UTTAR PRADESH JOURNAL OF ZOOLOGY

Aquatic ecosystems are increasingly threatened by anthropogenic organic contaminants arising from industrialisation, urbanisation, and inadequate waste management. Polycyclic aromatic hydrocarbons (PAHs) are of particular concern because of their persistence, bioaccumulation potential, and toxicity. Estuarine environments receive benzo[a]pyrene (BaP) from industrial effluents, urban runoff, oil spills, and atmospheric deposition, creating risks for resident organisms. The mud crab Scylla serrata is a useful sentinel species because of its broad estuarine distribution, semipelagic behaviour, sensitivity to contaminants, ease of laboratory acclimatisation, and commercial importance. This study evaluated the effects of sublethal BaP exposure on selected haematological and immunological biomarkers in S. serrata. Crabs were exposed to 10 or 20 μg L⁻¹ BaP for 40 days, followed by a 20-day depuration period. Total haemocyte count (THC), differential haemocyte count (DHC), phagocytic activity, and haemolymph antibacterial activity were assessed. THC and DHC were altered after BaP exposure, while phagocytic activity declined significantly relative to the control, with the greater reduction occurring at 20 μg L⁻¹. The optical-density-based antibacterial assay also changed across concentrations and exposure periods, although the pattern did not consistently indicate reduced immune activity. Depuration produced partial recovery, particularly in the lower-exposure group. These findings indicate that sublethal BaP exposure can disrupt circulating haemocyte profiles and cellular immune function in S. serrata, supporting the value of haemolymph-based biomarkers for assessing immunotoxic stress in estuarine crustaceans.