The nematodes parasitic in frogs in southern Western Australia are closely related to those found in South Australian hosts, with two cosmocercoid species common to both regions and at least two of the three Western Australian species of Parathelandros forming pairs with species in South Australia.
The host distribution of the Western Australian species of Parathelandros suggests a host specificity, at generic level, with one species restricted to the genus Hyla, one largely restricted to the genus Helioporus and one, more widespread, in the genera Neobatrachus, Helioporus and Limnodynastes.
Studies of the hosts have demonstrated that the four species of Helioporus in which P. carinae is known to occur do not represent a local radiation but are the result of at least three host invasions from the eastern side of the Nullarbor Plain. Because of this it is argued that the speciation of the parasites was independent of that of the hosts, was allopatric and probably involved intra-host competition.
The importance of using the divisions of their environment recognized by the parasites, rather than those recognized by their hosts, in studies of host: parasite relationships is stressed. As a consequence it is argued that although the theoretical basis of using parasites as indicators of host relationships is weakened their practical value remains unimpaired. Attention is drawn to the danger of circularity in the use of parasites in establishing host relationships.