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Is Mytilaster lineatus a suitable bioindicator of microplastic pollution? Insights from field surveys, laboratory experiments, and bioaccumulation indices in the Caspian Sea.
Summary
Scientists studying tiny mussels in the Caspian Sea found that smaller mussels absorb more microplastic per body weight, and mussels mostly pick up plastic from water rather than sediment—meaning these mussels could serve as an early warning system for plastic pollution, but the results depend heavily on mussel size and location, making them an imperfect single measuring tool. This matters because mussels and similar shellfish are commonly eaten by people, so understanding how they absorb microplastics helps researchers better estimate our own exposure through seafood and improve pollution monitoring methods.
We examined microplastic (MP) distribution in water, sediment, and Mytilaster lineatus at eight sites in the South Caspian Sea, plus a lab ingestion experiment. Bioconcentration Factor (BCF) and Individual Based Accumulation Factor (iBAF) separated environmental exposure from organism effects. Water MPs were highest at Sefidroud, Chamkhaleh, and Sisangan. MPs per individual were greatest at Sisangan and Sefidroud, where mussel biomass and density were highest - consistent with density dependent ingestion in lab. At Chamkhaleh (lower biomass), high water MPs did not yield high MPs per individual. At Amir Abad, MPs/gww peaked where biomass was lowest. A strong negative correlation (ρ = -0.78) between MPs/gww and biomass showed smaller mussels accumulate higher MP per unit weight. The correlation between water MPs and MPs per individual was not significant, modulated by size dependent accumulation. No significant differences among stations in iBAFwater (minimizing body size) suggested similar accumulation across sites. BCFwater exceeded 1 at all stations while BCFsediment remained below 1 at most stations, indicating water as primary uptake pathway. Ingested MPs were mainly 10-100 μm, confirming selective ingestion. The highest biomass at Sisangan and Sefidroud, despite peak water MPs and no mortality in lab at far above field concentrations, suggests MP concentrations are not limiting; physicochemical conditions appear more determinant. Thus, although M. lineatus is a useful sentinel for biological exposure, size dependent accumulation and selective ingestion restrict its ability as an absolute indicator - especially given decline in mussel biomass in southern Caspian Sea. This highlights the need for complementary bioindicators.