1890 | Wiesbaden, Germany

Testate amoebae (Cryptodifflugia oviformis) weapon use

Today’s post looks at our tiniest tool user ever. But it has big implications for how we understand animal tool use—because this creature doesn’t just have a small brain, it has no brains at all.

Shell shock

We begin in Wiesbaden, just west of Frankfurt, in a pond. It’s the late nineteenth century, and Swiss biologist M. Eugene Penard is fresh off a stint tutoring a Russian prince in St Petersburg. His passion is not teaching, however. It’s amoebas.

An amoeba is a single-celled organism, able to extend bulges or filaments of cytoplasm to move and interact with its environment. They don’t have cell walls, but many kinds build a protective shell or test, and are hence known as testate amoeba. It used to be thought that all amoeba were closely related to each other as a group, although molecular work has now found that there are amoeboid versions of algae, fungi, and even animals. It’s a simple form of life, abundant and effective.

Penard was the first to describe and name numerous types of amoeba, which he often found amid moss and ponds around Wiesbaden. The star of our story, Cryptodifflugia oviformis, is one of them—here’s Penard’s description of finding it, from his 1890 monograph ‘Rhizopodes D’eau Douce’:

Wiesbaden, étang.

J’ai trouvé cette espèce intéressante dans une bouteille remplie de l’eau d’un étang, et au fond de laquelle était une légère couche de champignons mélangée d'organismes animaux (rhizopodes, héliozoaires, monades); mais quoique les individus en fussent extrêmement nombreux, ce n’est que six mois aprés la pêche que j’en ai vu, et alors en nombre assez grand.

[Wiesbaden, pond.

I found this interesting species in a bottle filled with pond water, at the bottom of which was a light layer of fungi mixed with animal organisms (rhizopods, heliozoans, monads); but although the individuals were extremely numerous, it was only six months after fishing that I saw them, and then in fairly large numbers.]

In detail, what Penard saw was this, from a diagram by Hedley and colleagues in 1977:

Like I said, it’s all one cell. The large eye-like object is the nucleus, surrounded by protoplasm holding various other necessary biological bits. The whole is enclosed by its test, with an opening at one end for manipulative extensions, called pseudopodia or ‘false feet’. (Remember those feet, they are important.) The whole creature is only a few tens of micrometers long, invisible to humans for our entire coexistence until the invention of sufficiently powerful microscopes. And sufficiently motivated Swiss biologists willing to get their feet wet.

Actin up

So, why are we looking at an amoeba here? A single celled life form can’t be much of a tool user, surely.

Welcome to the stage Kenneth Dumack, an ecologist and amoeba expert at the University of Colognes. In January 2024 Dumack and his colleagues published a provocative paper in the European Journal of Protistology titled ‘It’s time to consider the Arcellinida shell as a weapon’. To clarify, protist is the name for a hugely varied group of single-celled, nucleated life forms, and the Arcellinida are a large group of shelled protists.

This is where tool use comes in. Dumack and his team reported how their test subject, C. oviformis, attacks prey that is too large to fit through the hole or aperture in their shell. This happens when the amoeba tries to consume long material like fungal strands or hyphae. While C. oviformis can directly ingest small yeast cells, those larger materials need to be broken up or punctured if the amoeba is to feast on them.

Looking even more closely, the researchers found that the amoeba initially grabs hold of a fungal thread with its pseudopodia, but then pulls these back to leave small circular bits of itself called—honestly—‘blebs’. The reason it pulls back is because the protein that normally supports the movement of those pseudopodia is redirected into the interior of the amoeba shell. That protein is actin, and you have versions of it in your own cells and muscles, doing a similar job as it does in our tiny protist cousin.

From the paper, this is the state of play:

The key is those spiky green bits of actin that have attached to the opposite wall of the test, essentially anchoring the grip that the amoeba has on its fungal prey. Those internal spikes have only been seen during this kind of feeding, giving this single cell enough leverage to break apart what would otherwise be an inaccessible food source. The red circles highlight the points of peak mechanical force that the amoeba exerts to crack its prey, before sucking out the juicy fungal interior.

If this was a case of C. oviformis anchoring itself to the outside world, it would be interesting, but not tool use. A non-tool parallel would be New World monkeys that use their prehensile tails to anchor themselves while feeding in trees. However, the amoeba is using the shell that it has built for itself, creating its own force-amplifying weapon. Here is the result of that weapon, seen in use in a 2023 study also involving Dumack, led by fellow Cologne University scientist Antonie Estermann:

In this microscopic sequence, the amoeba attaches itself to a fungal hypha (A), and begins to damage it before ingesting some of the contents (B and C; the arrows are pointing to the location of fungal innards that are ingested). Finally, it breaks the strand altogether (D), leaving the hypha partially digested. The actively manipulated shell is a vital part of that process.

