Icelandic fish 'hot tubs' reveal genetic adaptations to warming—and unexpected overeating
New research, led by the University of Glasgow reveals that adaptation to warmer waters causes fish to display a range of behavioral and biological differences, including obesity, overeating and lower metabolism.
The study, "Parallel adaptation to geothermally-warmed habitats due to common structural variation and functional developmental pathways" is published in Nature Communications.
The findings have important implications for our understanding of the potential impact of global warming on aquatic life.
To carry out the study, the research team headed to Iceland, where they used the country's naturally warm geothermal waters as a model for the impact of climate change.
There, the team studied the threespine stickleback, a species of fish that commonly lives in both Iceland's cooler, average-temperature waters and its geothermal waters, where indigenous fish have had to adapt to living in unusually warm conditions. These warm-water conditions reach up to 27.8°C (82°F) and have been described by the researchers as like "fish hot tubs."
Warmer waters changed feeding and fat storage
The researchers studied the behavior, appearance and genetics of the fish and found clear differences between those living in cooler and warmer waters. The threespine sticklebacks living in warmer waters were hyperphagic, a condition that causes excessive hunger and overeating.
The geothermal fish also put on more fat stores in the summer in preparation for winter and hold onto fat more efficiently during periods of low food availability.
Shared pathways beneath different DNA changes
In addition to differences in appearance and behavior, the warm-water fish also had a range of DNA adaptations. These genetic changes varied across different groups of geothermal fish, highlighting the unique and unpredictable nature of adaptation to specific environments.
However, the research team did discover cellular differences shared by all warm-water groups. A key genetic change in the mapk pathway, a signaling mechanism with implications for growth factors and stress, was found among all fish that had adapted to living in warmer waters but not in their cold-water cousins.
The geothermal fish harbor a flipped region of the genome (inversion) that is absent in cold-adapted fish, allowing them to live in geothermally warmed waters. Both the mapk pathway and the inversion work together to facilitate adaptation to warming environments.
Climate pressures may outpace migration
Dr. Kevin Parsons, co-lead author of the study from the University of Glasgow, said, "Our study is the first to show the specific genetic changes caused by adapting to warmer waters, offering key insights into the impact that climate change could have on aquatic ecosystems.
"We discovered that while the detailed DNA adaptations to warmer waters vary between fish populations, the overall impact of these changes was similar in all warm-water-adapted fish, with changes to their body size, appetite and metabolism."
Increasing global temperatures represent one of the biggest threats to wildlife, particularly cold-blooded animals that rely on their environment to regulate their body temperature. While some fish in the oceans may be able to migrate to avoid the impact of warmer waters, species in landlocked waters will either be forced to adapt or go extinct.
Dr. Matthew Brachmann, co-lead author of the study from the University of Glasgow, said, "Climate change is already causing rapid temperature increases in our waters, forcing some aquatic life to adapt to survive. However, predicting the extent and impact of adaptation remains challenging, as the extremes of global warming have not yet been experienced by most species.
"By focusing on indigenous fish living in different natural water temperatures in Iceland, we have been able to show the real-world impact of adapting to warmer waters, with adapted populations showing clear physical, behavioral and genetic changes."
Publication details
Matthew K. Brachmann et al, Parallel adaptation to geothermally-warmed habitats due to common structural variation and functional developmental pathways, Nature Communications (2026). DOI: 10.1038/s41467-026-75785-0. On bioRxiv (2025). DOI: 10.1101/2025.02.17.638600
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Citation: Icelandic fish 'hot tubs' reveal genetic adaptations to warming—and unexpected overeating (2026, July 29) retrieved 29 July 2026 from https://phys.org/news/2026-07-icelandic-fish-hot-tubs-reveal.html
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