Influence of reproductive condition of the Manila clam Ruditapes philippinarum on hemocyte parameters during early post-spawning period

Abstract

In marine bivalve aquaculture, mortality is a major problem for sustainability of the industry. Mortalities usually result from complex interactions in which the physiological status is suspected to play a major role. In many bivalves, spawning appears as a critical period. Spawning is an energy demanding process potentially resulting in immunodepression and higher susceptibility to pathogenic infection. In an effort to understand the actual impacts of spawning on the cellular defense system (i.e., hemocytes) of the Manila clam Ruditapes philippinarum, we collected clams on a one-time sampling during early post-spawning period. We then applied, for the first time, a histology–flow cytometry combined approach to these animals. Hemocyte parameters, including concentration, sub-populations, mortality, phagocytosis capacity and reactive oxygen species production were analyzed along with gender, condition index as well as the tissue protein and carbohydrate levels. Among the 200 clams analyzed, 60.5% were spent, 24% spawning, 12% resting and 3.5% ripe and ready for spawning. No gender differences could be observed among all parameters and no correlation was determined between the protozoan parasite Perkinsus olseni infection intensity and the immune parameters. CI and tissue protein level of spawning and post-spawning (spent and resting) clams were significantly lower than those of pre-spawning (ripe) clams. The concentration of hemocytes tended to decline from ripe to spent clams, partly because of a concomitant increase of cell mortality and, most of all, a decrease in the quantity of circulating granulocytes. The decline in circulating granulocytes also resulted in a reduced phagocytosis capacity (% of cells able to phagocytise). At the animal scale, these observations support a depression of the global immune defense of spawning and post-spawning clams. However, at the cellular scale, remaining circulating cells did not display signs of stress nor altered metabolism. Indeed, phagocytosis, as the number of beads ingested per cell, was normal and no oxidative stress could be detected. Further studies will be necessary to determine if these hemocytes already reached their limit of stress tolerance, and to potentially help develop immunoprotective food additives for the aquaculture industry.