Lanternfish as bioindicator of microplastics in the deep sea: A spatiotemporal analysis using museum specimens

Microplastics (MPs: <5000 µm) represent the largest share of plastic waste on the ocean surface, with an estimated abundance of 5 to 51 trillion particles. Although the water column below 200 meters represents over 90% of the Earth's biosphere. This lack of research leaves substantial gaps in our understanding of the fate of MPs in the deep ocean and raises significant concerns about the sublethal effects of hazardous chemical compounds released by these particles. Here we investigated MP ingestion in lanternfishes (Myctophidae), one of the most abundant vertebrates in the world. Several quality assurance and quality control procedures were implemented for MP extraction (detection limit of 20 µm) in a robust sample design, encompassing 14 Myctophidae species. Using archived samples from museum collections (from 1999 to 2017), we present the earliest records of MP ingestion for this group in the Atlantic, considering biological and environmental aspects, enabling temporal comparisons across multiple sampling campaigns in the Southwestern Atlantic. Microplastics were detected in 55% of the 1167 specimens analysed, with an average of 0.95 ±1.22 MP individual^-1. Despite the relatively short chronological gap between the sampling years and the present, global plastic production has increased by about 53% during this period. Interestingly, almost half of the lanternfishes analysed contained at least one particle in the gastrointestinal tract in the earliest data, whereas the incidence increased to two-thirds in the latest data available (2017). Although the shape and colour compositions of MPs followed a similar proportion. General Linear Models indicated that specimens collected in 1999, 2000, and 2010 had a 44%, 23% and 20% lower probability of MP ingestion, respectively, than those collected in 2017. Notably, the migratory patterns and the depth at which fish were captured emerged as more robust predictors of MP ingestion than temporal variation, underscoring the importance of vertical distribution in assessing MP exposure in deep-pelagic species. Likewise, studies have consistently shown stable or minor increases in MP contamination in marine biota. Further analysis of specimens collected in 1999-2000 revealed a pronounced depth-related pattern in the number and size of MPs detected. Specimens captured in the bathypelagic zone (> 1000 m depth) had the lowest number of particles, while those from the upper mesopelagic zone had an eightfold higher probability of MP ingestion. Feeding activity also played a significant role, as evidenced by the slight positive correlation between the gastrointestinal tract weight of fish and the number of detected MPs. Specifically, each additional gram in gastrointestinal tract weight was associated with a 19% increase in the likelihood of MP ingestion. The total polymer concentration detected by Py-GM-MS/MS was at ppm levels, this concentration was one order of magnitude higher than the amount detected in M. edulis from the North Sea in the same period. Lanternfishes were generally more likely to ingest high-density polymers, although polyethylene had the highest concentration (445.5 ± 526.4 µg g^-1 gastrointestinal tract). Finally, lanternfishes are valuable bioindicators of deep-sea environments, and museum collections can serve as essential tools for monitoring plastic pollution.