Transcriptome Changes in <i>Eriocheir sinensis</i> Megalopae after Desalination Provide Insights into Osmoregulation and Stress Adaption in Larvae

<div><p><i>Eriocheir sinensis</i>, an extremely invasive alien crab species, has important economic value in China. It encounters different salinities during its life cycle, and at the megalopal stage it faces a turning point regarding the salinity in its environment. We applied RNA sequencing to <i>E. sinensis</i> megalopae before (MB) and after (MA) desalination, resulting in the discovery of 21,042 unigenes and 908 differentially expressed genes (DEGs, 4.32% of the unigenes). The DEGs primarily belonged to the Gene Ontology groups “Energy metabolism,” “Oxidoreductase activity,” “Translation,” “Transport,” “Metabolism,” and “Stress response.” In total, 33 DEGs related to transport processes were found, including 12 proton pump genes, three <i>ATP-binding cassettes</i> (<i>ABCs</i>), 13 <i>solute carrier</i> (<i>SLC</i>) family members, two <i>sweet sugar transporter</i> (<i>ST</i>) family members and three other substance transporters. Mitochondrial genes as well as genes involved in the tricarboxylic acid cycle, glycolytic pathway, or β-oxidation pathway, which can generate energy in the form of ATP, were typically up-regulated in MA. 11 unigenes related to amino acid metabolism and a large number of genes related to protein synthesis were differentially expressed in MB and MA, indicating that <i>E. sinensis</i> possibly adjusts its concentration of free amino acid osmolytes for hyper-osmoregulation. Additionally, 33 salinity and oxidative stress induced genes were found to be differentially expressed, such as the <i>LEA2</i>, <i>HSPs</i>, <i>GST</i> and coagulation factor genes. Notably, <i>LEA2</i> is an extremely hydrophilic protein that responds to desiccation and reported for the first time in crabs. Therefore, we suppose that when the environment is hypo-osmotic, the megalopae might compensate for ion loss via hyper-osmoregulation by consuming more energy, accompanied by a series of stress induced adaptions. This study provides the first genome-wide transcriptome analysis of <i>E. sinensis</i> megalopae for studying its osmoregulation and stress adaption mechanisms.</p>