Green-lipped mussel (Perna canaliculus) hemocytes: A flow cytometric study of sampling effects, sub-populations and immune-related functions.

Green-lipped mussels (Perna canaliculus) are a commercially and culturally important bivalve species in New Zealand (NZ). As the highest value export aquaculture product in NZ, understanding and safeguarding the health of this species is imperative. The identification and characterization of hemocytes can provide useful information regarding the health of this species. Using flow cytometry (FCM), the present study assessed for the first time the use of different antiaggregant solutions and storage times on the immune-related parameters of hemocytes from cultured adult P. canaliculus. In addition, characterization of the immune-related functions of hemocyte sub-populations within the hemolymph were assessed. The two antiaggregant solutions tested (Modified Alserver's, MAS, A and B) maintained similar numbers of hemocytes in circulation over a 60 min period but, reduced the viability (MAS A) and increased the ROS production (MAS B) of the hemocytes compared to hemocytes diluted in cold filtered seawater (FSW). Hemocytes diluted in FSW and kept on ice showed significant aggregation after 2 h and a reduction in viability from 4 h. Three different hemocyte sub-populations were identified, discernible by their relative size and internal complexity: blast-like cells, hyalinocytes and granulocytes, which accounted for approximately 4, 67 and 29% of the total hemolymph population respectively. Granulocytes showed significantly higher reactive oxygen species production, phagocytic capabilities and neutral lipid content compared to hyalinocytes and blast-like cells. Results indicate that maintaining extracted hemolymph in cold FSW, completing analysis of fresh samples within 2 h of extraction and FCM assay incubation times of no longer than 30 min are best to obtain accurate results. Formalin fixation can also be used for future determination of hemocyte sub-populations and internal structures. Results from this study will allow effective future study of the effects of various stressors on P. canaliculus health and lead to improved management and production strategies in this species.