Behavioural and biochemical responses of the freshwater snail Lanistes carinatus to toluene, ethylbenzene, and xylene.

The freshwater apple snail Lanistes carinatus (Olivier, 1804) is an established sentinel species for Egyptian freshwater biomonitoring. Toluene, ethylbenzene, and xylene (TEX) are pervasive freshwater contaminants of petroleum origin, yet their sub-lethal effects on freshwater gastropods remain poorly characterised. This study integrates SLEAP-based automated locomotor phenotyping of 12 individual animals per treatment group with three enzymatic biomarkers, glutathione S-transferase (GST), acetylcholinesterase (AChE), and superoxide dismutase (SOD), across fifteen treatment groups under semi-static conditions (22 ± 2 °C, pH 7.8 ± 0.3). Each compound was tested at 10.8, 19.4, 35.0, and 63.0 mg/L alongside a solvent-free negative control (C-), a solvent control (SC), and a field-collected water sample (P). Nineteen of the twenty-two reported behavioural endpoints differed significantly among groups (Kruskal-Wallis, p < 0.05). Toluene at 63 mg/L produced the most severe locomotor suppression, reducing percentage time mobile from 75.3% to 37.7%, a compound-specific effect, since it differed significantly from both the solvent-free and the solvent control (Dunn's post-hoc, p < 0.005) while the vehicle itself did not affect this endpoint. Average speed and trajectory area were also lowest under toluene (82.0% and 98.4% below the solvent-free control), but for these two endpoints the vehicle contributed substantially, and a toluene-specific increment was not statistically resolvable. All three enzymatic biomarkers showed highly significant treatment effects (ANOVA, p < 0.0001; eta-squared 0.92-0.95): GST was induced up to +62.4%, AChE reduced up to -56.6%, and SOD depleted up to -42.5%, all maximal under xylene. The dissociation between behavioural and biochemical potency underscores the complementary information provided by these endpoint classes.