Climate change is forcing all sorts of species to change their ways. For example, certain tropical lizards have swiftly learned to deal with the heat, shape-shifting plants are physically changing their appearance, and crabs are going into survival mode. Now, new research shows that when it comes to water fleas, they are using genetics to adapt to climate change.
The water flea Daphnia, a zooplankton organism typically found in shallow ponds and lakes, normally reproduces asexually by cloning itself. But in difficult living conditions - during food shortages or heat waves, for instance - these fleas switch to a different type of procreation: they mate and lay dormant eggs. These eggs are in fact encapsulated embryos that are resistant to harsh conditions. Remarkably, a dormant egg can remain in the sediment of a pond for dozens of years and still be able to hatch.
So biologists from KU Leuven, Belgium, compared "resurrected" water fleas - hatched from 40-year-old eggs - with more recent specimens to find out how they evolved amidst a changing climate.
"When water fleas reproduce asexually, their offspring is genetically identical to the mother. But when they mate, this results in genetic variation. The genetically fittest water fleas - the ones that are best adapted to the environment - survive and lay dormant eggs," biologist Aurora Geerts explained in a statement.
"When we hatch the dormant eggs of water fleas from the past and compare them with the contemporary population," she continued, "we can reconstruct the evolutionary changes that occurred in that population and examine how they have adapted to the rising temperature of the water in which they live."
The biologists decided to use dormant eggs from Felbrigg Hall, a shallow lake in England, where both the water flea population and changes in lake temperature are well-documented. It's also a site that has seen some serious climate changes. For example, over a period of 40 years the average temperature near Felbrigg Hall has risen by 1.15 degrees Celsius, and the number of heat waves has tripled.
"This causes stress to animals that live in such shallow water," Geerts said.
To take a closer look, the team collected dormant eggs from sediment layers matching the periods 1955-1965 and 1995-2005. They then "resurrected" the eggs by hatching them and examined the heart tolerance of the resulting populations from these two time periods, by scoring the temperature at which the water fleas lost motor function and fainted. It turns out the critical maximum temperature for activity for the water fleas from the recent sediment layer is half a degree more than 40 years ago.
In another experiment, the biologists examined whether current populations of the water flea Daphnia can genetically adapt to higher temperatures. For two years, they exposed a population of water fleas to two temperature treatments: ambient temperature and ambient +4°C.
After resurrecting and hatching these fleas, they measured their heat tolerance to that of water fleas under standardized lab conditions.
"For the water fleas that had been exposed to a heated environment the critical temperature for activity was on average 3.6 degrees higher than for water fleas from the control group," Geerts said.
This suggests that water flea populations can adapt quite rapidly to rising temperatures. While previous studies have shown concern for various animal species around the world in the face of climate change, this study is the first to show that some populations can adapt and already have adapted to higher temperatures and increased heat wave frequencies - two results of climate change.
While the water flea has been successful in evolving to stand the heat, the capacity for genetic adaption is, however, not enough to guarantee success, Geerts adds.
"Climate change may have an impact on other factors as well," she said. "The water flea might be exposed to more enemies, less food, or an increased sensitivity to parasites. But our results show that we need to take into account the evolutionary dynamics of a species if we want to predict how it will respond to climate change."
The results were published in the journal Nature Climate Change.
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