Effects of high pH on molting and calcium absorption of Pacific white shrimp (Litopenaeus vannamei)

Rising pH in culture water has become an increasingly prominent problem in high-density aquaculture systems. Researchers have found that an increase in pH level can affect the molting process of prawns. However, studies of the effects of different pH environments on the molting and calcium absorption of Pacific white shrimp ( Litopenaeus vannamei ) are relatively scarce. The goals of this study were to explore the effects of different pH levels on the molting, calcium absorption, ion regulation, and hormone changes of this species. The results showed that high pH (9.0–9.25) significantly reduced shrimp survival, with the highest mortality observed at pH 9.25. At high pH levels, the calcium ion concentrations in hemolymph, gills, and midgut decreased and showed tissue specificity. Scanning electron microscopy results indicated that high pH damaged the shrimp shells and reduced their thickness. Energy dispersive spectroscopy further revealed that the calcium content in the shells decreased under high pH conditions. Additionally, the ion content and related ion enzyme activities decreased at high pH. The assessment of carbohydrate metabolism-related hormones showed that high pH caused an increase in thyroid-releasing hormone, while the concentrations of corticotropin-releasing hormone and insulin 2 were significantly reduced. Expression levels of genes related to molting (e.g., chitin synthase , chitinase 5 , ecdysteroid receptor , retinoid X receptor , and calcium-calmodulin dependent protein kinase I) were significantly downregulated, while the expression of molt-inhibiting hormone was upregulated. In summary, we found that high pH values affect calcium absorption, ion balance, hormone regulation, and molting processes in shrimp through multiple physiological and molecular pathways. These findings provide a theoretical explanation for the changes in molting and calcium ion regulation caused by high pH in high-density shrimp farming, and they provide important references for subsequent calcium supplementation measures in shrimp aquaculture, for example by supplementing soluble calcium salts to enhance exoskeleton mineralization.