Can Fishes Adapt To Our Warming Oceans?

Low potential for evolutionary rescue from climate change in a tropical fish (2020) Morgan et al., PNAS, https://doi.org/10.1073/pnas.2011419117

The Crux

As the planet warms thanks to climate change, the massive bodies of water that are our oceans grow hotter. Since they’re larger, and much poorer conductors of heat, they don’t tend to vary in temperature as much as the land does, which means many species will have to get used to longer, warmer periods.

If species can adapt to hotter temperatures through thermal acclimation, ecosystems may not be too harshly affected. However if they’re unable to adapt, marine ecosystems may undergo rapid changes as they lose native species. Today’s researchers looked at a key study species – the zebrafish – in order to figure out how well fish can respond to increasing temperatures.

What They Did

The team sourced 5,000 zebrafish (Danio rerio) from different sites in West Bengal, India. After the transport and acclimation to their new home in Norway, 1,800 of these fish were selected for the experiment. Initially, they were allowed to interbreed freely in order to decrease the chance of spatial bias. By doing this, the researchers could be sure that there weren’t differences between fish purely as a result of the area they came from.

The resulting generations were then tested for their thermal tolerance. This involved placing them in tanks and slowly raising the temperature until they lost their equilibrium and passed out. The fish were then split into four different ‘lines’. One line was consistently made up of the offspring of fish which had high thermal tolerances (up-selected fish), another was made up for fish which had lower tolerances (down-selected). These two lines (plus a third control line made up of mixed response fish) were held at 28 degrees the entire time.

A fourth line made up of up-selected fish were additionally acclimated to higher temperatures (32 degrees) before being tested for their thermal tolerance over each generation.

Did You Know: Fish Redistribution

Rising temperatures are leading to species shifting further poleward in every ecosystem. One might think that since the sea offers fewer barriers than the land, this climate driven range-shifting might proceed at a pretty uniform rate across all species. But just because a species can move unmitigated, doesn’t mean it will. We’re already seeing different rates of migration among fairly similar species, meaning that as a warming climate drives species to new grounds, a global redistribution of life occurs.

What They Found

The up-selected acclimated lines consistently showed the highest thermal tolerance, especially at the first generation. The up-selected line also had significantly higher thermal tolerance than the down-selected fish, and this tolerance increased over the six generations, at a higher rate than that of the acclimate fish. As you’d expected, the down-selected fish had lower thermal tolerance than the other groups.

This might seem promising, but when the acclimated fish were placed at the same temperature as the up-selected fish for a longer period, their thermal tolerances didn’t differ. This suggests that acclimation didn’t affect the actual evolution of the fish at all.

Lead author Dr. Rachel Morgan sampling zebrafish (Image Credit: Fredrik Jutfelt, CC BY 2.0)

Problems

One of the only problems with this incredibly comprehensive study is the classic lab versus field conundrum. The genetic mixing brought about by allowing fish from different sites to interbreed accounts for spatial bias, BUT it’s unlikely to be how the species breeds in the wild. Likewise, the short generation times make zebrafish an excellent study species, but they’re not typical of most fish species. This study is a great indication of how species may adapt in the future, but it’s hard to know how accurately it can predict ecosystem changes in the future.

So What?

There was evolution towards a higher thermal tolerance here, however it was very slow, and the particularly slow increase in thermal tolerance of the acclimated species indicates that there may be what the authors call a ‘hard ceiling’ for evolution of thermal tolerance. This is worrying, since there is no evidence of a hard ceiling when considering the temperature increase brought about by climate change.

Studies like this paint a worrying picture of the future, but at least it prepares us somewhat. Knowing that fish species can only tolerate so much before having to find cooler temperatures gives us an indication of what marine ecosystems might look like in the future, and allow us to prepare and mitigate the resulting ecosystem shifts.


Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who is currently sitting on an uncomfortably warm train wishing he could get some of this temperature acclimation he keeps reading about. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.

Title Image Credit: Per Harald Olsen, CC BY 2.0, Image Cropped

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