The scientists analyzed samples of fish living in the Mariana Trench in the Pacific Ocean and trenches in the Indian Ocean. (Image credit: Han Xu et al. (2025) Evolution and genetic adaptation of fish to deep-sea environments. Cell, Volume 188, Issue 5, 1393–1408.e13. Retrieved from: https://doi.org/10.1016/j.cell.2025.01.002 (CC BY 4.0))
Researchers say fish surviving in extreme deep-sea environments have developed the same genetic mutation, despite evolving separately and over different timeframes.
Scientists have also found industrial chemicals in both fish and the soil of the Mariana Trench, indicating that human-made pollutants can reach the deepest levels of the planet's environment.
Deep-sea fish have developed unique adaptations to survive in conditions of high pressure, low temperatures, and near-total darkness. These species have adapted to the harsh conditions with specialized skeletal structures, altered circadian rhythms, and either vision tuned to low light or relying on other 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 deep-sea zone — an area of water about 19,700 feet (6,000 meters) and below — to better understand how they adapted to such extreme conditions.
Researchers used manned submarines and remotely operated vehicles to collect samples at depths ranging from 3,900 to 25,300 feet (1,200 to 7,700 m) below the surface of the Pacific Ocean's Mariana Trench and other trenches in the Indian Ocean.
By tracking the evolution of deep-sea fish, the researchers conducted an analysis that showed that the eight groups of fish 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 more recently, in the Neogene period (23 to 2.6 million years ago).
Despite the differences in how long it took the deep-sea fish to adapt to their environment, 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 is responsible for coding and expressing DNA. This 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-sea environment, rather than sharing a common evolutionary ancestor, highlighting how deeply deep-sea conditions shape the biology of these species.
“This study shows that deep-sea fishes, despite originating 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 characteristics 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 challenging conditions of the deep sea,” Betancourt added.
The expeditions also revealed anthropogenic pollutants
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