"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

October 23, 2015

Goussia ameliae

The fate of parasites are often inextricably linked to that of their hosts, and when there are changes in the host population, the effects cascade onto their parasites. The study featured today is focused on Goussia amelia - it is a newly described single-cell protozoan parasite which infects alewives and is known to cause erosion in the intestinal wall of their fish host.
Image modified from Figure 2 and 3 of the paper
Alewife is a species of herring native to the east coast of North America. They are anadromous fish that live in the coastal marine environments as adults, but enter freshwater streams to breed, much like salmon. Sometimes populations of alewives become trapped in lakes for one reason or the other during their migratory journey. These isolated fish eventually become adapted to the freshwater environment and evolved on divergent paths to their anadromous relatives. This is a relatively common occurrence which has happened multiple time in the last few thousand years, and it is also the origin for the population of alewives found in Lake Hopatcong. This lake was originally connected via a canal to the Delaware River and alewives from the coast of New Jersey used to migrate to Lake Hopatcong to spawn. But during the start of the 1900s the canal was blocked off, and the alewives that were in the lake at the time became isolated from their relatives on the New Jersey coast.

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.

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