Summary This study quantified and compared hypoxia tolerance and swim performance among cyprinid fish species from rapid-flow, slow-flow and intermediate-flow habitats (4 species per habitat) in China. In addition, we explored effects of short-term acclimation on swim performance, maximum metabolic rate (Mo2 max) and gill remodelling to detect habitat-associated patterns of plastic response to hypoxia. Indices of hypoxia tolerance included oxygen threshold for loss of equilibrium (LOE50) and aquatic surface respiration (ASR50) and critical oxygen tension for routine metabolic rate (Pcrit). Critical swimming speed (Ucrit) and Mo2 max were measured under normoxic and hypoxic conditions after 48 hours acclimation to normoxia and hypoxia, and gill remodelling was estimated after 48 hours of hypoxia exposure. Both traditional ANCOVA and phylogenetically independent contrast (PDANOVA) analyses showed that fish species from rapid-flow habitat exhibited lower LOE50 compared to fish from intermediate and slow-flow habitats. Habitat-specific difference in Pcrit and Ucrit were detected using PDANOVA but not traditional ANCOVA analyses, with fish species from rapid-flow habitat exhibited lower Pcrit but higher Ucrit compared to fish from intermediate and slow-flow habitats. Fish species from rapid-flow habitats were also characterized by less plasticity in swim performance and gill morphology in response to hypoxia acclimation compared to species from slow-flow habitats, but a greater drop in swim performance in response to acute hypoxia exposure. The study detected a habitat-specific difference in hypoxia tolerance, swimming performance and its plasticity among fish from habitats with different flow conditions possibly due to the long-term adaptation to the habitat caused by selection stress. The phylogenetically independent contrasts were more powerful than traditional statistical analyses according to the habitat effects in both hypoxia tolerance and swimming performance in this study.
Theory indicates that the anti-predation behavior of a fish species will depend on predation threats in a way that tends to improve the probability of surviving predation. Pale chub (Zacco platypus), a small Asiatic cyprinid, is widely distributed in both high- and low-predation reaches of the Wujiang River. To test whether the anti-predator behavior of pale chub along the Wujiang River varied in fish living in habitats under different levels of predation pressure, we measured spontaneous swimming activity, risk-taking behavior, utilization of shelter and boldness in pale chub collected from both high- and low-predation habitats in the Wujiang River. The fish from high-predation populations showed less spontaneous swimming activity and risk-taking behavior compared with those of the fish from low-predation populations. However, neither utilization of shelter nor boldness exhibited any significant differences between high- and low-predation populations. One reason for this result may be that in their daily lives, the pale chub in the high-predation population primarily respond to the presence of predators by decreasing spontaneous swimming activity and risk-taking behaviors when threatened rather than increasing utilization of shelters and showing less bold behavior. The lack of an increase in shelter utilization and the lack of a decrease in boldness in response to predation pressure may be a compromise to allow foraging and growth.
Abstract Limited aerobic scope (AS) during digestion might be the main constraint on the performance of bodily functions in water‐breathing animals. Thus, investigating the postprandial changes in various physiological functions and determining the existence of a shared common pattern because of possible dependence on residual AS during digestion in freshwater fish species are very important in conservation physiology. All species from slow‐flow habitats showed impaired swimming speed while digesting, whereas all species from fast‐flow habitats showed strong swimming performance, which was unchanged while digesting. Only two species from slow‐flow habitats showed impaired heat tolerance during digestion, suggesting that whether oxygen limitation is involved in the heat tolerance process is species‐specific. Three species from slow‐ or intermediate‐flow habitats showed impaired hypoxia tolerance during digestion because feeding metabolism cannot cease completely under hypoxia. Overall, there was no common pattern in postprandial changes in different physiological functions because: (1) the digestion process was suppressed under oxygen‐limiting conditions, (2) the residual AS decreased during digestion, and (3) performance was related to residual AS, while digestion was context‐dependent and species‐specific. However, digestion generally showed a stronger effect on bodily functions in species from slow‐flow habitats, whereas it showed no impairment in fishes from fast‐flow habitats. Nevertheless, the postprandial change in physiological functions varies with habitat, possibly due to divergent selective pressure on such functions. More importantly, the present study suggests that a precise prediction of how freshwater fish populations will respond to global climate change needs to incorporate data from postprandial fishes.
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