Guest post by Kenneth Dumack, based on a thread on Bluesky.
Did you ever wonder why there is such a large morphological diversity in shell-bearing amoebae and what is the function of shells anyway? It was long assumed shells are for defense, well let me try to convince you that their purpose is attack!
Arcellinida (Amoebozoa) produce persistent shells that can form fossils. Accordingly, there is a huge body of research on shells, but why do they look so different? I thought, if they are for defense, their shape should be quite conserved, but it isn't. Many Arcellinida may attack, perforate, or cut large prey, like nematodes, algae, and fungi. Such behaviour is not observed in shell-lacking Amoebozoa. Is this a coincidence or does the shell have an involvement in prey manipulation?
See for instance this Cryptodifflugia oviformis attached to fungi hyphae:
Here you see, that it could cut the hypha and ingest it's cell content (see the grey arrows pointing out to an empty hyphal piece). How does this work? Why do only shell-bearing amoebae do this? Most protists are size-limited to consuming only smaller prey.
Well, let's see the involvement of the cytoskeleton in the process: Actin (green) is used by amoebae to attach to substrate. This is why pseudopodia are heavily stained in green. A moving amoeba needs actin in the pseudopodia.
How does it look like during predation? Suddenly there is actin in the cell body! This actin forms these conical bundles that attach with the prey.
These actin bundles also connect to the opposite sides of the shell. It seems therefore that Arcellinida use actin to pull their prey in! For this, the shell is needed as an anchor point.
See this individual pulling a yeast cell through the opening of the shell. You see strongly pulsating circular blebs, indicating that actin is repurposed, and the yeast is heavily deformed, confirming mechanical manipulation.
The related Phryganella paradoxa partially inserts its diatom prey into the opening of its shell and breaks the shell of its prey. I hypothesized that the partial insertion is necessary to exert force. Here the shell is not only an anchor point, but also a fracture point.
My conclusion: The shell is an anchor point for actin to pull on prey, the edges of the shell opening are breaking points, where the prey is broken if it can not be pulled entirely through the shell's opening.
We knew for a long time that Arcellinida are masters of the cytoskeleton as they use it to build their elaborate shells, it was unknown that they use both, the cytoskeleton and shell, in combination to function as small apex predators in their systems.
Papers:
Fungivorous protists in the rhizosphere of Arabidopsis thaliana – Diversity, functions, and publicly available cultures for experimental exploration, Antonie H. Estermann et al., 2023.
It's time to consider the Arcellinida shell as a weapon, Kenneth Dumack et al., 2024.
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