Tiny Killifish Show How Animals Adapt to Changing Environment

In a warming world where oceans are becoming more acidic, polar ice is melting, and extreme weather is becoming the new norm, numerous plant and animal species are going to have to learn to adapt. And the tiny killifish, a new study shows, hints at how animals may deal with their changing environments.

Fundulus heteroclitus, a species of killifish commonly known as the mummichog, can be found along the Atlantic coast of North America, according to Animal Diversity Web (ADW). And while tidal creeks and freshwater lagoons are their habitat of choice, they also swim in saltwater marshes and estuaries where ocean tides flow into.

The fish's upper limit for salinity is 106-120.3 parts per thousands (ppt), while the average salinity of seawater is 32-33 ppt.

So how is it that this tiny fish can survive in both freshwater and saltwater? According to a new study published in the journal Molecular Biology and Evolution, the answer lies in a phenomenon known as phenotypic plasticity. Certain organisms can change their observable characteristics in response to their environment.

"They have to be able to make those changes very rapidly, and to do that it [killifish] physically changes the structure of its gills," Joe Shaw, a professor at Indiana University's School of Public and Environmental Affairs, told Nature World News.

"It's one way organisms are able to evolve to live in rapidly changing environments," he added.

Phenotypic plasticity, the researcher explains, can also be seen in certain microcrustacea that, when a predator is around, change their appearance to appear more "evil" to avoid being eaten, and in moths and butterflies that alter their color patterns as a means of camouflage.

In the study, Shaw and his co-author Tom Hampton led a team in studying the Atlantic killifish and how it modifies its gills to live in both freshwater and seawater. Specifically, by activating or deactivating channels that secrete salt. While researchers already know certain mechanisms involved in salt balance, the new findings shed light on the functional and structural changes killifish gills undergo to transition from one environment to the other.

Gene expression that normally enables this phenotypic plasticity is inhibited by arsenic, which can be found in saltwater. Shaw's team used this fact to identify which genes are responsible for shifting between the two types of water, and found that there is unusually accurate and precise control of these genes.

This not only suggests that killifish evolved to have certain individual genes, but that natural selection also targeted the regulatory networks that facilitate the phenotypic plasticity response.

The killifish's ability to adapt to salt and fresh water suggests that they might be able to handle any other environmental changes that come their way as a result of climate change. Not only that, but their resolve may just be one example of how all animals learn to adjust to a changing environment.

"The spin on climate change is the idea that we're understanding some basic mechanisms of how this ability to change phenotype on a dime has evolved," Shaw said. "Understanding these processes is important."

What's more, these remarkable fish can withstand warm waters - above 34 degrees Celsius - more so than any other. ADW says it would take a little over an hour before the killifish succumbs to heat shock. It can also tolerate temperature fluctuations from six Celsius to 35 Celsius. This hardiness is just another example of why the killifish will most likely handle climate change better than most without so much as a blink.