Bathynellacea are found in the groundwater nearly all around the globe. They are absent in the Antarctic and in the Northern Hemisphere in those areas that had been covered by ice during the last glaciation. Recolonization has been observed at a few places but has not progressed very far beyond the old ice frontier. Bathynellacea are not known from Central America and are also absent from volcanic islands and several islands of continental origin (e.g., New Caledonia, Fiji, Caribbean islands).
Both families have a worldwide distribution, but Ba-thynellidae are more common in temperate regions of the world than in the tropics. Parabathynellidae have a more even distribution but do not reach as far north as Bathynellidae in the Northern Hemisphere. All recolonizers of previously ice-covered areas belong to the Bathynellidae.
There are two hypotheses to explain the distribution of Bathynellacea. Both agree on marine ancestors as the starting point of the adaptational process and on plate tectonics as a major factor. The first hypothesis maintains that the ancestors of Bathynellacea invaded fresh surface waters and that from their larvae the Bathynellacea arose to become inhabitants exclusively of the groundwater. Subsequent spread in the groundwater itself led to today's worldwide occurrence. According to the second hypothesis, as a first step a marine ancestor became adapted to an interstitial life in littoral sands and in a second step had to switch to freshwater conditions as a result of marine shoreline regression. Multiple such invasions of the groundwater caused by repeated sea-level changes at different geological times together with plate tectonics led to the currently observed worldwide distribution.
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