Microplastic accumulation in local dominant shellfish from the Khwae Noi Basin in Western Thailand and its environmental factors

Microplastics (MP) have accumulated in environmental components including aquatic organisms. In this study, we investigated MP accumulation in the local dominant shellfish, i.e. 150 individual assassin snails (Anentome helena) and 60 individual clams (Pilsbryoconcha exilis compressa), sampled from 39 sites in headstream, midstream and downstream areas of the Khwae Noi Basin in western Thailand. MP accumulation was found to be only the filamentous type in approximately 26.0% and 38.3% of the occurrences in assassin snails and clams with averages of 0.5±1.0 and 2.5±3.3 pieces/individual, respectively. MP colors found in assassin snails were blue and black with 54.8% and 45.1% occurrences, and those found in clam were blue and green with 64.5% and 35.4% occurrences, respectively. Principal Correspondence Analysis (PCA) demonstrated that MP accumulation was associated with 9 environmental factors; for example, downstream, land-use area and population size were positively related to MP accumulation, whereas headstream, elevation and pristine area were negatively related to accumulation. Besides, Sorensen Distance of 65.0% similar index was classified into three groups of MP accumulation in the shellfish. Fourier-transform infrared analysis illustrated four types of MP accumulation in the shellfish, comprising polydimethylsiloxane, polyamide, polyester, and polyvinyl chloride. All MP types found in the shellfish were related with those found in water and sediment.