Dietary Microplastics are Eliminated Through Feces, Causing Metabolic Impairment in the Marine Amphipod Parhyale hawaiensis

The toxicity of microplastics (MPs) can vary depending on the route of exposure, yet the effects of dietary exposure remain poorly understood. In this study, we evaluated the effects of chronic dietary exposure to MPs on the marine amphipod Parhyale hawaiensis (Dana, 1853) over 35 days. Amphipods were exposed to 53-63 µm polyethylene microspheres (18 ± 4 MPs per food unit) through their diet. We assessed survival, reproduction, molting frequency, condition factor, oxygen consumption, MP accumulation, and fecal elimination. P. hawaiensis readily ingested and excreted MPs via feces. Exposure to MPs significantly increased oxygen consumption in exposed individuals (0.21 ± 0.053 mg O₂/g/h) compared to controls (0.12 ± 0.07 mg O₂/g/h), suggesting elevated metabolic costs potentially associated with MP processing and elimination. Despite this physiological stress, no significant effects on reproduction or growth were observed. These results indicate that, although ingested MPs can be eliminated through feces, their presence may still impose sublethal metabolic stress on marine detritivores.