Fish adapted to higher temperatures without noticeable sacrifice
As fish evolve to cope with higher temperatures, scientists have assumed that it comes with a price. A new study by researchers from the University of Gothenburg shows that this may not be the case. They see no noticeable changes in the zebrafish included in the study.
Global warming has a major impact on fish, whose body temperature is controlled by the water temperature around them. Unless fish can move to areas with colder water, their ability to cope with higher water temperatures needs to be developed. Adaptation occurs through evolution, with fish that cannot cope with higher temperatures dying and the genes of those that do survive being passed on to the next generation.
Scientists have long assumed that this kind of adaptation to warmer water has other undesirable consequences.
Seven generations
In a new scientific article in Nature Climate Change, researchers from the University of Gothenburg and elsewhere have studied zebrafish bred over seven generations to cope with higher water temperatures. When the researchers studied how well the zebrafish reproduce, grow and perform in the warmer water, they found no negative effects on the fish's physiology.
“Very few research groups have been able to test how fish might evolve in the face of climate change, as it requires thousands of fish and many years of careful experimentation. Our experiment is unique,” says Fredrik Jutfelt, professor of zoophysiology at the University of Gothenburg and senior author of the study.
Surprising results
The study was designed to see if the higher heat tolerance would affect other aspects of the fish's metabolism and ability to survive. Much to their surprise, the researchers discovered that the fish bred to cope with warmer temperatures were also more tolerant of colder temperatures.
“What we normally expect, if we compare species in the wild that are warm adapted, is that they have lower cooling tolerance. But we saw a higher cooling tolerance in the zebrafish, which was surprising,” says Anna Andreassen, a researcher at the Technical University of Denmark and first author of the scientific paper.
As in humans, fish metabolism increases at higher temperatures, causing individuals to require more oxygen. The researchers therefore hypothesised that fish that could cope with higher temperatures would be better at absorbing oxygen from the water. This assumption also failed.
“We can't see that the oxygen actually helps the fish to tolerate the warming in this case, and that is important knowledge, because it goes against previous hypotheses,” says Anna Andreassen.
Does not apply to all species
The ability of tropical zebrafish to adapt to a warming world is impressive but does not translate to other species. Because zebrafish reach sexual maturity after one year, there are frequent generational changes in the species. On average, zebrafish could tolerate 0.04 oC more heat with each new generation, which still is estimated to be too slow to withstand warming in many places. In addition, many fish species have longer generation times, which means that they take too long to adapt to new living conditions in an increasingly warming world.
“Fish living in Nordic waters often have generation times longer than one year and do not have time to adapt when water temperatures rise. So, they have to move to colder water, and if they can't, they face problems such as reduced growth, infections and fewer eggs laid,” says Fredrik Jutfelt.
Looking into the genes
Researchers are now looking further into how zebrafish are affected by warmer waters. The next study will look at how genes change. All cells contain ‘heat shock proteins’ that help other proteins to function normally even in abnormally high temperatures, the genes that control these proteins will be switched off in a group of fish to see how it affects their heat tolerance.
Scientific article in Nature Climate Change: Evolution of warming tolerance alters physiology and life history traits in zebrafish
Contact info: Fredrik Jutfelt, Professor of Zoophysiology at the Department of Biological and Environmental Sciences at the University of Gothenburg, phone: 076-618 48 48, e-mail: fredrik.jutfelt@bioenv.gu.se
