Assessing Molecular Mechanisms of Stress Induced Salinity Adaptation in the Juvenile Ornate Spiny Lobster, Panulirus ornatus.

Panulirus ornatus, the ornate spiny lobster, is a stenohaline weak hyper-osmoregulator, yet its osmoregulatory response to salinity stress remains poorly understood. This study investigated six osmoregulatory genes-Na+/K+-ATPase (nka), V-type H+-ATPase (vhe), Na+/HCO3- exchanger (nbc), Na+/K+/2Cl- co-transporter (nkcc), Na+/H+ exchanger (nhe), and carbonic anhydrase (ca)-in juvenile gills exposed to 25 ppt, 34 ppt (control), and 40 ppt salinities during acute (48 h) and chronic (>38 d) phases. Transcriptome analysis revealed that all genes were unresponsive following either 25 ppt or 40 ppt salinity acute exposure. However, nkcc showed a tendency toward for upregulation under 25 ppt salinity during acute exposure. Additionally, glutathione S-transferase and putative ferrous reductase 1 were upregulated under 25 ppt salinity, suggesting increased metabolic demand. In contrast, glutathione peroxidase and an ammonia transporter were upregulated in 40 ppt salinity, indicating protein catabolism. Quantitative PCR confirmed nkcc- and nka upregulation under chronic 25 ppt salinity. Vhe, nbc, nhe and ca showed no response, and 40 ppt salinity did not affect the six target genes. These findings suggest P. ornatus relies on nkcc- and nka-mediated ion transport and lacks mechanisms to tolerate high salinity, resulting in reduced growth and survival. These findings define optimal salinity range for aquaculture (25-34 ppt), highlighting the need to avoid high-salinity stress in lobster water quality management.