EXPLORING DECAPOD CRUSTACEAN IMMUNE RESPONSES THROUGH PATHOGEN CHALLENGES IN AMERICAN LOBSTER (HOMARUS AMERICANUS)

Abstract

The American lobster (Homarus americanus) fishery is a significant economic driver in coastal communities across North America. Researchers increasingly recognize pathogens as important factors affecting population ecology and the subsequent management of the lobster fishery. Two infectious pathogens, including Aerococcus viridans var. homari (a Gram-positive bacterium) and Anophryoides haemophila (a ciliated protist), threaten both wild and captive lobsters, while Vibrio splendidus (a Gram-negative bacterium) has been found in lesions associated with lobster shell disease. Despite their economic impact, little is known about the host immune responses elicited by these pathogens. This thesis investigated the transcriptional responses of hepatopancreatic tissue in H. americanus following experimental challenges with each pathogen. The RNA-Seq analyses revealed specific transcriptional changes. over 11,000 differentially expressed genes (DEGs) were detected in response to A. haemophila, over 1,800 to A. viridans var. homari, and 1,250 to V. splendidus. Of these, 87 (A. viridans var. homari), 152 (A. haemophila), and 72 (V. splendidus) DEGs were associated with immune or stress responses. Key immune-related DEGs included anti-lipopolysaccharide factors (ALFs), serum amyloid A-like (SAA-like) proteins, lectins, Toll-like receptors, cytokines, relish, cactus, and genes involved in antioxidant defence and coagulation. This study provided further evidence that ALFs and SAA-like proteins play essential roles in lobster immune defence. Several ALF genes, including HA-ALF-L1, HA-ALF-L1-like, and HA-ALF-L2, were significantly upregulated by all pathogen types. The HA-ALF-L1 appeared to have the highest induction in response to the Gram-negative bacterium, while HA-ALF-L4 has been shown to respond more strongly and selectively to the Gram-positive bacterium. In contrast, HA-ALF-L5 was specifically upregulated by V. splendidus, but not by the Gram-positive bacterium or the ciliate. Additionally, one transcript related to HA-ALF-L6 (HA-ALF-L6-like-a) was also upregulated by both Gram-positive and Gram-negative bacteria, but not by the ciliate. These findings highlight pathogen class-specific regulation of immune genes and enhance our understanding of crustacean immune specificity. The consistent upregulation of SAA-like genes across all pathogen types further supports their role in the crustacean acute-phase immune response.