Mussel and crab as bioindicators for assessing microplastic contamination in a tropical freshwater urban river

Microplastic (MP) pollution threatens freshwater ecosystems; its effects in tropical urban rivers remain unclear. This study examined MP contamination in the Surma River, Bangladesh, using histological biomarkers and contamination levels in two invertebrate species with different feeding strategies to assess ecological harm. Sampling was conducted during the pre-monsoon at three previously classified sites with low, moderate, and high MP loads. Two sentinel species, the freshwater mussel Lamellidens marginalis and the crab Sartoriana spinigera were collected to quantify MP ingestion and assess tissue-level toxic effects. MPs were extracted via alkaline-oxidative digestion, identified microscopically, and confirmed by ATR-FTIR spectroscopy. Ecological risk was evaluated using the Pollution Load Index (PLI) and the Polymer Hazard Index (PHI), along with histopathological analysis of gill, kidney, and hepatopancreatic tissues. MPs were found in every organism examined and were consistently higher in S. spinigera (2.71 – 5.86 items/g) than in L. marginalis (1.86 – 2.89 items/g), peaking at the most urbanized site. Fiber and fragment morphotypes constitute the dominant particle classes, with a majority of particles exhibiting size below 200 µm. The main polymers identified were PP and PE. All sampling sites showed PLI values above 1, indicating ecological decline, with PVC yielding high hazard (PHI) level. Histological analysis revealed mild to severe damage in vital organs of respective species, including gill tissue rupture, renal granule anomalies, and hepatopancreatic cell lysis. These findings demonstrate that integrating histomorphological biomarkers with ecotoxicological profiles can detect impacts, establishing these invertebrates as bioindicators for freshwater MP monitoring.