Salinity change impairs pipefish immune defence.

Global change is associated with fast and severe alterations of environmental conditions. Superimposed onto existing salinity variations in a semi-enclosed brackish water body such as the Baltic Sea, a decrease in salinity is predicted due to increased precipitation and freshwater inflow. Moreover, we predict that heavy precipitation events will accentuate salinity fluctuations near shore. Here, we investigated how the immune function of the broad-nosed pipefish (Syngnathus typhle), an ecologically important teleost with sex-role reversal, is influenced by experimentally altered salinities (control: 18 PSU, lowered: 6 PSU, increased: 30 PSU) upon infection with bacteria of the genus Vibrio. Salinity changes resulted in increased activity and proliferation of immune cells. However, upon Vibrio infection, individuals at low salinity were unable to mount specific immune response components, both in terms of monocyte and lymphocyte cell proliferation and immune gene expression compared to pipefish kept at ambient salinities. We interpret this as resource allocation trade-off, implying that resources needed for osmoregulation under salinity stress are lacking for subsequent activation of the immune defence upon infection. Our data suggest that composition of small coastal fish communities may change due to elevated environmental stress levels and the incorporated consequences thereof.