Impacts of methylmercury exposure on host–parasite size relationships between Bluegill Sunfish (Lepomis macrochirus) and Posthodiplostomum minimum

Methylmercury (MeHg) is a globally pervasive and persistent contaminant that threatens freshwater ecosystems. Because MeHg biomagnifies through food webs and accumulates in animal tissues, it can impair organismal health, physiology, reproduction, and survival. Although MeHg toxicity has been widely documented in fishes and wildlife, most research emphasizes direct host effects, leaving its influence on host–parasite interactions poorly understood. This gap is important because parasites represent a substantial component of aquatic biodiversity and influence food webs and energy flow. Harrison’s Rule predicts a positive relationship between host and parasite body size, yet this pattern may shift if parasites experience contaminant exposure differently than their hosts. Uneven MeHg distribution among host organs may create tissue-specific contaminant microhabitats that affect parasite development and size. This study tests whether MeHg exposure could challenge Harrison’s Rule by influencing parasite size and abundance in tissues with differing contaminant burdens. Posthodiplostomum minimum metacercariae were extracted and quantified from the hearts and livers of Bluegill Sunfish (Lepomis macrochirus; n = 25) collected from Lake Strom Thurmond, GA. Parasite abundance by organ was recorded, and a standardized subset (up to five metacercariae per organ per fish) was measured microscopically using ImageJ. Host morphometrics were collected to evaluate host–parasite size relationships, and tissues will soon undergo MeHg analysis to determine organ-specific exposure. We propose that contaminant heterogeneity within hosts may decouple host–parasite size relationships. Integrating ecotoxicology and parasitology improves predictions of ecological responses to contamination and highlights the importance of including parasites in contaminant risk assessments.