This study investigates the digestion and degradation of polystyrene foam debris by the benthic clamworm *Perinereis vancaurica*. Analysis of excreted fragments reveals evidence of polystyrene digestion, leading to microplastic formation. 16S rRNA gene sequencing identified a diverse gut microbiome dominated by *Acinetobacter* and *Ruegeria*. Gut isolates of *Acinetobacter johnsonii*, *Brevibacterium casei*, and *Ruegeria arenilitoris* demonstrated polystyrene degradation in vitro, exhibiting slight weight loss, chemical group changes, and the production of metabolic intermediates over 30 days. The findings suggest that polystyrene-degrading bacteria in the clamworm gut influence plastic fragmentation and degradation in marine ecosystems.
