Wikipedia currently lists 62 species in the genus Bipalium. Of these, maybe 5 have become invasive. Given that the nonivasive ones are exclusively found in South and Southeast Asia, that means that around 57 are known from their native habitats, so these habitats cannot be said to be unexplored.
And yet, the invasive ones all have mysterious origins:
- Bipalium kewense: “believed to be native to Southeast Asia”
- Bipalium adventitium: “believed to have been introduced in the last century to the United States from Asia.” Another source: “likely native to Asia like other Bipalium species, although its native range is not currently known.”
- Bipalium vagum: described from Bermuda, but “certainly originated elsewhere.”
- Bipalium pennsylvanicum: when described, “known only from Pennsylvania.” In another source: “native to Asia, possibly Japan, although its native range is not entirely certain.”
- Bipalium nobile: " started to be found in Tokyo by the end of the 1970s. As it was not reported in previous surveys in the area, it is assumed to be introduced from elsewhere"
So that leaves only Bipalium kewense with the specific statement, “native to Vietnam and Cambodia,” although the Penn State Extension article was unhelpfully vague as to which of its sources provided that information.
iNaturalist data do not elucidate the matter much:
Observations of B. pennsylvanicum are exclusively in the United States – no clue as to origins there.
Observations of B. adventitium are also exclusively North American, except for one
observation in India – is India the country of origin, or is it a new invasive there?
B. nobile is interesting because it has been observed in coastal China, Korea, and Japan – possibly a progressive expansion? Was China its country of origin?
B. kewense has been observed worldwide, but has only three observations in northern Vietnam and none in Cambodia – this provides little support for these countries as its place of origin.
B. vagum is the only one for which the observation map seems likely to include the native range.
This seems very strange to me, that here we have a genus with a known native distribution, yet several of its invasive members are known only from their invaded range. Given the large number of species in this genus which are known from their native range – 57 of 62, or 92% – I would think that native populations of the invasive species could be found.