Species-specific impacts of fibrous microplastics on behavior, survival and oxidative stress in freshwater zooplankton

Fibrous microplastics (MPs) pose a growing threat to zooplankton through ingestion, entanglement, and physical interference. However, species-specific interactions with MPs remain understudied, particularly in relation to zooplankton morphology, size, and locomotory traits. This study investigates the contrasting responses of the segmented Thamnocephalus platyurus and the smooth-bodied Brachionus calyciflorus. Both species were exposed to fibrous MPs at concentrations of 1, 10, 100 and 1000 items mL⁻¹. The endpoints assessed included physical entanglement, swimming speed, movement patterns, reactive oxygen species (ROS) levels and survival. In T. platyurus, higher concentrations of MPs resulted in physical entanglement at multiple appendages, disrupting locomotory dynamics, altering movement trajectories, and causing physical damage. Physical effects were accompanied by elevated ROS levels and a reduction in survival at higher concentrations. In contrast, B. calyciflorus did not exhibit visible entanglement or impaired locomotory dynamics, likely due to its morphology. However, ROS levels increased at the highest concentration, coinciding with reduced survival. While the segmented morphology of T. platyurus increased susceptibility to entanglement-related stress, B. calyciflorus remained vulnerable to oxidative stress, despite avoiding prolonged physical interaction. These results demonstrate that fibrous MPs induce species-specific sublethal and lethal effects depending on zooplankton morphology and behavior.