Microgromia is a tiny amoeba with an organic shell who, much like a spider, lays down a sizeable spread of thread-like pseudopods (filopodia) lines with sticky extrusomes, waiting for the unfortunate bacterium or eukaryotic flagellate to stroll by. Unlike a spider, Microgromia does not need to wander off to apprehend trapped prey — its ‘web’ actively delivers the food straight to its mouth, seemingly digesting some of it along the way. While slower, quieter and much smaller than the truly-web-forming foraminifera, it can nevertheless handle some impressive flagellates on its own, relative to its size. You can usually find a ton of them by floating a coverslip atop some pond water samples, for a couple of days. The cell body itself appears to be quite determined to stay where it is, but you can watch the extrusomes and various tasty victims be slowly transported by the filopodia.
The test (‘shell’) has a bent ‘neck’ that is characteristic of this genus. Bacteria get drawn through that neck and into their final resting place in an acidic digestive vacuole within the cell body proper. As one would expect of a freshwater species, Microgromia has a contractile vacuole complex that gradually expands as its components fill up and fuse, and then expels its contents somehow — this appears to happen inside the test. The test itself generally starts out colourless and darkens to a brown with age, as a result of iron oxide (rust) accumulation. The nucleus is lined with strange granules, described as a ‘chromidium’ in old literature. That term seems to have fallen out of use, but primarily because people stopped talking about it altogether. The identity of these granules is unclear, as there are no ultrastructural studies of these organisms to date, to my knowledge.
Shelled filose amoebae like Microgromia are diverse and fairly common, but close to nothing is known about them. Small and restricted to growing on top of substrates, they have largely escaped human attention. Don’t think we even know who their relatives are via sequence data, yet. Presumably, they are granofilosean cercozoans like Limnofila, but one can’t know for sure without molecular phylogenies (or even with molecular phylogenies, for that matter!). The most comprehensive reference I know of is on arcella.nl, in addition to a couple old! papers. Microgromia and its friends are nevertheless quite interesting to watch, and may well carry a fascinating idiosyncrasy or three in their cell structure and molecular biology. (This specimen is from a local pond)
PS: There were old reports of a species where multiple individuals fuse their filopodia into a single network, called Microgromia socialis. It was a popular example in discussions of biological individuality concepts back in the late 19th and early 20th centuries (eg. in this book on the nature of human personality, of all things), and subsequently completely forgotten. (this discussion may not even refer to the same thing, for all we know) It sounds like some non-socialis species are also capable of fusing together into a network, or otherwise forming a colony, so M. socialis may have just been a specific life stage of some other Microgromia species. That said, I have come across clearly fused networks of Microgromia-looking things, whose individuals appear rather different from the solitary forms I find. The shape differs, and the ‘chromidium’ around the nucleus seems to be absent. The colourless appearance could be owed to a younger age, though this species may well not get rusty. Might not even be a Microgromia sp, as it’s considerably faster (but not as much as a foram). In any case, a weird bug. This one also came from a freshwater pond, but a bigger one — which is now unfortunately drained until further notice.