Upper thermal and hypoxia stresses affect the metabolism of fishes and these limits are likely, in turn, to affect how species re-distribute across the globe in response to climate change. One key question is how invasive species gain the upper hand in novel environments over the indigenous residents that have evolved in harmony with the environment. Perhaps the answer is hidden in how animals cope with extreme climate events that induce rapid changes in temperature and oxygenation. Rick Stoffels and his team at La Trobe University, Australia, used an experimental physiology approach to understand the physiological limits of the global invader – the mosquitofish (Gambusia holbrooki) – and a threatened native freshwater species – the pygmy perch (Nannoperca australis) – when they both experienced high temperatures and hypoxic stress.
The team used respirometry to measure the routine and the standard metabolic rate – required to maintain essential bodily functions – of mosquitofish and pygmy perch. The fishes then experienced a period of hypoxia, before the team reaerated the water and measured the metabolic cost of recovery from hypoxia, to learn more about the fishes’ capacity to supress metabolism when oxygen is scarce. Repeating the procedures at low and high temperatures, the team was able to build a profile of the fishes’ responses to hypoxia under different thermal regimes.
They discovered that the invasive mosquitofish exhibited a greater tolerance of hypoxia than the native pygmy perch. More importantly, the hypoxia tolerance of the mosquitofish was not as compromised as that of the perch at higher temperatures. The team reasoned that the mosquitofish's greater hypoxia tolerance was due to its four times greater ability to suppress metabolism than the perch and this capacity was also maintained at the higher temperatures. Additionally, the standard metabolic rate of the mosquito fish increased less at higher temperatures than it did in the perch. In other words, the perch had a harder time meeting their metabolic demands at higher temperatures during hypoxia.
Having found that the mosquitofish appear to be more robust than the indigenous perch when oxygen is scarce, the team wondered how this strength gives mosquitofish the edge in the same habitats. Stoffels and his colleagues monitored the dissolved oxygen and temperature in habitats shared by the two species. They discovered that the dissolved oxygen throughout the water column drops below the threshold where both species can support their standard metabolic rate when heat waves occur. However, the mosquitofish's ability to suppress its metabolism allows it to thrive in conditions where the perch would suffocate. In addition, high water temperatures throughout the summer will further favour the mosquitofish.
Thus, the invading mosquitofish will have an advantage over the pygmy perch as the frequency of heat waves and hypoxic episodes increases. However, this is merely one case of a species re-distribution where global climate change may tip the balance in favour of an imposter and the reality for entire ecosystems will be far more complex. Will invasive species still have the edge if the indigenous species are already well adapted for naturally occurring, physiologically harsh extremes? It's hard to say, but currently the march of the mosquitofish through Australia's freshwater habitats seems inexorable.