Polystyrene microplastics impede growth and induce behavioural toxicity in endangered juvenile horseshoe crabs (Tachypleus tridentatus)

Polystyrene microplastics are ubiquitous, ecologically concerning pollutants in aquatic ecosystems and have become increasingly prevalent in recent years, and their persistent exposure has been demonstrated to adversely affect aquatic organisms through growth retardation, metabolic dysregulation, oxidative stress potentiation, and behavioural abnormalities. The horseshoe crabs, an Endangered species in the International Union for Conservation of Nature's (IUCN) Red List, are already experiencing precipitous population declines, primarily attributed to coastal habitat degradation, exposure to microplastics, and unsustainable harvesting practices, including biomedical blood extraction. To investigate the effects of polystyrene microplastics exposure on the growth and behavioural responses in 2-year-old horseshoe crabs, individuals were exposed to four different polystyrene microplastics concentrations (0 μg/L, 2 μg/L, 20 μg/L, 200 μg/L) for 14 days, followed by a 7-day recovery phase to assess potential post-exposure recuperation. The results revealed that exposure to polystyrene microplastics at concentrations of 20 μg/L and 200 μg/L induced significant growth retardation and aberrant behavioural responses, including reduced locomotion activity, diminished feeding frequency, and disruption of sensory-mediated behaviour, in juvenile horseshoe crabs. In addition, horseshoe crabs exhibited abnormal hyperactivity in behavioural activities even after exposure to polystyrene microplastics at 2 μg/L, demonstrating non-negligible toxic effects of polystyrene microplastics. This study provides foundational and novel data for understanding the impacts of polystyrene microplastics on the behavioural responses of organisms. The integrated effects of polystyrene microplastics exposure on the growth and behavioural patterns of horseshoe crabs necessitate further systematic investigation.