Diet influences salinity preference of an estuarine fish, the killifish Fundulus heteroclitus

Understanding the interplay among the external environment, physiology and adaptive behaviour is crucial for understanding how animals survive in their natural environments. The external environment can have wide ranging effects on the physiology of animals, while behaviour determines which environments are encountered. Here, we identified changes in the behavioural selection of external salinity in Fundulus heteroclitus, an estuarine teleost, as a consequence of digesting a meal. Fish that consumed high levels of dietary calcium exhibited a higher preferred salinity compared with unfed fish, an effect that was exaggerated by elevated dietary sodium chloride. The mean swimming speed (calculated as a proxy of activity level) was not affected by consuming a diet of any type. Constraining fish to water of 22 p.p.t. salinity during the digestion of a meal did not alter the amount of calcium that was absorbed across the intestine. However, when denied the capacity to increase their surrounding salinity, the compromised ability to excrete calcium to the water resulted in significantly elevated plasma calcium levels, a potentially hazardous physiological consequence. This study is the first to show that fish behaviourally exploit their surroundings to enhance their ionoregulation during digestion, and to pinpoint the novel role of dietary calcium and sodium in shaping this behaviour. We conclude that in order to resolve physiological disturbances in ion balance created by digestion, fish actively sense and select the environment they inhabit. Ultimately, this may result in transient diet-dependent alteration of the ecological niches occupied by fishes, with broad implications for both physiology and ecology

Acid-base responses to feeding and intestinal Cl- uptake in freshwater- and seawater-acclimated killifish, Fundulus heteroclitus, an agastric euryhaline teleost

Marine teleosts generally secrete basic equivalents (HCO(3)(-)) and take up Na(+) and Cl(-) in the intestine so as to promote absorption of H(2)O. However, neither the integration of these functions with feeding nor the potential role of the gut in ionoregulation and acid-base balance in freshwater have been well studied. The euryhaline killifish (Fundulus heteroclitus) is unusual in lacking both an acid-secreting stomach and a mechanism for Cl(-) uptake at the gills in freshwater. Responses to a satiation meal were evaluated in both freshwater- and seawater-acclimated killifish. In intact animals, there was no change in acid or base flux to the external water after the meal, in accord with the absence of any post-prandial alkaline tide in the blood. Indeed, freshwater animals exhibited a post-prandial metabolic acidosis ('acidic tide'), whereas seawater animals showed no change in blood acid-base status. In vitro gut sac experiments revealed a substantially higher rate of Cl(-) absorption by the intestine in freshwater killifish, which was greatest at 1-3 h after feeding. The Cl(-) concentration of the absorbate was higher in preparations from freshwater animals than from seawater killifish and increased with fasting. Surprisingly, net basic equivalent secretion rates were also much higher in preparations from freshwater animals, in accord with the 'acidic tide'; in seawater preparations, they were lowest after feeding and increased with fasting. Bafilomycin (1 micromol l(-1)) promoted an 80% increase in net base secretion rates, as well as in Cl(-) and fluid absorption, at 1-3 h post-feeding in seawater preparations only, explaining the difference between freshwater and seawater fish. Preparations from seawater animals at 1-3 h post-feeding also acidified the mucosal saline, and this effect was associated with a marked rise in P(CO(2)), which was attenuated by bafilomycin. Measurements of chyme pH from intact animals confirmed that intestinal fluid (chyme) pH and basic equivalent concentration were lowest after feeding in seawater killifish, whereas P(CO(2)) was greatly elevated (80-95 Torr) in chyme from both seawater and freshwater animals but declined to lower levels (13 Torr) after 1-2 weeks fasting. There were no differences in pH, P(CO(2)) or the concentrations of basic equivalents in intestinal fluid from seawater versus freshwater animals at 12-24 h or 1-2 weeks post-feeding. The results are interpreted in terms of the absence of gastric HCl secretion, the limitations of the gills for acid-base balance and Cl(-) transport, and therefore the need for intestinal Cl(-) uptake in freshwater killifish, and the potential for O(2) release from the mucosal blood flow by the high P(CO(2)) in the intestinal fluids. At least in seawater killifish, H(+)-ATPase running in parallel to HCO(3)(-):Cl(-) exchange in the apical membranes of teleost enterocytes might reduce net base secretion and explain the high P(CO(2)) in the chyme after feeding

Polysaccharide synthesis in vitro from cellulose-producing model organisms

1. 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) if changes in the microbiome are concomitant with changes in the physiology of the host, including the immune system and response to cold. 2. We exposed laboratory populations of the spring field cricket, Gryllus veletis (Orthoptera: Gryllidae), to simulated overwintering winter and spring, we extracted and sequenced 16S bacterial genomic DNA from cricket guts, to capture seasonal variation in the composition of the microbiome. 3. The composition of the gut microbiome was similar between microcosms, and overall me, 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. 4. 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. Specifically, haemocyte concentrations increase but survival of fungal infection decreases in the winter, whereas the ability to clear bacteria from the haemolymph remains unchanged. 5. 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 if these changes in the microbiome contribute to host overwintering success

Gastrointestinal assimilation of Cu during digestion of a single meal in the freshwater rainbow trout ( Oncorhynchus mykiss)

Gastrointestinal processing and assimilation of Cu in vivo was investigated by sequential chyme analysis over a 72 h period following ingestion of a single satiation meal (3% body weight) of commercial trout food (Cu content = 0.42 μmol g − 1 ) by adult rainbow trout. Leaded glass ballotini beads incorporated into the food and detected by X-ray radiography were employed as an inert marker in order to quantify net Cu absorption or secretion in various parts of the tract. Cu concentrations in the supernatant (fluid phase) fell from about 0.06 μmol mL − 1 (63 μM) in the stomach at 2 h to about 0.003 μmol mL − 1 (3 μM) in the posterior intestine at 72 h. Cu concentrations in the solid phase were 10 to 30-fold higher than in the fluid phase, and increased about 4-fold from the stomach at 2 h to the posterior intestine at 72 h. By reference to the inert marker, overall net Cu absorption from the ingested food by 72 h was about 50%. The mid-intestine, and posterior intestine emerged as important sites of net Cu and water absorption and a potential role for the stomach in this process was also indicated. The anterior intestine was a site of large net Cu addition to the chyme, probably due to large net additions of Cu-containing fluids in the form of bile and other secretions in this segment. The results provide valuable information about sites of Cu absorption and realistic concentrations of Cu in chyme fluid for future in vitro mechanistic studies on Cu transport in the trout gastrointestinal tract