The effects of car tyre additives (6PPD-quinone, DPG, and MBT) on activity behaviours of marine and freshwater amphipods

Tyre-derived pollutants (TDPs), such as 6PPD-quinone, N,N′-diphenylguanidine (DPG), and 2-mercaptobenzothiazole (MBT), are entering aquatic ecosystems and their effects on invertebrate behaviour are largely unknown. This study examined the effects of three TDPs at four nominal concentrations (0.01, 0.1, 1 & 100 µg/L) and standard/solvent controls on the activity of freshwater (Gammarus pulex) and marine (Marinogammarus marinus) amphipods over a 14-day exposure, under alternating light and dark phases. Significant species-specific and phase-dependent effects were observed. M. marinus exhibited reduced activity at environmentally relevant concentrations, by approximately 75% at MBT 0.01 µg/L by day 14, and DPG 0.01 µg/L causing a ∼64% reduction by day 7 compared to controls. In contrast, G. pulex displayed variable responses, including hyperactivity at high MBT concentrations (MBT 100 µg/L, day 4: ∼42% increase), followed by cumulative inhibition over time (day 14, MBT 0.01 µg/L: ∼47% reduction). For 6PPD-quinone, G. pulex showed limited and phase dependent behavioural effects, whereas activity in M. marinus was significantly reduced during the second light phase at 1 µg/L. These findings highlight the importance of varying light/dark conditions and species-specific assessments in these types of experiments. While M. marinus showed heightened sensitivity to low concentrations and prolonged exposures, G. pulex exhibited greater reactivity to high concentrations, highlighting distinct behavioural and environmental contexts (e.g., freshwater vs. seawater). This study underscores the need for species-specific and phase-inclusive ecotoxicological assessments, advocating for regulatory frameworks to incorporate behavioural endpoints under varying environmental conditions.