Isopods are a large group of crustaceans. They don't all roll up into "pills", nor is the whole group terrestrial. Indeed, not all are even free-living, with various species and families parasitising both vertebrates and other invertebrates.
Not all parasitic isopods are obligate parasites for their whole life and, like most parasitic animals, generally each type of isopod specialises in abusing a particular host. There are entire taxa of isopods which parasitise a specific taxa of animal, with each species bound to another species. Cymothoidae is a family of fish parasites. Some of these are ectoparasites – meaning they attach to the outer surfaces of the fish – and others are endoparasites, nourishing themselves from inside the fish.
Below is a photograph of a fish which somebody caught in Payamino River (Image 1), just in front of the station, a couple of weeks ago...