Effects of spawning stress on the immune capacity of blood cockle Tegillarca granosa occurring on the south coast of Korea

Spawning in marine bivalves is a great energy-demanding process, and it often results in lethal and sublethal stresses during the post-spawning period, including depressed immune capacity. The blood cockle Tegillarca granosa (Linnaeus, 1758) distributes widely in silty-mud tidal flats on the south coast of Korea, and they spawn in late summer. To understand the impacts of spawning on immune parameters, we analyzed the total hemocyte count (THC), hemocyte mortality, phagocytosis capacity, and reactive oxygen species (ROS) production of T. granosa in pre-, and post-spawning condition using a flow cytometer. Histology indicated that the blood cockles occurring on the south coast of Korea ripe and ready to spawn in July, and they spawned in August and September. The THC in the blood cockle hemolymph declined from pre-spawning (1.2 × 108 cell mL−1) to post-spawning (0.9 × 108 cell mL−1), possibly due to the spawning stress and the massive infiltration of hemocytes in the gonad to phagocytose and resorb the residual gametes during the post-spawning period. The hemocyte mortality increased linearly from August (4.1%) to November (9.1%), as the histology revealed that the blood cockle completed spawning, and they resorbed the relict gametes. The granulocyte phagocytosis capacity declined dramatically from July (12.7%) to September (6.0%), when the cockles were engaged in active spawning. The flow cytometry revealed that the production of reactive oxygen species (ROS) from the granulocytes and the erythrocytes type II increased linearly from August (0.8–0.9 × 105 A U.) to December (2.1–2.8 × 105 A U.), which may cause stresses at a cellular level during this period. As the data indicated, spawning is a stressful activity inducing depressed immunological capacities in the blood cockles.