Phreatogammarus

P. fragilis was the first species to be described, when Charles Chilton named it Gammarus fragilis in 1882; it was moved to the genus Phreatogammarus by T. R. R. Stebbing in 1899, becoming the type species for the group.[2] It lives in wells and groundwater, mainly in Canterbury and Nelson, and is not known to occur on the North Island.[1]

Chilton's second species, P. propinquus was described in 1907 from a single specimen collected near the summit of Mount Anglem on Stewart Island/Rakiura. Chilton originally thought this specimen was a male, but later realised that it was in fact female.[2] Other specimens have been collected near sea level, but have not been described in the literature.[1]

When he realised that he had mis-sexed his Mount Anglem specimen of P. propinquus, Chilton also realised that the material he had described under that name from Rona Bay, Wellington, was a third species, which he named P. helmsii in 1918.[2] It is a coastal freshwater species, only occurring further inland near Hamilton on the Waikato River system.[2] It is widespread, occurring in many parts of both the North and South Islands.[2]

Few scientists have worked on the genus Phreatogammarus after Charles Chilton, and it was not until 2003 that a fourth species was described. M. A. Chapman, in a revision of the genus, established a new species, P. waipoua for populations of Phreatogammarus living in two forests in Northland Region.[2]

There are at least two further undescribed species which live in Canterbury, and many collections from Nelson have not yet been examined.[1]