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Keywords:

  • biogeography ;
  • Cypridoidea ;
  • discovery record ;
  • ubiquity

The idea that free-living minute organisms have ubiquitous distributions has been recently revitalized, causing significant controversy. The ubiquitous model predicts that a threshold where ubiquity leaves room to biogeography might exist somewhere along the animal body-size range. In the present study, such a prediction is tested by analysing body-size frequency distribution, species distribution, and local-to-global species ratio at the scale of biogeographical realms in cypridoidean non-marine ostracods, a group with a body-size range in the ubiquity–biogeography (U-B) boundary. Data were gathered for all described extant cypridoidean ostracod species (N = 1761), with body-size recorded for 1134 of them. Although local-to-global species ratios show significant over-dispersal of small-body ostracods for the Palaearctic and the Australasian regions, there are explanations alternative to the ‘Everything is Everywhere’ model that can account for such a result. Indicators of taxonomic structure do not support the hypothesis of a random distribution of cypridoidean species among realms. Nevertheless, the strong biogeography signal occurring at a large scale vanishes at the local scale (country-level within the Palaearctic), and suggests wide dispersion within biogeographical realms. Additional factors, including inconsistent taxonomic criteria for species recognition, uneven sampling effort, and an excess of ‘single-report’ occurrences, have been identified too as potential distorters of the observed patterns. Taxonomic harmonization, open databases of biogeographical data, and better ecological information are suggested as critical goals that need to be achieved for further understanding of ostracod global distribution patterns. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 409–423.