Integrated Haemological, Histological, and Transcriptomic Analysis of Immune Responses in Atlantic Rock Crab (Cancer...

The Atlantic rock crab, Cancer irroratus, is ecologically important in Atlantic Canada, yet its immune response to bacterial challenge remains poorly characterized. We examined haemological, histological, and transcriptomic responses of C. irroratus following injection with 1.5 × 10⁶ CFU per crab of a locally isolated Vibrio splendidus strain. Total haemocyte count, refractive index, haemolymph bacterial culture, histopathology of the gills, hepatopancreas, and heart, and hepatopancreatic RNA sequencing were assessed at 1, 3, 7, and 14 days post-infection. Total haemocyte count and refractive index did not differ significantly between challenged and control crabs. Culturable bacteria were detected only at 1 day post-infection and at very low abundance, indicating rapid removal or effective control of V. splendidus in circulation. Histopathology nevertheless revealed marked localized responses, including haemocyte infiltration, nodulation, and melanization. Nodule density increased significantly in the hepatopancreas at 1, 3, and 7 days post-infection and in the heart throughout the 14-day experiment, whereas no significant increase occurred in the gills. Transcriptomic analysis identified 744 differentially expressed genes, with the strongest response at 1 day post-infection. Early responses included increased expression of relish, inhibitor of kappa B, immune-regulatory factors, anti-lipopolysaccharide factors, prophenoloxidase-activating factors, transglutaminase II, crustacean hyperglycemic hormone, and Cu/Zn-superoxide dismutase. These findings demonstrate that C. irroratus mounts a rapid, tissue-specific innate immune response characterized by minimal systemic haemolymph disruption, sustained localized defence in the heart and hepatopancreas, and strong but transient transcriptional reprogramming. This study provides the first controlled transcriptome-level characterization of Atlantic rock crab immune responses to V. splendidus infection.