The effects of sulfide on the energy metabolism of Boleophthalmus boddaerti in normoxia and hypoxia were examined. The 24-, 48-, and 96-h LC50 values of sulfide for B. boddaerti with body weight ranging from 11.6 to 14.2 g were 0.786, 0.567, and 0.467 mM, respectively. The tolerance of B. boddaerti to sulfide was not due to the presence of a sulfide-insensitive cytochrome c oxidase. There was no accumulation of lactate in the muscle and liver of specimens exposed to sulfide in normoxia. In addition, the levels of ATP, AMP, and energy charge in both the muscle and the liver were unaffected. These results indicate that B. boddaerti was able to sustain the energy supply required for its metabolic needs via mainly aerobic respiration when exposed to sulfide (up to 0.4 mM) in normoxia. Exposure of B. boddaerti simultaneously to hypoxia and 0.2 mM sulfide for 48 h resulted in decreases in the ATP levels in the muscle and liver. However, the energy charge in both tissues remained unchanged, and the level of lactate accumulated in the muscle was too low to have any major contribution to the energy budget of the fish. Our results reveal that B. boddaerti possesses inducible mechanisms to detoxify sulfide in an ample supply or a lack of O2. In normoxia, it detoxified sulfide to sulfate, sulfite, and thiosulfate. There were significant increases in the activities of sulfide oxidase in the muscle and liver of specimens exposed to sulfide, with that in the liver being >13-fold higher than that in the muscle. However, in hypoxia, sulfide oxidase activity in the liver was suppressed in response to environmental sulfide. In such conditions, there were significant increases in the activities of sulfane sulfur-forming enzyme(s) in the muscle and liver that were not observed in specimens exposed to sulfide in normoxia. Correspondingly, there were no changes in the levels of sulfate or sulfite in the muscle or liver. Instead, B. boddaerti detoxified sulfide mainly to sulfane sulfur in hypoxia. In conclusion, B. boddaerti was able to activate different mechanisms to detoxify sulfide, producing different types of detoxification products in normoxia and hypoxia.
Tropical ectotherms are generally believed to be more vulnerable to global heating than temperate species. Currently, however, we have insufficient knowledge of the thermoregulatory physiology of equatorial tropical mammals, particularly of small diurnal mammals, to enable similar predictions. In this study, we measured the resting metabolic rates (via oxygen consumption) of wild-caught lesser treeshrews (Tupaia minor, order Scandentia) over a range of ambient temperatures. We predicted that, similar to other treeshrews, T. minor would exhibit more flexibility in body temperature regulation and a wider thermoneutral zone compared with other small mammals because these thermoregulatory traits provide both energy and water savings at high ambient temperatures. Basal metabolic rate was on average
1.03 ± 0.10
mL O2 h-1 g-1, which is within the range predicted for a 65-g mammal. We calculated the lower critical temperature of the thermoneutral zone at 31.0°C (95% confidence interval: 29.3°-32.7°C), but using metabolic rates alone, we could not determine the upper critical temperature at ambient temperatures as high as 36°C. The thermoregulatory characteristics of lesser treeshrews provide a means of saving energy and water at temperatures well in excess of their current environmental temperatures. Our research highlights the knowledge gaps in our understanding of the energetics of mammals living in high-temperature environments, specifically in the equatorial tropics, and questions the purported lack of variance in the upper critical temperatures of the thermoneutral zone in mammals, emphasizing the importance of further research in the tropics.