Histopathological and histochemical analysis of the digestive tract of adult horseshoe crabs under mercury stress

Mercury (Hg) contamination is a major environmental concern that continuously impacts marine organisms, including the mangrove horseshoe crab (Carcinoscorpius rotundicauda). As a vulnerable species, C. rotundicauda holds significant ecological and evolutionary value, making it particularly susceptible to Hg pollution and deserving of focused conservation efforts. This study presents the first comprehensive histopathological and histochemical analysis of mercury toxicity in the digestive tract of adult C. rotundicauda. Through both qualitative and quantitative analyses of histopathological and histochemical alterations, we evaluated the effects of acute Hg exposure (0.5 mg/L and 5 mg/L) on the esophagus, stomach, and intestines over time. The results revealed significant dose- and time-dependent tissue damage, with the stomach exhibiting the most pronounced alterations, including epithelial detachment, vacuolation caused by fibers degradation in the loose connective tissue, and muscle layer disruption, followed by the esophagus and intestines. Additionally, mucous cell density in the hindgut submucosa decreased by 30 % after 96 h of acute Hg exposure (0.5 and 5 mg/L HgCl₂), indicating a weakened protective barrier. The tissue damage of the digestive tract may further have a negative impact on the health of the adult horseshoe crab, thus threatening the survival of C. rotundicauda population and reducing the biodiversity of the mangrove ecosystem. These findings underscore the critical threat posed by Hg pollution to the digestive system of C. rotundicauda and offer novel insights into the toxicological impact on marine invertebrates. Considering the tissue-damaging effects of Hg on adult horseshoe crabs, this study underscores the importance of regular monitoring of Hg contamination in natural habitats. The results offer valuable guidance for future habitat restoration and effective management of existing habitats.