The polyp hydra is ubiquitous in freshwater and is highly variable, with many species names assigned to different strains. Types of hydra do fall into four morphologically recognizable groups but many of the species determinations are confusing. To assess the diversity of hydra we collected 101 strains from six continents and built a phylogeny using three genetic markers. Each of the four well-defined groups of species represents a clade in our phylogeny. The green hydra group diverged first, followed by the braueri group and finally the sister groups vulgaris and oligactis. Each of eight species easily definable by morphological criteria represents a distinct clade in our phylogeny. Hydra of two clades, the green and the vulgaris hydra, are found on all continents (except Antarctica) and many islands, whereas hydra of the other two groups (braueri and oligactis) are restricted to the Northern Hemisphere. Our best estimate of the time of origin of hydra is about 60 Ma, long after the breakage of Pangea into northern and southern landmasses. Hydra appear to have diversified in the Northern Hemisphere, and their current diversity is greatest here. Two species were then able to disperse to the Southern Hemisphere, perhaps due to their thermal tolerance.
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