| Summary: | Global diversity of inland water cnidarians is low, containing <40 species belonging to phylogenetically distinct groups representing independent invasion events: the common and cosmopolitan hydras (12-15 species); the sporadically occurring freshwater medusae (6-16 sp.); the Cordylophorinae (2 sp.); the parasitic Polypodium (1 sp.); the medusae occurring in saline lakes (4 sp.). Freshwater cnidarians inhabit nearly all types of freshwater on all continents (except Antarctica), but only a few species have cosmopolitan distributions. Due to uncertainty in species knowledge, fine scale regions of endemicity are not yet clear.
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