A new way of looking at the shells of arcellinid amoebae

Netzelia, Netzelia corona, Protist Homes, Protistology, Testate amoebae Add comments

Dec162024

Netzelia corona is a testate amoeba with “teeth” around the aperture of its shell. It also has some impressive spines jutting out from its posterior, which have obvious advantages as a defensive apparatus. But it’s the teeth that have always puzzled me. They’re very neatly made, featuring uniform lobes of organic cement from which tiny, sharp mineral particles protrude.

What is their purpose?

Netzelia corona, collected in Mer Bleue Bog, Ottawa. Image by Bruce Taylor.

A recent article by Kenneth Dumack and colleagues offers a possible explanation, but it requires a shift in our usual view of the arcellinid shell. Amoeba tests are conventionally seen as protective structures, designed to thwart predators and parasites, and provide shelter in times of drought. However, according to Dumack and his collaborators, the shell is not merely a refuge, but an instrument of predation. As the authors put it in their provocative title: “It’s time to consider the Arcellinida shell as a weapon.”

This insight hinges partly on the behaviour of a protein called “filamentous actin”, a substance that enables amoeboid cells to move forward by extending pseudopods. When a pseudopod forms at the leading edge of the cell, long chains of f-actin molecules rapidly polymerize within it, forming an inner scaffolding of filaments that push the pseudopod out. Then, as the pseudopod is retracted, this scaffolding is quickly disassembled and sections of filamentous actin are pulled back into the body of the cell, with the help of myosin motor proteins.

During locomotion, there is a concentration of f-actin in the pseudopods themselves, which can be seen with the help of a stain made of phalloidin. This substance–originally derived from the Death Cap mushroom, Amanita phalloides–binds to the actin in a cell, and can be combined with fluorescent dyes, so that, in a properly equipped microscope, it takes on a vivid green fluorescence, revealing the exact regions in which actin is concentrated. This was beautifully recorded in an earlier paper (Estermann et al., 2023), using the testate amoeba Cryptodifflugia oviformis. When that amoeba is in motion, we see a high concentration of stained actin in the pseudopods, but nearly none in the body:

Cryptodifflugia oviformis in locomotion, showing a concentration of green-stained actin in the pseudopods. Source: Estermann et al., 2023

During predation, however, the distribution of actin is quite different. When the same species of amoeba is feasting on fungal hyphae we see filaments of stained actin extending up into the body of the cell.

Cryptodifflugia oviformis preying on strands of fungi. Green stain indicates the presence of actin. Source: Estermann et al., 2023

According to the authors, the actin filaments are anchored on the walls of the shell itself, within the strands of cellular matter traditionally called epipodia (attachment points for the organism, within its test). This is where the shape and structure of the shell–including its aperture–becomes important. In the model proposed by Dumack and associates, as prey is dragged into the shell to be consumed it is broken across the lip of the aperture. Here’s a helpful pair of images, showing the mechanism by which Cryptodifflugia oviformis breaks open fungal hyphae:

Evidently, the conical bundles of actin shown in the images on the left are used “to exert force on the cell walls of the amoeba’s prey while being anchored to the inner shell surface.” The mechanism of the force is not fully explained at the molecular level, but it seems these actin bundles act somewhat like cables in a winch (presumably driven by contractile proteins, such as myosin?). One end of the actin bundle is firmly attached to the inside of the shell, and the other is affixed to a tasty prey organism. As the victim is pulled into the shell, its membranes or cell walls are broken open on the rim of the aperture.

As for the Netzelia corona I posted, above, I am not sure what organisms it shreds on the spines of its “denticulate lobes”. I’ve never stopped to watch it feed.

However, there is a closely related species, the free-floating carnivorous amoeba Netzelia tuberspinifera, whose shell has a very similar design. This one-celled hunter is capable of taking down large prey, including rotifers, a behaviour that was caught in an interesting sequence of photographs published in a study of “carnivory and active hunting” in that species (Han et al., 2008). The sequence shows N. tuberspinifera penetrating the gelatinous sheath around the rotifer Collotheca, then dragging the animal itself into its shell, to be devoured:

Netzelia tuberspinifera preying on the rotifer Collotheca. Source: Han et al., 2008

The amoeba gains access to the rotifer by breaching the gelatinous sheath that surrounds it (identified in the fourth panel by a dotted line). This does seem quite similar to the feeding strategy described by Dumack et al., and to me it seems likely that the denticulate structures around its aperture play a role in perforating the sheath.

In other words, these “teeth” might actually serve as teeth!

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Posted by Bruce Taylor at 1:01 am

One Response to “A new way of looking at the shells of arcellinid amoebae”

1. Harry Mueller says:

December 18, 2024 at 3:28 pm

An interesting and very plausible proposition. Once I get my mind around the fact that these little one celled creatures possess the blueprints and materials to build their fledgling daughters a new home then small embellishments to that home are not so farfetched. Maslow’s basic heirarchy of needs coupled with natural selection provides a solid basis for some form of improved food gathering strategy. Will the future produce a Pac-Man like testate amoeba with articulating jaws powered by actin filaments? 😃