Changes in complement responses in Gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) under crowding stress, plus viral and bacterial challenges

Gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) were subjected to either experimental infection with Photobacterium damselae subsp. piscicida or Nodavirus after a period of 2 weeks of crowding in which fish were subjected to a 5-fold increase in density (10–50 kg/m3). Samples were obtained before the crowding period (0 h or control) and at 24 h and 72 h after crowding from both groups of infected fish. The Complement haemolytic activity and the expression of the C3 gene were evaluated in blood and liver samples respectively. The bacteriolytic and lysozyme activities were also assessed. The results showed that Complement haemolytic activity was reduced at 72 h with both bacteria and virus in high density Gilthead seabream, and a similar increase was observed at low density. Bacteriolytic activity under both bacterial and viral challenges for both species was increased at 24 h, under low density. At high density, the bacterial challenge did not induce significant changes. C3 mRNA abundance was substantially increased after pathogen treatments in low density groups at 24 h but no significant changes were detected at high densities. results support the idea of the suppressor effect of stressors on the immune system since a reduction of Complement activity under virus and high density, or lack of response in C3 expression under high density were observed.