This is another short essay on some of the fascinating creatures that live in the water of the rather intriguing pond to be found at the Cite du Lac in Vaour. In this case, a unicellular worm that lives on the floor of that pond.
When thinking about fast animals one’s mind usually springs to the cheetah chasing its prey across the Serengeti at speeds of up to 120 km/h though those familiar with the behaviour of raptors may point to the fact that in its dives the Peregrine Falcon can reach up to 380 km/h. Some may consider the latter a bit of cheat because the falcon isn’t using its musculature to achieve those speeds, rather it adopts a minimum drag conformation to drop from the sky like a stone. Perhaps the least likely place to find something that can go really fast is at the bottom of a pond? Something that can move 50 times faster than the eye can blink! I thought so but…
I was reviewing a short movie that I had made of a rotifer – in itself a fascinating creature that can move surprisingly fast, when I noticed something that happened in the interval between two movie frames. As ever, “the closer you look, the more you see” but in this case, the first time around, I hadn’t been looking close enough! My camera was running at 25 frames a second so I knew that the change I had witnessed had occurred in less than 1/25th of a second – a Spirostomum, a unicellular ‘worm’ that can reach lengths of up to 2 or 3mm had contracted to a less than half of its length. This rapid movement that occurred between two movie frames led me to read more about the contractile behaviour of this extraordinary organism. It turns out that because the contraction is so rapid, scientists have been drawn to study it. High speed cameras have shown that the contraction occurs much faster than my camera could catch. When mechanically stimulated as would occur during an attack by a predator, Spirostomum can go from its maximum to its minimum length in less than 1/200th of a second! Though its shrinking speed isn’t that fast, Spirostomum‘s acceleration is extraordinary. Contracting at 140 metres per second squared, the cell changes shape faster than a cheetah can reach its top speed. Here are the two frames that drew this amazing behaviour to my attention:
Spirostomum is an extraordinary organism not only is it very fast, tolerant of low oxygen tensions, and huge for a single cell, it appears that by virtue of the rapidity of its contraction, it can communicate with others of its species. Its contraction is so rapid that it induces a shock-wave in the water around it. This shock-wave leads other nearby individuals to contract, whole groups of them appearing to shrink simultaneously. While the reason for the contraction of an individual Spirostomum may seem obvious in that it enables it to avoid a predator by causing it to suddenly discover that a tasty morsel has disappeared from its reach, it turns out that there are more subtle events associated with this disappearing trick. When Spirostomum contracts it releases toxins from little pockets on the cell’s surface. The contraction also generates a vortex that spreads the toxins into the surrounding water much faster than diffusion alone could do. The coordinated contraction of neighbouring individuals results in a higher concentration of the toxin and may aid the group in avoiding the danger represented by a predator. The toxins, the ‘spirostomins’, drive predators like flatworms away.
Since Spirostomum doesn’t contain the contractile proteins found in the muscles of higher animals, scientists were drawn to study the cell’s cytoskeleton. As the name implies, the cell ‘cytoskeleton’ includes proteins that are a bit like tent poles that give a cell its shape. Some of these can be assembled and disassembled rapidly enabling cells to change shape. Yet others can work together to act like tiny muscles. In Spirostomum’s case, there is a network of contractile filaments (centrins) organised in a network just under the cell membrane and tiny tubes (microtubules) that form a spiral spring. When the cell is stimulated to contract an influx of calcium causes the filaments to shorten and the spiral microtubule spring is compressed. It is the energy stored in the microtubules that causes the cell to lengthen again. The lengthening process is slow by comparison to the contraction. Beautiful pictures of these microscopic components can be found here: https://www.biorxiv.org/content/10.1101/854836v1.full. The effects of some of these cytoskeletal components on the cell’s shape can even be seen in some of my photographs of living Spirostomum. Interestingly, centrins are found in the cells of nearly all animals and plants and are responsible for pulling the chromosomes apart when they divide.
In some good news for the people of Vaour and the tiny creatures living in its pond, the presence of Spirostomum is an indication of the water’s purity. In the 1990’s, Spirostomum was discovered to be a sensitive indicator of the presence of heavy metals, fungicides, certain other pharmaceuticals and agrochemicals. Indeed, Spirostomum gave its name to a bio-assay for water purity – the Spirotox test. Vaour’s pond water has passed the exam!
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