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Feeding selectivity drives fine microplastic uptake and persistence in sand bubbler crab Scopimera globosa on an estuarine tidal flat
Summary
Tiny crabs living in coastal mudflats actually pick out the smallest microplastic bits (10-20 micrometers) from sand when they feed, rather than just accumulating whatever plastic happens to be around them—though they do clear most of it from their bodies within about a week. Since these crabs sit near the bottom of the food chain in coastal ecosystems, this selective uptake matters for understanding how tiny plastic particles might work their way up to fish and shellfish that humans eat, even though this study didn't directly test human health effects.
Microplastics (MPs) are widespread in coastal ecosystems. However, field evidence on the uptake of fine MPs by deposit feeders remains limited. We investigated the occurrence, size composition, and biological processing of MPs in the sand bubbler crab, Scopimera globosa, on a sandy tidal flat in the Tama River estuary (Tokyo Bay, Japan). Field surveys were conducted in the spring, summer, and autumn of 2024 to quantify MPs in surface sediments, crab-produced sand pellets, and crab tissues, and laboratory experiments were conducted to test ingestion size selectivity and elimination using fluorescent polystyrene beads. Sediment MP abundance did not significantly vary seasonally, whereas MP abundance increased in both sand pellets and crab tissues in the autumn. The mean size of MPs in sand pellets (25.1-28.1 μm) was larger than that in sediments (19.5-23.2 μm) and crab tissues (17.8-23.1 μm), consistent with particle sorting during feeding. In the laboratory, crabs preferentially ingested small MPs (10-20 μm), despite an equal supply of three size classes. Following exposure to fine MPs, internal MP counts decreased over time and were described by an exponential decay model, yielding an elimination half-life of 1.39 days and estimated times to 90% and 99% elimination of 4.61 and 9.21 days, respectively. These results indicate that MP burdens in S. globosa are governed by feeding activity and size-selective ingestion, and cannot be inferred from sediment MP loads alone.