1. With few exceptions, copepods dominate over other crustacean and non-crustacean invertebrate groups in ground water. They have colonised a vast array of habitats in continental ground waters, where they are represented by over 1000 species in six orders: Platycopioida, Misophrioida, Calanoida, Cyclopoida, Harpacticoida, Gelyelloida. However, members of only the last four orders entered genuine fresh ground water.2. Stygobiotic copepods show a wide range of morphological and physiological adaptations to different groundwater habitats. They frequently exhibit simplifications in body plans, including reductions in appendage morphology, which is regarded as a result of paedomorphic heterochronic events.3. Copepod distributions at small spatial scales are most strongly affected by habitat type and heterogeneity, with sediment grain size and availability of organic matter being important habitat characteristics. Large-scale spatial distributions (biogeographical) are mainly related to past geological, climatic and geographical processes which occurred over medium to long time scales.4. Such processes have affected colonisation patterns and diversification of copepods in ground water, leading to a number of phylogenetic and distributional relicts and a high degree of endemism at different taxonomic levels. This is reflected in the composition of groundwater copepod communities characterised by distantly related species in the phylogenetic tree.5. Copepods dominate the species richness of groundwater fauna in all regions and on all continents where more than cursory surveys have been carried out, i.e. in Europe, North and Latin America as well as in Australia.6. Species-specific microhabitat preferences, high proportions of local endemics, high proportions of phylogenetic and distributional relicts, and higher-level taxonomic diversity are all factors suggesting that copepods are a useful indicator group of overall species richness for defining conservation priorities in ground water.
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