|Image modified from Figure 2 and 3 of the paper
So how did this affect parasites like G. ameliae? A pair of scientists compared G. ameliae found in alewives from Lake Hopatcong to those found in the anadromous alewives from Maurice River and noted some key differences in the two forms. For example, G. ameliae from anadromous alewives have oocysts (the infective stage of the parasite) which are comparatively shorter and wider than those from landlocked hosts.
They also have different trends in their prevalence and distribution; adult anadromous alewives are more commonly and heavily infected with G. ameliae than young fish, possibly because adult fish become stressed while migrating upstream and dealing with changing salinity levels as they move from the marine environment to a freshwater one, making them more susceptible to parasitic infections. In contrast, G. amelia was very common in younger landlocked alewives, infecting over ninety percent of young fish, but it was only found in about a third of the adult fish, which may indicate that the landlocked alewives can acquire resistance to the parasite as they mature.
Given those differences, are the anadromous and landlocked G. amelia actually different species? The scientists compared the DNA of G. ameliae from the anadromous and landlocked hosts, focusing on the 18S RNA gene which can function like a barcode for distinguish different species of parasites. They found that despite the two form having slightly different morphology and ecology, it was not enough to make them separate species - their 18S RNA gene sequences were identical. But given their differences, much like their hosts, those separate populations might be in the process of diverging into two different species - it is just a matter of time.
Lovy, J., & Friend, S. E. (2015). Intestinal coccidiosis of anadromous and landlocked alewives, Alosa pseudoharengus, caused by Goussia ameliae n. sp. and G. alosii n. sp.(Apicomplexa: Eimeriidae). International Journal for Parasitology: Parasites and Wildlife, 4: 159-170.