Does sustained tolerance of reduced salinity seawater alter phagocytosis efficiency in haemocytes of the blue mussel Mytilus edulis (L.)?

Climate change driven increases in precipitation and extreme weather events bring with them periodic introductions of large volumes of freshwater to coastal waters and the possibility of extended salinity reductions capable of challenging marine organisms living there. Mytilus edulis is an osmoconformer, with no physiological capacity to maintain its haemolymph osmolarity above that of a dilute seawater medium. These mussels can however tolerate reduced seawater salinity to some degree by using intracellular volume control mechanisms to extend the range of seawater salinity they can tolerate, allowing them to continue to feed, respire and maintain general cellular function in fluctuating salinity conditions. When M. edulis are exposed to a falling salinity gradient, a critical salinity is reached that triggers closure of the valves, isolating their internal tissues from the surrounding environment and protecting them from further osmotic stress. This study investigated whether sustained tolerance of M. edulis to seawater with salinity just higher than the threshold level for valve closure affected haemocyte phagocytosis efficiency. Phagocytosis by haemocytes plays a significant role in the immune system of M. edulis by removing foreign bodies such as invading micro-organisms, together with dead and dying cells from the haemolymph. Valve gaping behaviour was recorded continuously in mussels exposed to a reducing seawater salinity gradient to establish the critical level initiating valve closure. M. edulis were then held for 5 d within a flow through seawater system at a salinity of 25, close to the trigger value for valve closure in the population of mussels investigated (Namsenfjord, Norway). The exposed mussels continued to hold their valves open and active throughout, though mean valve gape and movement between valves were generally lower than control mussels held at ambient seawater with salinity of 34.3. Post-exposure total haemocyte counts and haemolymph protein concentrations in exposed mussels were similar to the controls, while phagocytosis performance, measured using flow cytometry, improved in the reduced salinity exposed mussels. These findings support a conclusion that M. edulis have the physiological capacity to continue to irrigate their gills for at least 5 days when held continuously at seawater salinity close to their valve closure trigger level without detriment to haemocyte phagocytosis efficiency, a key component of their immune-response system. Such a capability will be important in the event of increasing persistent reduced salinity episodes.