Fish eyes make the perfect home for parasites

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A fish can be an ecosystem unto itself and fish eyes make ideal homes for parasites, according to research by Canadian scientists published in a recent issue of the journal Molecular Ecology.

Sean Locke, Daniel McLaughlin and David Marcogliese have discovered that parasite diversity in freshwater fishes are much higher than previously supposed by studying an intermediate larval stage of flukes known as metacercariae. 

Because metacercariae show up as indistinct white blobs in fish tissue and cannot be identified to species based on morphology, they have been treated as representing very few species parasitising a wide variety of fishes. 

Using DNA barcoding to identify the fluke metacercariae found in freshwater fishes caught from the St. Lawrence River in Canada, the authors detected 47 species of a particular group of flukes (diplostomoids) residing in freshwater fishes, almost four times as many species as previously known.

The authors discovered that the parasites found in most tissues, such as the muscle, gills, brains, and other internal organs were specialised in one or a few closely related species of freshwater fishes.  They also found the eyes of the fishes to be an ideal home for generalist parasites, with as many as five species that infest a broad variety of freshwater fishes and even frogs residing within the lens. 

One reason for this plethora of parasites in the eyes is because the lens is one area where the immune response is more limited in the event of an infection. A strong immune response would blind the fish, so evolution has favoured immunological restraint in the eye, according to lead author Sean Locke.

According to the authors, understanding the parasite diversity of freshwater fishes has important implications in wildlife management and aquaculture, where pathogen identification is a first step to control and treatment.

For more information, see the paper: Locke, SA, JD McLaughlin and DJ Marcogliese (2010) DNA barcodes show cryptic diversity and a potential physiological basis for host specificity among Diplostomoidea (Platyhelminthes: Digenea) parasitizing freshwater fishes in the St. Lawrence River, Canada. Molecular Ecology 19, pp. 2813–2827.