The hadal zone—Earth’s deepest oceanic trenches below 6,000 meters—has long been considered a lifeless abyss. Researchers from the Institute of Hydrobiology (IHB) and the Institute of Deep-Sea Science and Engineering (IDSSE), both under the Chinese Academy of Sciences, together with collaborators from Northwestern Polytechnical University, recently decoded how fish thrive in this extreme realm through two evolutionary pathways while uncovering alarming traces of human pollution in these pristine ecosystems. Their discovery was published in Cell on March 6, 2025.
By analyzing 11 deep-sea fish species sampled across trenches and hydrothermal vents (1,218–7,730 meters), the team revealed two distinct colonization routes: the “ancient survivors” predating the Cretaceous mass extinction and the “new immigrants” dominating post-extinction. This dual-pathway model explains how vertebrates adapted to crushing pressures and near-freezing darkness—a feat partly enabled by a convergent mutation in the rtf1 gene that boosts transcriptional efficiency under extreme pressure.
The study also overturned the long-held trimethylamine N-oxide (TMAO) hypothesis of deep-sea adaption—which claims organic osmolyte levels scale with depth to counteract the destabilizing effects of high hydrostatic pressure—by showing this trend breaks beyond 6,000 meters. While TMAO remains vital for survival at moderate depths, the discovery of pressure-resistant genetic mechanisms in hadal species reshapes our understanding of deep-sea adaptation. This highlights evolutionary flexibility, where organisms employ multiple strategies—biochemical (TMAO) and genetic (rtf1 efficiency)—to thrive under crushing pressures.
More critically, high concentrations of polychlorinated biphenyls (PCBs) were detected in Mariana Trench snailfish livers, confirming synthetic pollutants now permeate Earth’s remotest ecosystems—a stark reminder of humanity’s planetary footprint.
Sampling information and morphological characteristics of 11 deep-sea species. (Graphic: IHB)