Relating Microplastic Contamination to Trophic Ecology of the Highly Invasive Red Swamp Crayfish and Eastern Mosquitofish

The red swamp crayfish (Procambarus clarkii) and the eastern mosquitofish (Gambusia holbrooki), native to North America, are highly invasive in temperate aquatic environments worldwide, where they often coexist in a variety of habitats. We used stable isotopes of carbon (δ13C) and nitrogen (δ15N) to examine the trophic ecology of Procambarus clarkii and Gambusia holbrooki co-existing in the same coastal wetland in Italy and explore potential relationships with microplastic (MP) ingestion. Gambusia consistently occupied a higher trophic position than Procambarus, and ingested more MPs per individual. There was strong spatial variation in the degree of niche overlap (0%–70%) between habitats. MPs were found in all individuals of alien species with mean concentrations (±SD) of 5.1 ± 3.2 (items/ind) in Procambarus and 5.8 ± 3.30 (items/ind) in Gambusia. MPs were also found in their potential prey (macroinvertebrates), with concentrations ranging from 0.35 ± 0.2 to 69.8 ± 87.8 items/ind., depending on the taxon. Similar polymer colours (mainly blue) and shape (mainly fibres) were found across all samples. Invertebrates were the primary diet items for both species, with little contribution from detritus and plants. The importance of specific invertebrate prey taxa varied between alien species, as did the percentage of MPs potentially ingested from their prey. We found little evidence of biomagnification, since MP concentration was not correlated with trophic levels, but was higher in Gambusia. The use of stable isotopes, combined with the examination of MPs, provides valuable insights into the role of feeding ecology in MP contamination. Given that Procambarus and Gambusia are also key prey for many terrestrial and aquatic predators, they may act as vectors, facilitating the transfer of MPs across ecosystem boundaries. Further research is needed to better understand the long-term ecological impacts of microplastic contamination across various trophic levels, and to assess the potential risks to higher predators.