Plastic pollution poses a growing threat to marine biodiversity, and its biological effects may vary with particle size. Early life stages of the Atlantic horseshoe crab, Limulus polyphemus, inhabit coastal environments that can accumulate plastic particles across a broad size range. This study investigated the effects of small (0.2-9.9 μm) and large (53-63 μm) polyethylene particles on juvenile L. polyphemus exposed for 21 days to 5 μg L-1. Survival, somatic growth, oxygen consumption, muscle water content, locomotor performance, and biochemical biomarkers related to antioxidant defence, detoxification, and oxidative damage were assessed. Exposure to both polyethylene particle sizes significantly reduced somatic growth relative to control, with a significantly greater reduction under large particle size exposure. Survival, oxygen consumption, muscle water content, and locomotor performance did not differ significantly among treatments. Individual biochemical biomarkers, including CAT and GST, also showed no significant treatment effects, although exploratory multivariate analysis of overall biochemical profiles likewise showed no significant treatment effect. These findings indicate that particle size influenced the response of juvenile L. polyphemus, with large particles exerting the strongest effect on somatic growth under the tested conditions. However, this response was not accompanied by broader metabolic, osmoregulatory, behavioural, or biochemical impairment. Somatic growth may therefore represent a sensitive endpoint for detecting sublethal effects of polyethylene particles in juvenile Atlantic horseshoe crabs, with potential implications for their early development.
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