The effects of exposure to bacterial infection on behaviour and metabolic rate of a freshwater fish, Nannoperca vittata

Nuwandi U Pathirana

Infection is often associated with changes in host behaviour, particularly increased activity and boldness. Such behavioural changes have frequently been ascribed to adaptive manipulation of the host by the parasite but could also arise from a non-adaptive side effect of infection, such as increased energy demands on the host as a result of infection. As infection has also often been shown to alter the basal or standard metabolic rate (SMR) of hosts, there may be a link between behaviour, infection and metabolic rate, but the causal relationships among these factors have rarely been experimentally investigated. To undertake such an investigation, I first conducted a longitudinal study to explore the repeatability and correlations among behavioural traits in western pygmy perch Nannoperca vittata, an endemic freshwater fish of south-western Australia, and in the guppy, Poecilia reticulata, a commonly used model species in behavioural studies. Behavioural measures representing the traits of activity, boldness and sociability were significantly repeatable for both species, and there was also evidence for both correlations among-individuals (behavioural syndromes) and correlations within individuals (joint phenotypic plasticity) for activity and boldness traits in N. vittata. There was little concordance in phenotypic, within-individual and among individual correlations between traits, emphasising the importance of partitioning trait covariances to infer behavioural syndromes. Behavioural traits representing activity and boldness, and SMR were then measured in individual N. vittata before and after exposure to a bacterial parasite (Photobacterium damselae subsp. damselae). Prior to challenge, all traits showed moderate-high repeatability and there was no evidence of among-individual correlations between SMR and behavioural traits. Compared to uninfected (control) fish, fish which were exposed to infection showed a significant increase in activity, boldness and SMR. Standard metabolic rate of exposed fish remained high over a three-week post-challenge period, but behavioural changes were transient and returned to pre-challenge levels after two weeks. Exposure to P. damselae subsp. damselae decreased the repeatability of SMR, but not behavioural traits, and had little effect on correlations between traits. Photobacterium damselae subsp. damselae is transmitted through the water, rather than through predation or fish to fish contact, so the observed behavioural changes in fish are more likely to be a consequence of increased energy demands than manipulation by the parasite. To determine the cause of the increased energy demand, behavioural traits and SMR were measured in non-parasitized fish that were exposed to either an immune challenge (injection with lipopolysaccharide, LPS) or food deprivation. Changes in SMR and behavioural traits were found after food deprivation, but not after immune challenge. Food-deprived fish showed an increase in activity, but not boldness, immediately following the treatment period, while SMR increased three weeks post-treatment. From the results of these experiments, I hypothesise that loss of appetite and lethargy after infection leads to weight loss in fish, which is followed by a period of compensatory growth. Compensatory growth requires behavioural changes, such as increased activity to find food and this leads to physiological changes which increase maintenance costs and therefore SMR.

The effects of exposure to bacterial infection on behaviour and metabolic rate of a freshwater fish, Nannoperca vittata

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