Sea-ice retreat is rapidly transforming the Southern Ocean (SO), yet consequences for benthic biodiversity remain poorly resolved. Here, we show that declining sea ice is directly linked to reduced species diversity in benthic isopods, a dominant SO fauna (Pearson's r = 0.56). We analysed 214 isopod species from 78 genera in the Atlantic sector using a 14-year dataset integrating satellite and in situ observations across 60-77° S and 403-4976 m depths. Of these 214 species, 32% (68) occur exclusively at high sea-ice stations, and among them 53% are new to science and 75% are rare. Redundancy analysis (variance explained 15.3% and 6.5% for axes 1 and 2; p ≤ 0.002) and generalized linear models (p = 0.0025) identify sea-ice cover as a primary driver of species richness (Pearson's r = 0.56) and community composition. Declining sea ice is associated with lower diversity and shifts in assemblage structure, indicating strong coupling between cryosphere change and deep-sea biodiversity. Given that the SO is a global biodiversity hotspot, these findings reveal unquantified vulnerability of seafloor communities. With continued sea-ice decline projected, irreversible biodiversity losses are likely, underscoring an urgent need for climate mitigation as impacts reach the farthest marine frontiers.
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