) to the environment, a beneficial consequence of the osmotic challenge offered by the food for a freshwater hyperosmotic regulator.

SUMMARYWe investigated the potential acid-base and nitrogenous waste excretion challenges created by voluntary feeding in freshwater rainbow trout, with particular focus on the possible occurrence of an alkaline tide (a metabolic alkalosis created by gastric HCl secretion during digestion). Plasma metabolites (glucose, urea and ammonia) were measured at various time points before and after voluntary feeding to satiation (approximately 5% body mass meal of dry commercial pellets), as was the net flux of ammonia and titratable alkalinity to the water from unfed and fed fish. Arterial blood, sampled by indwelling catheter, was examined for post-prandial effects on pH, plasma bicarbonate and plasma CO 2 tension. There was no significant change in plasma glucose or urea concentrations following feeding, whereas plasma ammonia transiently increased, peaking at threefold above resting values at 12 h after the meal and remaining elevated for 24 h. The increased plasma ammonia was correlated with an increase in net ammonia excretion to the water, with fed fish significantly elevating their net ammonia excretion two-to threefold between 12 and 48 h post feeding. These parameters did not change in unfed control fish. Fed fish likewise increased the net titratable base flux to the water by approximately threefold, which resulted in a transition from a small net acid flux seen in unfed fish to a large net base flux in fed fish. Over 48 h, this resulted in a net excretion of 13·867·μmol·kg -1 more base to the external water than in unfed fish. The arterial blood exhibited a corresponding rise in pH (between 6 and 12 h) and plasma bicarbonate (between 3 and 12 h) following feeding; however, no respiratory compensation was observed, as Pa CO 2 remained constant. Overall, there was evidence of numerous challenges created by feeding in a freshwater teleost fish, including the occurrence of an alkaline tide, and its compensation by excretion of base to the external water. The possible influence of feeding ecology and environmental salinity on these challenges, as well as discrepancies in the literature, are discussed.

Seasonal changes in the environment, such as varying temperature, have the potential to change the functional relationship between ectothermic animals, such as insects, and their microbiomes. Our objectives were to determine: (a) whether seasonal changes in temperature shift the composition of the insect gut microbiome, and (b) whether changes in the microbiome are concomitant with changes in the physiology of the host, including the immune system and response to cold. We exposed laboratory populations of the spring field cricket, Gryllus veletis (Orthoptera: Gryllidae), to simulated overwintering conditions in both a laboratory microcosm and a field‐like microcosm containing soil and leaves. In summer, autumn, winter and spring, we extracted and sequenced 16S bacterial genomic DNA from cricket guts, to capture seasonal variation in the composition of the microbiome. The composition of the gut microbiome was similar between microcosms, and overall highly anaerobic. In both microcosms, we captured similar seasonal variation in the composition of the microbiome, where overwintering resulted in permanent changes to these microbial communities. In particular, the abundance of Pseudomonas spp. decreased, and that of Wolbachia spp. increased, during overwintering. Concurrent with overwintering changes in the gut microbiome, G. veletis acquire freeze tolerance and immune function shifts temporarily, returning to summer levels of activity in the spring. In a specific manner, haemocyte concentrations increase but survival of fungal infection decreases in the winter, whereas the ability to clear bacteria from the haemolymph remains unchanged. Overall, we demonstrate that the gut microbiome does shift seasonally, and in concert with other physiological changes. We hypothesize that these changes may be linked, and suggest that it will next be important to determine whether these changes in the microbiome contribute to host overwintering success. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13153/suppinfo is available for this article.

No abstract