Summary Parasites and pathogens can have an important effect on their host's thermal resistance. The impact of parasite infection on host physiological performances has traditionally been studied in controlled laboratory conditions, and much less is known about its actual effects in wild populations. Nonetheless, such knowledge is critical when assessing the effect of climate change on the future survival of the host. Tetracapsuloides bryosalmonae is a myxozoan endoparasite causing proliferative kidney disease (PKD) in salmonids. Infection and clinical symptoms of PKD are dependent on environmental temperature and PKD has become an emerging disease of primary importance for farmed and wild salmonids in the last decades. Despite important achievements in understanding PKD pathology in recent years, there are still crucial gaps in the knowledge of the disease ecology, notably in how the parasite affects host performance in the wild. We sampled juvenile (0+) brown trout (Salmo trutta) from the wild during early and late summer and assessed relative parasite load (DNA quantification with qPCR) and disease severity (kidney hyperplasia). We also measured haematocrit, leucocyte formula, aerobic scope and upper thermal tolerance in a field‐physiology approach in order to better understand the relationships between PKD severity and host performance. By using wild‐caught individuals and performing measurements directly on location, we aimed to gain insights into host physiology in a natural environment while avoiding biases caused by laboratory acclimation. We found that most physiopathological symptoms in the wild were strongly correlated with kidney hyperplasia, but more weakly linked to parasite load. Disease severity was positively correlated with anaemia and abundance of circulating thrombocytes, and negatively correlated with aerobic scope and thermal tolerance. Our results suggest that impaired aerobic performances and thermal tolerance in infected fish may potentially result in decreased host survival in the wild, especially in relation with predicted higher average summer temperatures and increased frequency of extreme events (summer heatwaves) in the context of global climate change. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12701/suppinfo is available for this article.

Telomeres protect eukaryotic chromosomes; variation in telomere length has been linked (primarily in homoeothermic animals) to variation in stress, cellular ageing and disease risk. Moreover, telomeres have been suggested to function as biomarker for quantifying past environmental stress, but studies in wild animals remain rare. Environmental stress, such as extreme environmental temperatures in poikilothermic animals, may result in oxidative stress that accelerates telomere attrition. However, growth, which may depend on temperature, can also contribute to telomere attrition. To test for associations between multitissue telomere length and past water temperature while accounting for the previous individual growth, we used quantitative PCR to analyse samples from 112 young-of-the-year brown trout from 10 natural rivers with average water temperature differences of up to 6°C (and an absolute maximum of 23°C). We found negative associations between relative telomere length (RTL) and both average river temperature and individual body size. We found no indication of RTL-temperature association differences among six tissues, but we did find indications for differences among the tissues for associations between RTL and body size; size trends, albeit nonsignificant in their differences, were strongest in muscle and weakest in fin. Although causal relationships among temperature, growth, oxidative stress, and cross-sectional telomere length remain largely unknown, our results indicate that telomere-length variation in a poikilothermic wild animal is associated with both past temperature and growth.

Cohabiting microorganisms play diverse and important roles in the biology of multicellular hosts, but their diversity and interactions with abiotic and biotic factors remain largely unsurveyed. Nevertheless, it is becoming increasingly clear that many properties of host phenotypes reflect contributions from the associated microbiome. We focus on a question of how parasites, the host genetic background, and abiotic factors influence the microbiome in salmonid hosts by using a host-parasite model consisting of wild brown trout (Salmo trutta) and the myxozoan Tetracapsuloides bryosalmonae, which causes widely distributed proliferative kidney disease. We show that parasite infection increases the frequency of bacteria from the surrounding river water community, reflecting impaired homeostasis in the fish gut. Our results also demonstrate the importance of abiotic environmental factors and host size in the assemblage of the gut microbiome of fish and the relative insignificance of the host genotype and gender.

Early growth conditions can have profound impacts on individuals' development, growth and physiology, with subsequent long-term consequences for individuals' fitness and life expectancy. Telomere length (TL) has been suggested to indicate both individual fitness and life expectancy in wide range of species, as the telomere attrition rate at early age can be accelerated due to exposure to various stressors, including parasites and inflammatory diseases, which increase production of reactive oxygen species (ROS) and influence antioxidant (AO) levels. We investigated impacts of Tetracapsuloides bryosalmonae infection, a causative agent of proliferative kidney disease (PKD), on AO status and TL in a natural population of juvenile brown trout (Salmo trutta). The fish with higher parasite load showed more severe kidney hyperplasia, anemia and smaller body size compared to less parasitized fish. Furthermore, fish with severe PKD symptoms had lower SOD-, CAT- and GST activity than fish with milder kidney hyperplasia. However, parasite load was not directly correlated either with AOs or with TL. Smaller fish showed shorter TLs, potentially reflecting lower individual quality. The fish, which were less sensitive to parasite-induced impaired growth, quantified as parasite load-adjusted fork length, showed also longer TLs, lower GR- and GST activity and less GSH compared to more sensitive fish. These results provide novel knowledge about the impacts of the PKD in brown trout at the molecular level and support the idea that TL may reflect individual quality and ability to cope with parasitic infections.

Widespread epibiontic relationship is best known as a commensal relationship, but several studies have shown also harmful influences on hosts by epibionts. A laboratory experiment was carried out to investigate the effect of peritrich ciliates (Epistylis sp.) as epibionts on the survival of Acartia bifilosa (Copepoda). Copepods were collected from the Gulf of Riga and the trial was made in seawater taken from the same location as the zooplankton samples. Adult copepods were divided into four groups by sex and by infection with epibionts. Results of the trial suggest that animals infested with epibionts were less viable than the uninfested ones. This pattern was significant in the case of females, but not males. Comparison of performance by sexes showed that the survival rate was significantly higher among females than males. The results further suggest that epibiontic protozoans may also influence egg production of copepods, because of shorter life span of females, and therefore may play a substantial role in copepod production and community dynamics.