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The Ocelloid

The Ocelloid

Through the eye of a microbe

Gluttonous ciliate postscript: even large predators fall prey to amoebae

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I may have mentioned a few times to never mess with amoebae. Let me reiterate how lucky we are to be a few orders of magnitude of size removed from those creatures. Remember yesterday's prey-stuffed ciliate, Frontonia? Well, while procrastinating and perusing random literature, I came across this title: "Observations on Amoeba Feeding on the Ciliate Frontonia" (Dale Beers 1923, J Exp Biol). It still boggles my mind as to how typically slow-moving amoebae can catch and devour fast and hyperactive ciliates -- despite having observed it live myself, where an amoeba clung to another raptorial ciliate, Litonotus, gradually engulfing it.

The amoeba, in this case a large Amoeba proteus, first comes along a ciliate and attaches itself to it. Frontonia often hang out in the detritus on the bottom, and are thus especially prone to encounter amoebae. The amoeba rides the frantic ciliate while extending its pseudopods, and eventually attaches itself to a surface, anchoring the ciliate to its doom. The pseudopods continue to extend, and form a cytoplasmic ring that constricts and begins to pinch the ciliate into two halves. The ring extends into a tube as it exerts more pressure, and the ciliate is ultimately torn in half: one half engulfed and stuffed into a food vacuole, the other half swimming away (but mortally wounded). The dying half will probably be shortly sucked dry by scavenging Coleps ciliates. The terrifying ciliate predator has met its end, by an organism often used colloquially to represent slowness, laziness and general weakness.

Eight minutes. In eight minutes a large, tough ciliate is cleaved in half by a squishy meek-looking beast. As with other members of its group, Peniculids (including Paramecium), Frontonia has a fairly stiff cortex (or "skin"), and plenty of turgor pressure within to make it relatively rigid. In fact, they manage to contain bent cyanobacterial filaments -- which, you can probably imagine, exert quite a bit of tension themselves. Imagine the force the cytoplasmic ring must exert to pinch the ciliate apart! Furthermore, Amoeba proteus is a generalist, and does not specilialise in Frontonia or even ciliates. It eats everything from bacteria to small flagellates and large ciliates... to nematodes and rotifers. Yes, rotifers. Each of those prey requires a different strategy to capture them, and the amoeba is smart enough to know which one to use (presumably using chemical signals). In other words, as I probably mentioned too many times before... single cells do have behaviours! (as an aside -- much like spiders, some of whom apparently make silky nets to trap certain prey! (article here) [/irrelevant fact])

Much like a python after devouring a small sheep, the amoeba requires about four days of digestion to negotiate the [half of] Frontonia. I wonder if in the wild, it finds a place to hide and rest during the process. After such a feat, it surely deserves to.

The views expressed are those of the author and are not necessarily those of Scientific American.

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