Classification and phagocytosis of circulating haemocytes in Chinese mitten crab (Eriocheir sinensis) and the effect of extrinsic stimulation on circulating haemocytes in vivo.

Eriocheir sinensis (Henri Milne Edwards 1854) is one of the most important aquaculture species in China. In this investigation, we characterised the different types of haemocytes of E. sinensis using light and electron microscopy combined with cytochemical analysis and determined the in vivo phagocytic ability of different haemocyte types by injecting polystyrene beads. The haemocytes of E. sinensis were divided into three types: hyalinocytes, semigranulocytes and granulocytes. The hyalinocytes had no or few cytoplasmic granules; the semigranulocytes contained abundant small granules and a few large refractile cytoplasmic granules; and the granulocytes contained numerous large refractile cytoplasmic granules. The hyalinocytes were demonstrated to be the most abundant circulating haemocytes and the most avid phagocytic haemocytes, accounting for approximately 88.7% of the total phagocytes. The haemocyte-containing granules displayed limited phagocytic ability, with approximately 5.0% of granulocytes and 6.3% of semigranulocytes displaying positive phagocytic ability against the invading polystyrene beads in vivo. After injection with Aeromonas hydrophila, Bacillus subtilis and different concentrations of lipopolysaccharide for 0.25, 0.5, 1, 2, 4, 6 and 8 h, all three types of haemocytes experienced dramatic decline and then rapid recovery to their initial levels. A high concentration of lipopolysaccharide and A. hydrophila were extremely toxic to the crabs, as they induced a more serious loss of haemocytes compared with a low concentration of lipopolysaccharide and B. subtilis. Overall, the results obtained in this study indicate that a small proportion of the haemocytes of E. sinensis contributed to the phagocytic process, and the migration of haemocytes and haemocyte lysis were most likely a prominent pathway for pathogen elimination.