At 72 years of age one comes to reflect upon why it is that one chose a particular path through life. There are only a few things of which I am certain. The first is that I am a scientist and a firm believer in the Scientific Method. In a world where the irrational seems to have once again taken hold, science is my rock. Secondly, that I am in awe of ‘life’ by which I mean the limitless ability of evolution to overcome seemingly impossible odds and for life to thrive in the most hostile of environments. My interest in biology and thereby Nature, began with my late brother’s enthusiasm for butterflies. His love of these insects was infectious and as children, we would go hunting for them together. The next important step in my development as a scientist came at secondary school where my biology teacher introduced me to the wonders of the microscope. After explaining the importance of how to set up a high-power microscope he gave each member of the class a dish of pond water and instructed us to place a drop on a slide and observe it under the microscope. In my sample there was an amoeba – something that I had heard of but never seen. I watched spell-bound as it made its way around under the coverslip. I could clearly see the nucleus, vacuoles and other organelles. To my amazement it engulfed food particles in the water around it and then after a while, it began to divide in two! That was it, I was hooked! My catch-phrase became “the closer you look, the more you see” and my career in science was assured. Now in retirement, I spend my time in macro- and micro-photography and have yet to take a photograph in which I couldn’t see something more than was visible to my naked eye.
Recently, a friend told me of the efforts that a group of volunteers in Vaour had made to resurrect a pond that had become choked with weed and fallen trees. My first visit to the pond at the Cite du Lac was to photograph some of the insects there. There were quite a few dragonflies but because the insects were perching in inaccessible locations, it wasn’t a good place for photographs. However, it occurred to me that the water looked like it would be rich in tiny plants and animals. I decided to return with that most scientific of all apparatus, a jam jar on a stick and to try to repeat the experience I had at school more than 60 years ago!
Carefully placing a drop of the pond-water on a slide, I was amazed that it was absolutely teeming with life. There were literally hundreds of motile organisms visible in the tiniest of volume of water. There were some that were immediately identifiable. There were at least three members of the genus Spirogyres, the green, filamentous algae of the family Zygnemataceae, and recognisable by virtue of their shiny and intricately patterned ‘shells’ (frustules), several different species of diatom (Bacillariophyta), there were water fleas (Daphnia), and many different kinds of unicellular organisms including one with which I later became fascinated, Stentor. Stentor sp. are unicellular filter-feeding ciliates, that can reach lengths of two millimeters; as such, they are among the largest known extant unicellular organisms.
This picture has been created by combining many frames from a movie. The water flea in the centre was trapped under the coverslip and remained still while the many smaller organisms remained free to move creating the trails that you can see around the flea.
A rotifer – sometimes referred to as ‘wheel animalcules’, rotifers feature a characteristic circular arrangement of cilia at their front ends that when they are in motion is reminiscent of a turning wheel . They are multicellular and unlike Stentor have a brain and true organs such as a gut etc. In 2021, biologists reported the restoration of delloid rotifers after being frozen in permafrost for 24,000 years. Pond-life is tough!
Stentor caught my eye not only because it is large but also because its behaviour and structure are fascinating. It can take on different shapes. When feeding it attaches itself to a surface and extends into the water taking on a horn-shaped appearance – cilia at the end of the trumpet create a current that wafts food into its reach which it then engulfs. At some point, it will detach, round up and powered by its cilia, swim away to a new location.
Stentor, a unicellular organism – in this photomicrograph, internal organelles, a unicellular organism’s equivalent of organs, can clearly be seen. These include the nucleus, digestive vacuoles and a clear vacuole that is Stentor’s osmoregulatory ‘organ’. Stentor pumps water from its cytoplasm into this and then pushes it out into the water of the pond thus preventing swelling and its biochemistry from becoming too dilute.
I have recently been reading some of the works of early microscopists. The heyday of microscope development was in the Victorian era (La Belle Époque in France), when superbly engineered brass microscopes were produced for demanding and usually wealthy, gentlemen. Their observations were often very accurate and accompanied by detailed and skilled, drawings. However, what fascinated many of them was the complex behaviour of organisms like Stentor – was it an automaton or did it have ‘will’? Could it ‘learn’ and as it had no obvious brain, how was it that it could suddenly ‘decide’ to contract and swim away? Was it in some sense intelligent? These questions require a definition of the words involved. For example, what is ‘learning’? Most scientists would agree that learning is the adaptive updating of a system based on new information and that intelligence is a measure of the computational process that facilitates it. Learning is usually associated with animals that possess complex nervous systems, but as with Stentor, there is increasing evidence that life at all levels, down to single cells, can display intelligent behaviors. So, how can learning in Stentor be studied?
Above, I have described how Stentor can suddenly ‘decide’ to contract or swim away. I have made a video of part of this process – you can find it here (https://www.youtube.com/watch?v=95t86PUUEVo). One assumes that in its natural environment the decision to contract is caused by the presence of noxious stimuli or threats from predators. However, in the laboratory Stentor can be made to contract by touching it, puffing irritating substances or by electrical stimulation. If any of these stimuli is repeated, Stentor shows a progressive reduction in the probability of its contraction. This is a process known as ‘habituation’; a low level example of learning. Stentor can of course also ‘forget’…it completely loses habituation 1 hour after cessation of the noxious stimuli and again begins to contract with the same probability as before. There are now dozens of scientific papers on learning and memory in Stentor. It has been shown that there are molecules present in Stentor’s cell membrane that open protein channels leading to a voltage change across it. These voltage changes can be graded or take the form of what is known as an ‘action potential’ – a large all-or-none, nearly digital, voltage change. It is the latter that triggers the rapid contraction. These electrical signals are not unique to Stentor. As you read this article, the nerve cells in your brain are using both graded and action potentials to process the words on this page.
A series of frames from a movie showing Stentor contracting. It took only ~1/20th of a second for Stentor to contract and then about 30 seconds for it to go back to its ‘trumpet’ form.
So, science has brought us to the point where we can answer the questions that fascinated the amateur microscopists of La Belle Époque. Stentor can learn and to the extent that intelligence is the process that enables learning and adaptive behaviour, it is intelligent. Not only that, but the underlying molecular machinery is not dissimilar to that employed in our own brains. Sadly we cannot replicate its ability to fully regenerate after being cut in half and perfectly preserve its original structure!
An amoeba – I find them very beautiful. They are quite tricky to find, some hiding amongst the benthos while others hide inside a sort of shell that they secrete around themselves and from which the processes exude and enable the teste to walk around – a bit like a tortoise!
To end, I am going to go back to the amoeba I saw as a school child. Encouraged by my teacher I placed a cotton fibre soaked in dilute acetic acid at one corner of the coverslip under which the amoeba lay. To my amazement and that of the class, the amoeba began to move away from it. That seems such a simple result and yet it requires something that only occurred to me much later in my career as a scientist. In order to move away from such a stimulus requires that an organism can compare the concentration of a chemical in one location to that in another. In short, it has some memory of what the concentration was at point A and is later at point B. Only then can it ‘know’ that it is moving away from or toward, danger. If only we did too!
I originally wrote this article for a local nature group – Lichen Agile.
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