Increased Sea-Surface Temperatures Decrease Hypoxia Tolerance of Coral Reef Fishes, Ostorhinchus doederleini and Pomacentrus moluccensis

Since tropical organisms live in relatively constant environmental conditions, they are especially sensitive to change in global temperatures. Nilsson et al. (2010) studied the effects of increased sea-surface temperature on the resting oxygen consumption and the maximum oxygen uptake rate (aerobic scope) of Doederlein’s cardinalsh (Ostorhinchus doederleini) and lemon damselsh (Pomacentrus moluccensis) in the Great Barrier Reef, Australia. Their results showed that the increased sea-surface temperatures decreased the fishes’ hypoxia tolerance—the ability of fish to survive in oxygen-deficient conditions—and increased their basal metabolic rates. Even after prolonged exposure to high surface temperatures, the fishes’ basal metabolic rates were consistently elevated, indicating that they did not acclimate to the change in temperature. Thus, coral reef fishes will be severely affected by an increase in global temperatures unless they develop phenotypic plasticity, which could enable them and their progeny to survive in warmer temperatures. .— Sachi Singh
Nilsson, G. E., Ostlund-Nilsson, S., Munday, P. 2010. Effects of elevated temperature on coral reefshes: Loss of hypoxia tolerance and inability to acclimate. Comparative Biochemistry and Physiology, Part A 156, 389393.

Previous studies have shown that resting metabolic rates of five species of coral reef fishes increased when the fishes were exposed to temperatures 2–4 °C higher than the average summer surface temperatures; this consequently, led to a decline in the aerobic scope of the fishes. An increase in resting metabolic rate can also reduce the fish’s hypoxia tolerance, because a fish with a higher metabolic rate needs more oxygen to maintain itself. The authors kept Ostorhinchus doederleini and Pomacentrus moluccensis fishes at 32 °C—which was 3 °C higher than the average summer sea-surface temperature—for 7–22 days in outdoor aquaria at Lizard Island in the Great Barrier Reef, Australia; they studied the effects of the elevated surface temperatures on the resting metabolic rate, hypoxia tolerance and aerobic scope of the fishes. Their results showed the the adult fishes could not acclimate to the change in sea-surface temperature, as their basal metabolic rates continued to rise persistently even after 7 days of acclimation to 32 °C. In fact, the Ostorhinchus doederleini fish started to die after 8 days at 32 °C, which indicates that the cardinal fish are particularly sensitive to rises in sea-surface temperatures. The authors reasoned that as the coral fishes live in relatively stenothermal environments, they have probably lost their ability to acclimate to warmer surface temperatures. The increased temperatures also led to a decrease in hypoxia tolerance and aerobic scope of both the fishes. Since the ocean can be severely hypoxic at night, the decrease in hypoxia tolerance could force fish out of their protected refuge in the coral to look for food and consequently, make them prone to predation. This could have devastating effects on the population size and distribution of these coral reef fishes unless they develop a phenotypic mechanism to acclimate to elevated sea-surface temperatures. 

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