairment of locomotor activity, a behavioural alteration previously documented in Daphnia exposed to OFL and other fluoroquinolones [2, 3]. However, no significant changes were observed in total distance travelled. Furthermore, a significant decrease in body size was registered at both tested concentrations, aligning with morphological impairments reported for similar antibiotics [2, 3]. Conclusions: These findings revealed that OFL can impair swimming activity and highlighted that even environmentally relevant concentrations may interfere with the normal development in D. magna. Further studies are currently ongoing to elucidate the mechanisms underlying these sub-lethal effects and their ecological implications.