Phreatogammarus is a genus of amphipod crustaceans that are endemic to New Zealand. It is the only genus in the family Phreatogammaridae, and contains four species, three of which were described by Charles Chilton.
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. It lives in wells and groundwater, mainly in Canterbury and Nelson, and is not known to occur on the North Island.
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. Other specimens have been collected near sea level, but have not been described in the literature.
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. It is a coastal freshwater species, only occurring further inland near Hamilton on the Waikato River system. It is widespread, occurring in many parts of both the North and South Islands.
Few scientists 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.
- M. A. Chapman (2004). "A revision of the Phreatogammaridae (Crustacea, Amphipoda) of New Zealand. Part 2: Phreatogammarus fragilis and P. propinquus". Journal of the Royal Society of New Zealand. 34 (1): 59–79. doi:10.1080/03014223.2004.9517763.
- M. A. Chapman (2003). "A revision of the freshwater amphipod genus Phreatogammarus in New Zealand. Part 1: a re-description of P. helmsii Chilton, 1918 and a new species from Northland". Journal of the Royal Society of New Zealand. 33 (3): 633–661. doi:10.1080/03014223.2003.9517750.