Hemocyte diversity in crustaceans

Crustaceans is a diverse and ecologically significant invertebrate group, in which circulating hemocytes act as the main immune defense cells, also participating in blood clotting and wound repair. As a result, considerable research has focused on understanding hemocyte function in crustaceans, with species such as crayfish and shrimp being commonly studied models. Traditionally, hemocytes have been categorized based on their morphology, particularly the presence and type of cytoplasmic granules. Based on these features, they have typically been classified into hyaline, semigranular, and granular types. However, recent techniques such as single-cell RNA sequencing have revealed new molecular markers for different hemocyte populations, indicating that the traditional classification is too simplistic. These new findings suggest the existence of additional hemocyte types with distinct, specialized functions. In this review, we present a brief historical perspective and highlight recent progress in understanding the traits and roles of various hemocyte types.