Jeff writes: "[Do] squid or cephalopods have any weird parasites similar to the isopops that replace the tongues on a number of fish . . . ?"
(Heh. I am still chuckling over "isopops." Although probably coined accidentally, it is a perfect term for the tongue-replacing isopod. In fact, I'm envisioning a new line of shaped frozen treats . . .)
To answer: cephalopods often find themselves hosting the same classic parasites that plague the rest of us: tapeworms, roundworms, protozoans, and the like. In addition, they have some of the pickiest parasites in the world all to themselves. One entire phylum of worms, Dicyemida, lives exclusively in cephalopod kidneys*.
Like many other parasites, they have small, simple bodies, and long, complex life cycles. I won't go through the whole thing here, I'll just highlight my favorite dicyemid life stage: the infusoriform larva, which zooms around very quickly like a tiny flying saucer, using two large ciliated rings that look like headlights. This larval form is thought to be the "transmisson stage"--it exits the host via excreted urine, and presumably goes on to infect some other cephalopod.
I say "presumably" because the mechanism of infection for dicyemids is a huge black box. Are they transmitted vertically (parent to offspring) or horizontally (picked up from the environment)? Do they spend time in an intermediate host (probablyly non-cephalopod) as do many other parasites with similarly complex life cycles? How do they get into the kidneys?
We don't even know if they are actually pathogenic (harmful to the cephalopod host). If not, they aren't really parasites! But they've been labeled as such for so long, they'll have a hard time shaking the image, even if we discover evidence to the contrary.
* I use terms like "kidneys" and "urine" here, although a comparative anatomist might say that renal appendages are not kidneys and excreted nitrogenous waste is not urine. Not without just cause, but I feel that such concerns are superseded by the advantage of drawing analogies with familiar structures and substances.
