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...
A fair amount of disgust and horror were expressed when the colourless isopod emerged upon slicing the fish open to gut it (Image 2-3). I thought the little guy was quite cute and am amazed at the size of the isopod relative to its ichtheous host. Safe to say nobody wanted to eat the catch after the alien crawled out of it – as a vegetarian myself I must say: what I waste of life!
As impressive as this parasite is, a relative of their's leads – in my opinion – an even more interesting and bizarre lifestyle: as the tongue of the fish. Juvenile Cymothoa exigua attach to the gills of a fish, where they mature and mate. Generally, a female will then enter the fish via the gills and settle itself down. Quite literally. The isopod will starve the fish's tongue of blood by extracting it. Once the tongue has been rendered useless and decayed, the little lady will attach herself to the tongue stump and from thereon mechanically acts as the fish's tongue; though, granted, most tongues don't nourish themselves from the blood or mucus of their owner… But close enough!