Benthic hydroids are an important component of the Antarctic benthic ecosystem. This is characterised by extreme environmental conditions that undoubtedly affect organism distribution. In some cases, these conditions are related to depth (e.g. anchor ice or iceberg scouring). The Antarctic depth profile has special characteristics that reflect the depression of the continental crust by isostatic loading as well as glacial erosion during ice sheet expansion. Indeed, the Antarctic continental shelf is much deeper than others elsewhere in the world. This fact, along with the dynamic ice formation, affects a priori the bathymetric distribution of Antarctic benthic hydroids. This was first studied by Stepanjants in 1979, and later by Peña Cantero, in 2004, who summarised all available bathymetric information for Antarctic benthic hydroids, and established a series of bathymetric groups according to ecological features. The aim of this study was to review and update the present knowledge of bathymetric distribution of Antarctic benthic hydroids with data gathered in recent years in order to define hydrozoan bathymetric assemblages and depth-indicator species. To this end, all the valid records from the Antarctic (including the Scotia Arc) were compiled. Using similarity matrices (Sørensen coefficient), hierarchical clusters with SIMPROF test were performed. The results of the present study provide the division of the bathymetric profile into three main zones. The first one, which corresponds to the Shelf, extends from 0 to 700 m, and is clearly characterised by Eudendriidae and Campanulinidae. The second one, named Slope, associated with the continental slope, extends from 700 to 1500 m, and is characterised by a few exclusive species, like Halisiphonia prolifica Peña Cantero, 2014. Finally, the third zone, here named Abyss, extends beyond 2000 m, and includes a few species [e.g. Clathrozoella abyssalis Peña Cantero, Vervoort & Watson, 2003 (3083 m)]. The results also indicate a further division of the Shelf into three sub-zones: the anchor ice, the upper shelf and the lower shelf. The results show distinct boundaries between divisions, which could be linked to environmental factors, some of them likely related to the influence of ice.
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We would like to thank the three anonymous reviewers for their careful reading of the manuscript and their useful suggestions. We also acknowledge Stephen Archival and Adrian Serrano for their carefully revising the English Language. All of them improved the quality of this work.
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