Sex and physicochemistry modulate the acute toxicity of PAHs in the tropical amphipod model, Parhyale hawaiensis

Sex-specific physiological differences can modulate the toxicity of contaminants; however, the relationship between these differences and the physicochemistry of toxicants remains understudied. Our objective was to investigate sex-specific differences in the acute toxicity of nine priority polycyclic aromatic hydrocarbons (PAHs) with varying physicochemical properties (logKow: 3.30–6.63) in the tropical marine amphipod Parhyale hawaiensis. Adult males and females were exposed (96 h), toxicity thresholds (LC_x and EC_x) were determined, and Toxic Units (TUs), Hazard Quotients (HQs), and Risk Quotients (RQs) were computed to assess environmental risks. The results revealed a consistent pattern of heightened sensitivity in females for all the tested PAHs. For the most toxic mid-range compounds (logKow 4.4–4.95), females were significantly (p ≤ 0.05) more sensitive than males, with LC₅₀ values of 111.3 µg/L in females and 190.3 µg/L in males for phenanthrene. This heightened sensitivity was particularly evident at lower effect thresholds, where female EC₁₀ values for the most toxic PAHs were approximately two-fold lower than those for males. Toxicity was nonlinear with respect to hydrophobicity, and mid-range PAHs (logKow 4.4–4.95) posed the highest acute toxicity risk. Consequently, the calculated Risk Quotient for phenanthrene in females (RQ = 14.44) was nearly 30 times the Level of Concern (0.5). This study provides foundational toxicity data for a key tropical species, demonstrating that risk assessments that overlook both sex-specific vulnerabilities and the distinct threats of mid-range PAHs may fail to protect critically important tropical marine ecosystems.