|Halipegus occidualis in the gray tree frog (left), Southern leopard frog (centre), American bullfrog (right)
Photos from Figure 1 of the paper
A pair of researchers from Oklahoma State University conducted a series of experiments to find out more about Halipegus' specific preferences. They collected seven different species of frogs and toads from various locations in Oklahoma, and infected them with a species of fluke call Halipegus occidualis to observed how the parasite behaved and developed in those different hosts. To control for natural variations and to ensure that the parasites they are using is the right species and not some other similar-looking cryptic species (see this for example), they used parasites from a colony of H. occidualis which they have been maintaining in their lab.
They exposed the frogs to larval H. occidualis by feeding them with seed shrimps which they have previously infected with H. occidualis. This procedure through which the frogs are exposed imitates the process of how frogs in the wild become infected with this parasite. After exposure, they inspected the frogs' mouths everyday for the parasite's presence. When H. occidualis is initially swallowed by a frog, it does so as a tiny larva encased in the body of an arthropod. The digestive action of the frog's stomach free the fluke from the arthropod host, and it then migrate to the frog's mouth over the course of a few weeks to develop into a sexually-mature adult. Or at least that's what happens in most frog species.
For six of the frog species in the experiments, H. occidualis showed up as expected under their tongue as mature, egg-laying flukes about 6-8 weeks after they have been fed with infected seed shrimps. The parasite was most successful at establishing in the American toads (93%) and had comparatively lowest success with the southern leopard frogs (67%), but aside from that, there were no major differences among those six species in terms of how H. occidualis performed. But things were a bit different in the American bullfrog. In that host, H. occidualis never show up under the tongue - instead, they simply stayed in the stomach and developed to full maturity there.
It seems that not only is H. occidualis very specific are where it settles, it will also adjust accordingly if the host is different, to the degree that it would do so even if it has already developed into a fully-fledged adult fluke. When the researchers conducted further experiments where they transplanted adult flukes from under the tongue of gray tree frogs to other species of frogs, the flukes were quick to adjust. When the flukes were transplanted from a gray tree frog to yet another gray tree frog or a green frog, the fluke will move to its usual spot under the host's tongue, even though it is now in a new host. But if those flukes were transplanted to an American bullfrog, the flukes would migrate to the bullfrog's stomach. Furthermore, when those parasites were then extracted from the bullfrog and transplanted back to the tree frog, they went back to living underneath the host's tongue.
So what so special about the bullfrog, or specifically its stomach? At this point, it is not entirely clear. Perhaps the bullfrog stomach has some kind of chemical that encourage the fluke to stay instead of migrating to the host's tongue. While a parasite might be very specific about where it exactly it lives in the host, it might not always behave the same way when it finds itself in different host species. For a parasite like H. occidualis, not all frogs are equal.
Stigge, H. A., & Bolek, M. G. (2016). Anuran Host Species Influences Site Fidelity of Halipegus occidualis. Journal of Parasitology 102: 47-53