Are really males the sterner sex? The immune responses of the clam Tapes philippinarum as a case study

For the first time, gender-related differences in immune responses of the clam Tapes philippinarum were investigated. Haemocytes from male, female and sexually undifferentiated clams were collected, and total haemocyte count (THC), haemocyte volume, capability of haemocytes to assume the vital dye Neutral Red (indicative of endocytotic activity), acid phosphatase and lysozyme-like activities in both haemocyte lysate (HL) and cell-free haemolymph (CFH), were evaluated. No statistically significant differences in THC values were observed. However, differing haemocyte size frequency distribution was found: the fraction of larger haemocytes (6-8 μm diameter, 200 fl volume) markedly increased in females, whereas the fraction of smaller haemocytes (< 5 μm diameter, < 200 fl volume) increased in both male and undifferentiated clams. Significantly increased Neutral Red uptake was recorded in haemocytes from females. This was most likely related to the higher fraction of larger haemocytes in females, these cells being more actively involved in phagocytosis. No significant variations in lysozyme-like activity was observed in HL, whereas in CFH enzyme activity resulted significantly higher in females with respect to male and undifferentiated animals. HL acid phosphatase activity was significantly higher in males with respect to females and undifferentiated clams, whereas no significant variations in enzyme activity was observed in CFH. Overall, results obtained demonstrated that gender- related differences in immune responses occurred in T. philippinarum, and indicated that females had more active haemocytes than both male and undifferentiated clams, at least on the basis of the cellular parameters investigated.