Out in nature, you may notice that critters often like to be on top of one another, or inside one another. Of course, I'm talking about endo- and ectosymbioses (inside and on the surface, respectively). This is particularly true for microbes -- perhaps because they are far more diverse and numerous. Furthermore, many organisms use creations of others' for one thing or another, often to take up residence. Think hermit crabs. Many testate amoebae use diatom cases in constructing their shells, and foraminiferans are particularly notorious for specialising in materials like sponge spicules, coccoliths (algal scales made of calcium carbonate), the latter even carefully oriented in the right direction; and even other forams! How they pick out their favourite material -- and arrange them in a sensible manner -- remains a bit of a mystery.
On the other hand, some forams (colloquial for foraminifera) take advantage of homes that were already constructed by their amoeboid brethren. Forams reproduce by splitting up into many, many little 'baby' forams, after which the mother shell is abandoned. Thus, the ocean floor is completely littered by empty shells accumulated over the eons. Incidentally, a ton of research has been done on fossil forams, as they are good indicators for assessing geological layers -- something that oil and gas companies seem to care about for some mysterious reason. The actual biology of living forams remains largely ignored, which I find tragic. But a fair bit is known about their diversity, and what they can do (as opposed to how they do it), leading us back to hermit crabs. Well, their microscopic analogues.
First off, we'll look at a bryozoan-like foram test user -- Placopsilinella. By bryozoan-like, I mean similar to the colonial encrustations on seaweeds, rocks and other surfaces one can find in the ocean. Except these forams aren't colonial, but similar concept of chambers constructed on top of something else. I think it looks kind of neat: (has an echinoderm feel to it)
This specimen comes from a study of deep sea forams by Gooday et al. (2013, Mar Biol Res), which is conveniently open access -- so you can read it. The rest of the forams in question are internal dwellers, taking over empty rooms in vacant housing. Maybe they do drugs in there too, who knows. The orange thing in the image below is Hospitella, inhabiting an empty Globigerinid (planktonic foram) test. It's orange naturally, and looks rather un-foram-like. The chambers are connected by tubes, which also lead outside the opening. The amoeba itself occupies the space inside its chambers, as well as extruding a network of pseudopods outside its home, feeding on unfortunate prey who swim by. The pseudopods can't be seen here because the samples have to be fixed(=killed) before examination -- deep sea research is difficult that way. The squatter has a flexible organic wall, whereas the host shell is calcareous, meaning it contains calcite. It turns out that glycerol dissolves that, so you can see the soft-shelled inhabitants of calcareous forams in nude!
This is not the first time Hospitella has been seen: it was described by a German scientist, Rhumbler, in 1911 (back in the day when descriptive biology was popular... sigh). It was still orange, and full of tubules. If you're into old literature (with drawings!), this volume on forams is excellent -- although in German. But the pics are pretty, and this guy saw a lot of pretty badass stuff. There is another volume on the biology of forams, with really nice plates at the back, from which the following images are extracted. And if you're really interested in extensively procrastinating with whatever you have to do, there are more volumes on other marine organisms from the same expedition.
Of course, Gooday et al. themselves extracted those images, which I only noticed after going through the work myself. So y'all get a double dose. Below are more pictures of Hospitella, with C and D showing the organism once again in vivo. The scanning electron micrographs show detail of the flexible test, namely its flexibility, as well as a cross-section of its wall.
Below are some more scanning EMs of another specimen. The first image shows the intact host cell, as well as the inhabitant's tube to the outside world. B through D show the inhabitant after the loss of its home, with the glimpse of a chamber in C showing stercomata, or hard (sometimes structural) grains of its fecal matter. E and F show more shots of the critter inhabiting its home, in cross section of both.
Gooday et al. also describe a new squatter genus, Incola -- apparently derived from inculta, or Latin for 'neglected'. That description applies to far more than one foram... The new squatter has a stark difference from Hospitella -- its test is agglutinated, or composed of small particles stuck together. It somehow finds and uses coccoliths (recall that it has a network of pseudopods outside the test), the algal scales mentioned above, to supplement its construction. It's arguably a less lazy squatter than Hospitella, doing some of its own remodeling after moving in.
Looks a little like a potter wasp's nest!
A shell so good is hard to resist for other creatures also. Forams can be home for everything from mutualistic algae to parasitic nematodes. The study of living organisms consistently points back to a tendency for every available space and niche to be occupied by something or other -- not only on the macroscopic scale, but especially on the micro one. It's a theme found in human societies as well -- any available occupation and viable livelihood tends to be filled sooner or later by one of us trying to make ends meet in life. In way, we too spawn host environments and symbiotic relationships with each other -- some mutualistic, some parasitic... and some simply commensal. One'd hope the former would be most common, but evolution has no morals -- arguably, social evolution included.
And, of course, squatting is not unknown to us. Building a house inside a house, however... hmm, probably someone has, somewhere, but never heard of it. Perhaps the closest analogy would be the Pueblo cities in caverns like Mesa Verde, though those caverns weren't constructed by anyone. So yeah, it thus follows that forams are awesome -- of course!