Ingestion and retention of biodegradable vs. non-biodegradable microplastics in a tropical coral reef fish: The role of chemical and physical characteristics

Microplastic (MP) pollution poses growing risks to marine ecosystems, yet the influence of physical and chemical characteristics of MP on ingestion and retention in tropical reef fish remains understudied. This study assessed how polymer type, shape, size, and colour influence MP ingestion preferences and gastrointestinal transit time (GTT) in juvenile damselfish Acanthochromis polyacanthus. Fish were exposed to non-biodegradable polystyrene (PS) and biodegradable polycaprolactone (PCL; considered an environmentally safer plastic alternative) of varying shapes (films and fragments), sizes (100-400 μm diameter), and colours (transparent and blue). Ingestion was assessed after a 2 h exposure, while GTT and depuration rate were monitored for 72 h. Fish showed significant ingestion preference for larger, transparent fragments over films, irrespective of polymer type. Complete depuration of all MPs occurred within 48-72 h with fragments depurated slower than films and size strongly influencing depuration. Depuration of PCL showed a non-linear size-dependent pattern, with intermediate-sized fragments clearing faster than both smaller and larger items, highlighting complex gastrointestinal transit dynamics. Importantly, biodegradable PCL persisted longer than PS MPs. These findings provide novel evidence that polymer-specific physical traits, rather than biodegradability alone, modulate ingestion and retention dynamics in reef fish. Furthermore, it highlights the need to consider ingestion preferences and retention in addition to degradability when evaluating and developing "safer plastic alternatives".