P. cavaticus is unique among British Isopods in being a specialist stygobite (i.e. entirely associated with subterranean aquatic habitats). It appears to be restricted to southern areas, with the majority of records from carboniferous limestone cave systems in the Mendip Hills and South Wales.

It is typically recorded from underground streams, pools and wet surfaces in limestone caves and mines, where it tends to occur in small numbers in water films covering flowstones, within small seepages or in shallow pools.