Testing tools

Normally, the amoeba’s shell is a protective one. Testate amoeba use materials that are either self-secreted (idiosomes) or collected from their environment and cemented together (xenosomes). C. oviformis falls into the second group, binding its test using calcium phosphate, an inorganic material that forms an important structural part of our own human bones. Around that shell is a thin layer of organic material.

It’s the amoeba’s active use of its shell for leverage that brings it from the realm of construction behaviour into that of tool use, at least temporarily. There are several animals that build their own tools, including ones that use internally manufactured material as part of the construction. The ogre-faced or net-casting spider is one, spinning a small web that it then picks up and uses as a mobile net to catch flying insects. Human carry bags made of our own hair, and bubble-nets formed by feeding humpback whales similarly rely on internally-shaped material (hair or air) to corral their prey.

The fact that an amoeba’s test is primarily defensive doesn’t stop it being turned into a short-term tool. The F-actin that grips the internal shell to exert pressure on large prey isn’t a permanent feature, and in fact the actin is more often found outside the critter in its pseudopodia, helping the animal move and grip. It’s as if we as humans could form temporary internal fists that grabbed hold of our own skeleton for extra reinforcement and stability as we try to break open a stubborn food, like a coconut or a jar of peanut butter. Those actin grips create a mechanical interface between the shell and the prey, just as surely as if the amoeba had picked up a nearby bit of sand and pushed it against the fungus. The test is an enabling object, manipulated by the amoeba, which makes this clear-cut tool use.

How long has this kind of thing been going on? An early record of testate amoeba comes from the 407 million-year-old Rhynie chert in Scotland, which somewhat resembles Cryptodifflugia in having a globular shell with a single central opening, although other similar genera also have those features. There is no evidence that any ancient amoeba used their shell as a tool or weapon in the manner of C. oviformis, but the pseudopodia don’t preserve as fossils, and we’d have to be very lucky to find one encased in amber or otherwise caught in the act of blebbing onto a hapless hypha. What we do know is that testate amoeba have been found worldwide for countless millennia, especially in freshwater environments, so the opportunity for (re-)discovery of test tool use is extremely high.

Finally, I know that the idea of tool-using amoeba is a stretch for some. They don’t have brains, neurons, hands, muscles, or even more than one cell in their body. But it never pays to underestimate the ability of evolution to give even the smallest creatures surprisingly useful strategies for survival.

Take the example of Cryptodifflugia operculata, a soil-living amoeba cousin to C. oviformis. A study in 2015 led by another University of Cologne scientist, Stefan Geisen, found that packs of these tiny predators were attacking and consuming nematode worms many times their size.

This image shows hourly snapshots (over 12 hours) of an example of a pack attack—each of the little round things is a C. operculata individual, and the scale bar in the final image is 100µm:

At a tiny scale, this is the equivalent of a group of lions feasting on a downed antelope, or prehistoric humans around a mammoth carcass. It’s not clear how the amoebas coordinate their strike, but chemical signalling is a likely culprit. Life is life, no matter the scale. The researchers also suggest that these nematode feasts are happening across Europe, with potentially important ramifications for soil ecology.

The worm-hunting amoebas weren’t stained to look for actin, so we don’t know if they were killing their prey via tools. But over the past few decades we’ve found examples of technological life in most places that we’ve taken the time and care to properly look. That fact alone should alert us to the likelihood that tool-using animals (and amoebas) exist in greater abundance than we’ve assumed. Humans may be clever tool users, but the soils and ponds of the world hide technological struggles just as real—and just as valuable to survival—as our own.

Sources: Penard, M.E. (1890) Rhizopodes D’eau Douce. Memoires de la Société de physique et d'histoire naturelle de Genève 31:1-253. || Meisterfeld, R. (2000) Order Arcellinida Kent, 1880. In J. Lee, G. Leedale & P. Bradbury (Eds) The Illustrated Guide to the Protozoa 2nd Edition. Allen Press, USA; p.827-860.  ||  Dumack, K. et al. (2024) It’s time to consider the Arcellinida shell as a weapon. European Journal of Protistology 92:126051.  ||  Hedley, R. et al. (1977) Biology and Fine Structure of Cryptodifflugia Oviformis (Rhizopodea: Protozoa). Bulletin of the British Museum (Natural History). Zoology 30:313-328. || Estermann, A. et al. (2023) Fungivorous protists in the rhizosphere of Arabidopsis thaliana – Diversity, functions, and publicly available cultures for experimental exploration. Soil Biology and Biochemistry 187:109206. || Geisen, S. et al. (2015) Pack hunting by a common soil amoeba on nematodes. Environmental Microbiology 17:4538–4546.

Main image credit: Meisterfeld (2000) Fig. 76 || Second image credit: Hedley et al. (1977) || Third image credit: Dumack et al. (2024) || Fourth image credit: Estermann et al. (2023)