Objective Hemocyanin is a copper‐bearing protein in the hemolymph of many arthropods and mollusks and functions as an oxygen transport and important nonspecific immune protein. Methods In this study, complementary DNA of hemocyanin isoform 2 of the prawn Macrobrachium rosenbergii (MrHc2) was isolated by rapid amplification of cDNA ends and mRNA expression was characterized to elucidate molecular basis of its function. Result With a molecular mass of 77.3 kDa, MrHc2 contained three domains: hemocyanin‐all‐alpha, hemocyanin‐copper‐containing, and hemocyanin‐immunoglobulin‐like domains. Molecular phylogenetic analysis revealed that MrHc2 belongs to the γ‐type subunit and is closely related to hemocyanin subunit 1 of the palaemonid shrimp Macrobrachium nipponense. In addition, MrHc2 resided in a different clade relative to hemocyanin (MrHc) of M. rosenbergii (α‐type subunit) and in a different subclade relative to the hemocyanin proteins of penaeid shrimp. The messenger RNA transcript of MrHc2 was highly expressed in the hepatopancreas and weakly expressed in the gills, intestine, stomach, muscle, and hemocytes. Upon challenge with M. rosenbergii nodavirus (MrNV), the expression of MrHc2 was 1.96‐, 2.93‐, and 1.96‐fold on days 3, 4, and 5, respectively, and then gradually declined to basal levels on day 7. Conclusion This study suggests that MrHc2 plays an important role in the innate immune response of M. rosenbergii to MrNV.

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