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 (<i>Scylla paramamosain</i>) 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 (<i>spPiT1</i>) and carapace formation-related genes (<i>spBMP</i>) 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.