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The Arctic and Antarctic share many oceanographical features but differ greatly in their geological histories. These divergent aspects lead to similarities and differences between the sets of species inhabiting the poles. However, the patterns are not unambiguously homogenous throughout the tree of life. For the first time, Hydrozoa (Leptothecata and Anthoathecata) is used as a model group to study patterns of diversity, distribution, bathymetry and life history strategies between the polar regions. The analyses are based on a comprehensive literature survey of hydrozoan records. Subtle differences in species richness and contrasting values of endemism are found between the Antarctic (252 species and 58% endemics) and Arctic (233 species and 20% endemics) regions. Shared trends include the lack of a medusa stage in most of the representatives, a high percentage of rarity (Arctic: 49%; Antarctic: 63%), and few common species (18% in both regions). A few species (Grammaria abietina, Obelia longissima and Paragotoea bathybia) and genera (Bouillonia and Gymnogonos) might be tentatively considered bipolar, but further molecular investigation is recommended. The bathymetric distribution mirrors the geomorphological characteristics of each region. The highest species richness occurred in the continental shelves of both polar regions. Updated inventories from each polar region are provided as supplementary material. The present work establishes a fundamental step towards an integrated bipolar framework for the study of diversity and ecology of polar regions, laying the foundation for future approaches on a wide array of topics, from origin and diversification, to changes in the distribution of polar biota.