. . . both literally mean "living together", only they come from different roots.
co: Middle English, together; from Latin com, together, with; from Indo-European kom, beside, near, by, with
habit: from Latin habitare, to dwell; frequentative of habere, to have; from Indo-European ghabh, to give or receive
sym: from from Greek sun, together, from Indo-European ksun, with
biosis: from Greek bios, life, from Indo-European root gwei, to live
Of course, they have entirely different connotations. Imagine telling your parents that you and your significant other have decided to enter into a symbiotic relationship. Calling a relationship symbiotic has the additional effect of making most people think of it as mutually beneficial.
In a strict sense, symbiosis doesn't mean that at all. Ecologists use it for its literal definition, and then categorize the different kinds of symbioses based on the benefits to the participants. A symbiosis in which all parties benefit is called a mutualism. If one party benefits from the relationship and the other is indifferent, then it's a commensalism*. And if one party benefits at the expense of the other, we call it parasitism.
And today, my friends, I am here to talk to you about parasites.
Specifically, the phylum Dicyemida (or Rhombozoa, or Mesozoa... but that's a problem for the taxonomists). Dicyemids are little worm-like creatures that live exclusively in the renal appendages (read: kidneys) of cephalopods. Yeah, weird. That means they are basically swimming in squid urine. What possible benefit this could provide to either worm or squid is difficult to imagine.
The problem with all these symbioses is that, as in any relationship, it's often difficult to tell just who is getting what out of the association. Organisms are labelled mutualists, commensals, or parasites based on the perspective of the observer, which may be quite different from the participants. Certainly a number of cases are clear-cut parasites, such as malaria or tapeworms. Dicyemids, by contrast, are called parasites although we have yet to show any effect on the host cephalopods, benficial or detrimental. Not only that, but no one knows for sure how they infect their hosts in the first place, or for that matter, why they infected them in the way-back-then first place.
Dicyemids are extraordinarily simple. They're eutelic, which means each individual adult of a given species has exactly the same number of cells as all the other adults of that species. This has attracted a number of cellular biologists to study their cell lineages--tracing the history of each cell back to the zygote that gives rise to the whole organism. We're talking on the order of about twenty cells per organism here--these guys are not very big and they're not very complex. No organs or tissues of any sort disturb this perfect parasitic simplicity.
So they were originally classified as Mesozoa, which means "middle animals" and sticks them somewhere in between the Protozoa (first animals, single-celled critters) and the Metazoa (after animals, everything else). But then the geneticists got interested and did some sequencing work on dicyemids to show that they actually seem to be extremely simplified relatives of the Lophotrochozoa, a big group of metazoans which includes squid.
Parasitic simplification is just part of the larger phenomenon of symbiotic simplification. If you're living with someone else, particularly if you're living inside them, it turns out you can get them to do a lot of things for you, like digesting food and dispersing your offspring, towards which you no longer have to allocate energy. And this is why so many parasites can no longer live when separated from the hosts... I feel a tangent coming on.
The inability to reproduce independently is a derived character in parasites, while it is inherent to viruses. At least, that's the presumption. Might that be a little... presumptuous? Is it possible that the viruses we know today evolved from some RNA lifeform that could reproduce on its own, and secondarily lost the ability?**
Back to the point, which is: squid worms!
With all the geneticists and cell biologists mucking around with dicyemids, there aren't a whole lot of ecologists poking at them. And this means that we still don't know what I consider to be the really interesting aspects of their biology: how they interact with their hosts and to what end.
It seems a few experiments could at least give us some leads into this. First off, the infectious stage and method of infection need to be pinned down. That could pretty easily be done by raising octopuses in the laboratory and exposing them to various life stages of dicyemids at various points in their lives. Once the infection of hosts can be controlled, effect of infection of host fitness can be assessed. And if the dicyemids themselves can be cultured outside of the host, providing them artifically with various aspects of their normal environment could tell us how they sustain themselves in the cephalopod kidney.
Look for publications in a few years. Or decades.
* Middle English, sharing a meal; from Medieval Latin commensalis; from Latin com, with, and mensa, table.
**I'm totally making this up. I must remember to look it up and see if anyone else has had the same idea.