Reading this old thread made me wonder if there are different host species of fish for different variety of snails. For example maybe for a specific variety of snail GSF might be a suitable host but BG would not where a different variety of snail might have the reverse host needs. Perhaps that is why the BG had lots of the parasites with few on the hybrids. Perhaps the hybrids were not very good host candidates????
What made me think of this is host fish for mussels. Different variety of mussels have specific host fish. Here are a couple of links concerning mussels and their life cycles and one having information about specific fish hosts for specific mussels.
Mussels in Kansas If you click on a link to information on a specific mussel at the bottom of the description lists the suitable host fish
Life cycle of mussel Shows the parasitic life cycle
I have not done the research to see if specific varieties of snails only use specific fish for hosts like the mussels do.
