Does Immune Response Influence Mate Choice in Sex Role Reversed Pipefish?
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- Institutt for biologi 
Vibrio sp. is the most diverse and abundant pathogenic bacteria in aquatic environments. This pathogen gets more abundant at warmer temperatures and low salinity. The Baltic Sea is more affected by climate change than global averages. Since 1978, the surface temperature has increased 7 times more than the global rate and the salinity has decreased due to fewer inflows from the North Sea and is expected to decrease more with increased precipitation. These changes can lead to stronger parasite pressure on the species living there. Teleost fish have different ways to avoid infections by parasites, such as physical barriers that prevent the parasite from entering the body. However, if the parasites manage to overcome these barriers, it will face the innate immune response of the fish followed by an adaptive, specific immune response. Previous studies have used live infections to investigate whether parasitized individuals are avoided in mate choice. However, if mate preference is changed, we cannot know whether this change is caused by behavioural plasticity by the host or if it is driven by the parasite. We wanted to see if mate choice is driven by immune activation. An experiment has shown that pipefish treated with heat-killed Vibrio sp. show the same behavioural response as pipefish infected by live Vibrio sp. To activate the immune response of the fish, we therefore injected the fish with heat-killed Vibrio sp. (immune challenge). Our control group was sham-injected with the same volume of phosphate-buffered saline (PBS). We investigated whether immune activation affects the activity, attractiveness and choosiness in the two pipefish species Syngnathus typhle and Nerophis ophidion from populations in the Baltic Sea. A mate choice experiment was set up where a focal fish, either sham-injected or immune-challenged, had the choice between a sham-injected and immune-challenged fish. There was no difference in activity between immune-challenged fish and the sham-injected fish. Immune challenge may therefore not affect activity. Another possibility is that the immune-challenged fish invested energy to keep a normal activity level as a terminal investment into the current reproduction. In the experiment, the focal fish were provided with olfactory cues. The focal fish did not discriminate between immune-challenged and un-challenged fish. There is a possibility that the immune-challenge was not recognized. However, if the focal fish recognized immune-challenge, this could suggest that there was no difference in attractiveness between the groups and there could be advantages of choosing both a fish that has been exposed to an immune-challenge that can provide immunological information to the offspring as well as an un-challenged, healthy mating partner. We found no difference in how choosy the fish were between the two groups. Since attractiveness may not differ between the groups, the choosiness in immune-challenged individuals will be further investigated by the choice for compatible MHC.