The influence of parasites as an environmental factor on the immune response and haemolymph bacterial loads in New Zealand shore crabs
2010 The influence of parasites as an environmental factor on the immune response and haemolymph bacterial loads in New Zealand shore crabs DITTMER Jessica (Germany) email@example.com
Organisation: University of Poitiers, Poitiers (FR) - University of Otago, Dunedin (New Zealand) Supervisor : Mathieu Sicard (UP) & Robert Poulin (UO)
Summary: The immune system constitutes an animals last line of defence against parasite infection and proliferation. Important components of the crustacean immune system are different types of haemocytes and the proPhenoloxidase system involved in the melanisation of foreign particles. Focussing on two sympatric species of shore crabs, we analysed the variation in haemocyte concentration, the amount of circulating PO and the presence of bacteria in the haemolymph of crab species from two sites that experience different levels of parasitic pressure. Overall, the different immune parameters showed the same kind of variation in both host species depending on the presence of macroparasites in their environment. Thus, haemocyte concentrations were generally higher at the site experiencing a higher parasite prevalence, while circulating PO activity was lower. The enhanced production of haemocytes in crabs exposed to higher numbers of parasites is most likely an adaptive response to the higher level of parasitism. The fact that there is no higher investment in circulating PO activity suggests that humoral immune effectors are of lesser importance for the defence against macroparasites. Hence, while PO activity has been proven effective mainly against microbial pathogens, cellular defence mechanisms such as haemocytes involved in the encapsulation of foreign particles may be more efficient against large intruders. In addition, microphallid trematode metacercariae were found to cause a decrease in haemocyte concentrations in both species. These results underline the importance of parasites as an environmental factor and their capacity of exerting an important selective pressure on intermediate host populations. The bacterial loads in the haemolymph also varied significantly between sites in both species. however, the two species showed opposing trends in terms of septicaemia levels that could not be explained in this study.