The aquatic “earthworms” have apparently been derived from terrestrial ancestors and have secondarily adopted aquatic habits. They should not be confused with a number of families of small Oligochaeta collectively known as the “microdrili” and which are primarily aquatic animals.

Two native species, Diporochaeta aquatica and Pontodrilus lacustrus, have been collected from deep water, the former in Lake Manapouri and the latter in Lake Wakatipu. They apparently swim freely in the lake waters and feed on organic matter suspended or sinking through the water. The apparent absence of similar aquatic worms from other New Zealand lakes may be due only to lack of suitable sampling.

Four species, Decachaetus violaceus, Diporochaeta chatamensis, Eodrilus paludosus, and Perionyx helophilus, have been collected from swamps and bogs. The first two have been recorded only once, whilst the remaining two have been collected from various habitats including swamps. It cannot therefore be said that any native species are undoubtedly confined to swamps. The anaerobic conditions typical of subsurface horizons of swamps and bogs would prevent earthworms from living anywhere but very close to the surface.

Much work on the distribution and manner of dispersal of earthworms has been based on the assumption that they are unable to survive immersion in salt water, even for a short period, and consequently cannot have been transported across the sea by rafting. A number of native earthworms, however, live in the intertidal zone of seashores under stones and debris or in brackish water. Of seven species collected from such habitats, five (Microscolex aucklandicus, M. campbellianus, M. macquariensis, Rhododrilus cockaynii, and R. leptomerus) are found under stones and logs and in the soil under tussocks, forest, or other vegetation, as well as in the intertidal zone. They are therefore well fitted for the colonisation of the isolated sub-Antarctic islands on which they are found as the dominant section of the earth-worm fauna. Pontodrilus matsushimensis var. chathamianus, a Chatham Island seashore worm, is a local variant of a species widely distributed on the shores of Pacific islands.