Microplastic (MP) contamination represents a global threat to aquatic ecosystems, yet
its biological effects remain poorly understood. This study investigates the short-term
impacts of polyethylene (PE) microparticles on two amphipod species: the semi-terrestrial
Cryptorchestia garbinii and the aquatic Echinogammarus veneris. Amphipods are exposed to
MPs both in water and through dietary intake. After 24 h, C. garbinii ingested an average of
9.6 ± 1.2 particles per individual, while E. veneris ingested 12.5 ± 2.8 particles, confirming
an active uptake of microplastics. The mean particle size decreased from ≌50 μm in the
food tablets to 18–25 μm in the digestive tract, suggesting fragmentation during digestion
and highlighting the ecological role of amphipods in generating smaller, potentially more
bioavailable particles. Both species exhibited a marked increase in DNA damage, together
with variations in energy-reserve allocation (glucose, glycogen, and lipids) consistent with
acute metabolic stress. To our knowledge, this represents the first evidence of genotoxicity
ever reported in C. garbinii, expanding current understanding of the biological responses of
amphipods to plastic pollution. These findings highlight the vulnerability of detritivore
species to MPs exposure and, given their role in nutrient cycling, emphasize the need for
further research on the ecological implications of MPs contamination.