These are a pair of isopod parasites that I found in a fish while cleaning. The small male attaches to the gills, feeding on blood and casting its milt into the sea. The larger female has a worse lifecycle: she clips the fish's tongue and collects blood that way. As the tongue withers, she grows to replace the tongue and then some, eventually filling quite a bit of the fish's mouth. I never would have guessed I'd find these things, and was horrified when I first read of them. But I've now seen so many that I take them as a fact of life when you catch certain species.