Arsenic Impairs Crab Carapace Sclerotization after Molting by Interfering with Phosphorus Metabolism

Marine crabs accumulate arsenic (As) at substantially higher levels than other aquatic species; however, the toxicological consequences of such enrichment remain poorly understood. This study investigated the effects of As exposure on mud crabs (Scylla paramamosain) during the molting period. Environmentally relevant As elevations (8-fold in water and 3-fold in sediment) increased crab mortality by up to 60%, primarily due to impaired carapace sclerotization. Most deaths occurred within the first 8 days, coinciding with the peak inorganic As accumulation in the tissues. As exposure duration increased, mortality decreased, a trend associated with the biotransformation of inorganic As to arsenobetaine. Morphologically, As disrupted carapace mineralization, resulting in increased endocuticle pore density, shortened ribbon-like tubules, and disorganization of chitin-protein matrices. Concurrently, the phosphorus content decreased by approximately 30% in the hepatopancreas and 70% in the carapaces, showing significant negative correlations with As accumulation. The expression of phosphorus transporters (spPiT1) and carapace formation-related genes (spBMP) was down-regulated. These results demonstrate that As is highly toxic to mud crabs during molting, impairing carapace sclerotization through the interference of phosphorus metabolism. This study reveals a novel toxicological mechanism of As in marine crustaceans and highlights an urgent need to re-evaluate its ecological risks.