Immune function and transcriptome of hemocytes in northern quahogs Mercenaria mercenaria in response to acute salinity challenges.

The goal of this study was to evaluate effects of short-term acute salinity challenges on the hemocyte physiology in northern quahogs, Mercenaria mercenaria, an important aquaculture species in the U.S. The objectives were to: 1) challenge adult northern quahogs with salinities of 5, 15, 25 (control), 35, and 45 ppt; 2) evaluate cellular responses, including hemocyte concentration, viability, phagocytosis rate, ROS production, and lysosomal presence at 12 h, 24 h, 48 h, 72 h, 144 h, and 21 days post-challenge; and 3) evaluate the molecular response of hemocytes after 72 h challenge. M. mercenaria at 5, 15, 25, 35 ppt exposure showed no mortality, while at 45 ppt showed a 100 % mortality at day 7, reflecting better tolerance to hyposalinity. Changes of hemolymph osmolality at different salinities and exposure periods indicated M. mercenaria equilibrates with the environmental seawater within its tolerance range (15-35 ppt) as an osmoconformer. Hemocyte concentration increased significantly at 5, 15, and 45 ppt stress. Hemocyte immune functions exhibited a varied pattern in response to different salinity levels, enabling them to resolve stresses. The RNAseq of hemocytes revealed DEGs in response to salinity challenges, specifically enrichment of amino acid transporters (SLC2A5, SLC6A1, SLC5A8, SLC1A3, SLC25A38), immune response (CHAC2, MGST3, IRF1, and IRFD2) and cell signaling (Wnt, TRAIL-activated and Toll) pathway in hyper/hypo salinity groups, indicated their importance in salinity stress responses. This study provided critical insights into the cellular and molecular responses of M. mercenaria hemocytes to salinity challenges.