New insights into morpho-functional features of haemocytes from the blue crab Callinectes sapidus

In this study, we provided a comprehensive morpho-functional characterization of haemocytes in the blue crab Callinectes sapidus. Three haemocyte types were identified in the haemolymph: hyalinocytes (50 ± 4.7 %), lacking evident cytoplasmic granules; semigranulocytes (22.8 ± 2.02 %), containing a variable number of refractile granules; and granulocytes (27.2 ± 2 %), distinguished by their abundance of refractile granules. Haemocytes were predominantly oval or round. No significant size differences were observed among cell types, with granulocytes and semigranulocytes ranging from 7 to 22 µm, and hyalinocytes from 8 to 20 µm. Additionally, haemocytes were categorized into three cytochemical subpopulations: acidophils (38 %), basophils (36 %), and neutrophils (26 %). Notably, Neutral Red staining failed to reveal lysosomes in vivo, suggesting low membrane permeability under these conditions. Transmission electron microscopy corroborated the presence of the three haemocyte types. Both granulocytes and hyalinocytes exhibited phagocytic activity against yeast cells, although the phagocytic index remained low (∼4 %), implying that phagocytosis may not be the primary immune mechanism in C. sapidus. All haemocyte types generated superoxide anion and tested positive for several hydrolytic enzymes and phenoloxidase activity. Overall, these findings confirm the presence of three distinct haemocyte types in C. sapidus haemolymph and suggest that alternative immune pathways, beyond phagocytosis, may play a central role. Further research is needed to investigate additional immune functions, such as degranulation and inflammatory responses in C. sapidus.