Assessment of trace element contamination derived from coastal plastic debris using gastropods, bivalves, and decapod crustaceans on Zamami-jima Island, the Ryukyu Islands: Spatial, temporal, and species-specific variation

Plastic debris in coastal environments can serve not only as a physical pollutant but also as a carrier of trace elements. In this study, we analyzed 34 elements in four coastal invertebrate species—Nerita polita (Mollusca: Gastropoda), Cellana testudinaria (Mollusca: Gastropoda), Atactodea striata (Mollusca: Bivalvia), and Coenobita purpureus (Crustaeca: Decapoda: Anomura)—collected from two stations (Nita Beach and Furuzamami Beach) on Zamami-jima Island in the Ryukyu Islands. We assessed the uptake and accumulation of plastic-derived trace elements and interspecific differences. Trace element concentrations in muscle and internal organ tissues were measured using inductively coupled plasma mass spectrometry, and comparisons were made between stations, years, and species. The results revealed that concentrations of plastic additive-derived elements, such as As, Cd, Pb, and Cr, were higher at Nita Beach, where more plastic debris was observed, indicating plastic-derived contamination. In particular, As was detected at high concentrations across multiple tissues and species, suggesting ongoing exposure. A temporal comparison using C. purpureus between 2018 and 2023 revealed a decreasing trend for highly toxic elements such as Cd and Pb, whereas As showed a slight increase, raising concerns about persistent contamination. Interspecific comparisons showed that trace element concentrations were higher in gastropods than in bivalves, likely owing to differences in digestive physiology, such as the absence of microplastic egestion mechanisms like pseudofeces. These results indicate that monitoring of plastic-derived trace element contamination requires sampling across multiple species, years, and seasons. Furthermore, the phenomenon of minimal differences between stations suggests that even sites with seemingly little debris may experience unexpected exposures, possibly because of factors such as temporary heavy-pollution events followed by cleanups or accumulation of debris in coastal forests.