Abstract

We examined the hemocyte types and immune function of Ruditapes aspera from the sandy-pebble intertidal zone in Wimi Harbor on Jeju Island, using microscopy and flow cytometry. Three types of hemocytes were identified under light microscopy: granulocytes, hyalinocytes, and blast-like hemocytes. The ratio of nuclear size to cell size (N/C) of blast-like cells (0.77 ± 0.09) was slightly larger than that of granulocytes (0.52 ± 0.08) and hyalinocytes (0.65 ± 0.09). The N/C ratios of R. aspera hemocytes were similar to those of R. philippinarum (granulocyte 0.55 ± 0.06, hyalinocyte 0.61 ± 0.07, and blast-like cell 0.75 ± 0.07) collected from Jeju Island. Flow cytometry revealed that R. aspera contained 7.8 ± 1.1 × 105 hemocytes, which is similar to that of R. philippinarum (10.0 ± 1.4 × 105 cells). Among the three types of hemocytes in R. aspera, hyalinocytes accounted for 69.2% of the total hemocytes, followed by granulocytes (18.2%) and blast-like cells (10.2%). The lysozyme content, a proxy for the innate immune capacity of invertebrates, was highest in granulocytes (9.9 ± 3.2 × 104 A.U.), significantly higher than that in hyalinocytes (2.1 ± 0.4 × 104 A.U.), indicating that, like other marine bivalves, granulocytes are the main cells involved in defense. Phagocytosis rates measured by flow cytometry also showed that granulocytes in R. aspera are the primary defense cells, with a rate (58.5 ± 17.8 after 90 min) significantly higher than that of hyalinocytes (10.3 ± 5.5; ANOVA, P < 0.05). Similarly, reactive oxygen species (ROS) production in granulocytes (4.1 ± 3.0 × 106 A.U. after 120 min) was significantly higher than in hyalinocytes (4.1 ± 3.0 × 106 A.U. after 120 min), confirming granulocytes as the key hemocytes involved in defense.