Molting Alters the Microbiome, Immune Response, and Digestive Enzyme Activity in Mud Crab (Scylla paramamosain)

Molting is a crucial lifelong process in the growth, development, and reproduction of crustaceans. In mud crab (Scylla paramamosain), new exoskeleton, gills, and appendages are formed after a molting, which contributes to a 40 to 90% increase in body weight. However, little is currently known about the associations between molting and the dynamic changes of microbiota and physiological charac-teristics in mud crabs. In this study, the effects of molting on changes of the micro -biome, immune response, and digestive enzyme activities in mud crabs were investi-gated. The results showed dynamic changes in the abundances and community compositions of crab-associated microbiota harboring the gills, subcuticular epider-mis, hepatopancreas, midgut, and hemolymph during molting. Renewed microbiota was observed in the gills and midgut of crabs at the postmolt stages, which seems to be related to the formation of a new exoskeleton after the molting. A significant posi-tive correlation between the expression of two antimicrobial peptide (AMP) genes (SpALF5 and SpCrustin) and the relative abundance of two predominant microorgan-isms (Halomonas and Shewanella) in hemolymph was observed in the whole molt cycle, suggesting that AMPs play a role in modulating hemolymph microbiota. Furthermore, digestive enzymes might play a vital role in the changes of microbiota harboring the hepatopancreas and midgut, which provide suitable conditions for restoring and recon-structing host-microbiome homeostasis during molting. In conclusion, this study con-firms that molting affects host-associated microbiota and further sheds light on the effects on the immune response and the digestive systems as well. IMPORTANCE Molting is crucial for crustaceans. In mud crab, its exoskeleton is renewed periodically during molting, and this process is an ideal model to study the effects of host development on its microbiota. Here, multiple approaches were used to investigate the changes in microbial taxa, immune response, and digestive enzyme activity with respect to molting in mud crab. The results found that a renewed micro-biota was generated in the gills and midgut of crab after a molt. A significant posi-tive correlation between changes in the relative abundances of microbes (such as Halomonas and Shewanella) and the expression of AMP genes (SpALF5 and SpCrustin) was observed in the hemolymph of crabs during the whole molt cycle, suggesting the modulation of hemolymph microbes by AMPs. Furthermore, the di-gestive enzymes were found to participate in the regulation of microbiota in hep-atopancreas and midgut, consequently providing a suitable condition for the res-toration and reconstruction of host-microbiome homeostasis during the molting. This study confirms that molting affects the microbial communities and concomi-tantly influences the immune and digestive systems in mud crabs. This is also the first time the homeostasis of the host and microbiome, and the associations between molting and physiological characteristics in crustaceans, have been revealed.

Automated summary

This study found that molting in mud crabs leads to significant changes in microbial communities, particularly in the gills and midgut, and is closely associated with the expression of antimicrobial peptide genes and the relative abundance of microbes in the hemolymph. Additionally, digestive enzymes play a role in regulating the microbiota in the hepatopancreas and midgut, facilitating the restoration and reconstruction of host-microbiome homeostasis during molting.