Scientists collected samples of fish living in the Mariana Trench in the Pacific Ocean and in trenches in the Indian Ocean. (Image credit: Han Xu et al. (2025) Evolution and genetic adaptation of fish to deep-sea regions. Cell, Volume 188, Issue 5, 1393–1408.e13. Available at: https://doi.org/10.1016/j.cell.2025.01.002 (CC BY 4.0))
The researchers note that fish surviving in extreme deep-sea conditions developed the same genetic mutation, despite their separate evolution at different times.
Scientists have also found industrial chemicals in fish and in sediments from the Mariana Trench, suggesting that human-made pollutants can penetrate into the deepest layers of Earth's ecosystem.
Deep-sea fish have developed unique adaptations to survive in high pressure, low temperatures, and near-total darkness. These species adapt to extreme conditions through specialized skeletal structures, altered circadian rhythms, and either vision attuned to low light or relying on non-visual senses.
In a new study published March 6 in the journal Cell, scientists examined the DNA of 11 fish, including lipiformes, moor eels and lizards, that live in the extreme water zone — a region at depths of about 19,700 feet (6,000 meters) and below — to better understand how they evolved in such harsh conditions.
Researchers used manned and remotely operated vehicles to collect samples from 3,900 to 25,300 feet (1,200 to 7,700 m) below the surface of the Mariana Trench and other trenches in the Indian Ocean.
Tracing the evolution of deep-sea fish, the researchers conducted an analysis that showed that the eight groups of fish species studied entered the deep-sea environment at different times: the earliest probably appeared in the early Cretaceous period (about 145 million years ago), others reached it in the Paleogene period (66 to 23 million years ago), and some species as recently as the Neogene period (23 to 2.6 million years ago).
Despite the different times it took the deep-sea fish to adapt to their new home, all the fish studied that lived at depths of 9,800 feet (3,000 meters) showed the same type of mutation in a gene called Rtf1, which regulates how DNA is encoded and expressed. The mutation arose at least nine times in the deep-sea fish lineages at depths of 9,800 feet, study author Kun Wang, an ecologist at Northwestern Polytechnical University, told Live Science in an email.
This suggests that all of these fish evolved the same mutation independently as a result of living in the same deep environment, rather than as a result of sharing a common ancestor, highlighting how deeply the deep sea environment shapes the biology of these species.
“This study demonstrates that deep-sea fishes, despite coming from very different branches of the fish tree of life, have evolved similar genetic adaptations that allow them to survive the harsh conditions of the deep ocean — cold, dark, and high pressure,” Ricardo Betancourt, an ichthyologist at the University of California, San Diego, who was not involved in the new study, told Live Science in an email.
It's an example of convergent evolution, where unrelated species independently evolve similar traits in response to similar conditions. “It's a powerful reminder that evolution often reuses the same limited set of solutions when faced with similar problems — in this case, adapting to the extreme conditions of the deep sea,” Betancourt said.
The expeditions also revealed anthropogenic pollutants
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