The Conundrum of Colonies: More Questions Than Answers
Richard L. Howey, Wyoming, USA
For a number of years now, I have been wrestling with the issue of what constitutes a colony. The matter is complicated by the fact that we use the term in so many different senses such that, in some significant respects, it has made the concept elusive. We speak of bacterial colonies, mold colonies, of sponges as colonial animals, of colonial protozoa, colonial algae, colonial rotifers, colonial bryozoa, moss colonies, of corals as colonial animals, of colonial insects, of colonies of rock-boring worms, of colonial tunicates, and, of course, human colonies. Some biologists have argued that even complex vertebrates, including human beings, can be regarded as enormous colonies of cells organized to maintain the organism until it can propagate and transmit its genes–a view which we’ll consider later.
There are other kinds of cases as well. What about lichens? They constitute an interdependent relationship between an alga and 2 fungi and when we observe them on stones or trees, their patterns certainly suggest that to speak of them as colonies is an appropriate characterization. Then, there are the myxomycetes or true “slime molds” which, at least in certain stages can be regarded as colonial as, for example, when they form multicellular zygotes. This raises another interesting and difficult question: can zygotes truly be regarded as colonies? The true slime molds are a very peculiar case, because in the plasmodial stage they are acellular, multinucleate, and can cover areas of over 4 square feet!
Then there are cases of aggregate behavior wherein a group of animals that we would regard as composed of individuals appears to behave as a coordinated unit. Lobster marches are an example. There are certain times of the year when they form lines and play”follow the leader”. Army ants move in enormous columns with frightening precision attacking virtually anything edible within their path and they almost appear to be a gigantic organism which is biochemically coordinated. Two other more familiar examples are the flight formations of birds and the schooling behavior of fish. What is so striking to us are the remarkable behaviors that suggest a sort of fusion of awareness—a sort of Vulcan mind-meld or perhaps a Borg-like connection of consciousness—which results in splendid displays of choreography.
At the microscopic level, there is the amoeba Radiophrys. We have many highly alkaline lakes and ponds in this area and I took a sample from such a lake and placed it in culture dishes and added nutrient solution. Several days later I checked it, but there wasn't much of interest. After a couple of weeks, I decided to take another look and found something quite surprising. On the bottom were clumps of amoebas with shared pseudopodia which looked like miniature protoplasmic bridges and I could observe the movement within them. I later read that apparently Radiophyrs forms these temporary pseudo-colonies to increase the efficacy of feeding.
For the sake of clarity and sanity, let’s begin with some cases that have traditionally been regarded as legitimate examples of colonies. Anyone who has dabbled in bacteriology at all knows that many bacteria form colonies and that some of them are quite distinctive on agar plates. When, in the summers as a high school youth, I worked in the preparations laboratory for the bacteriology department at the University of Nebraska, I became acquainted with the professor who did water testing for the city of Lincoln to monitor bacterial levels. We always have to remember that life is a series of trade-offs and if you want almost completely bacterial-free water, then you either have to buy distilled water or drink water that is heavily dosed with chemicals. The number of colonies which grow on an agar plate from a standard quantity of a given water sample is one index for determining acceptability.
If you take a sample from such a colony and place it on a slide in an aqueous solution, you immediately discover that there seems to be no pattern or organization, but rather simply a collection of hundreds or thousands of bacteria randomly dispersed in the fluid. The aggregation into colonies is perhaps a primordial paradigm roughly analogous to the schooling behavior of fish. Often our first thought concerning bacteria is that they are pathogens that will attack us, our animals, or our fruits, vegetables, grains, cheese, etc. What we tend to forget is that the vast majority are harmless and many are not only beneficial, but essential in breaking down the vast quantities of organic natural and human generated waste. However, even more importantly, they are food for many other micro-organisms–just observe a Paramecium feeding around a clump of bacterial material. Also, in many, perhaps most, species there is a sense of biochemial identity; that is, you do not ordinarily find a mixture of species within the colonies anymore than you would find carpenter ants marching with army ants. If you do find bacterial colonies with mixed species, you can be certain that is because there is some kind of beneficial consequence of such an interaction.
There is also the stunning fact that there are more microbial life forms living in us that the number of cells we possess. We are in a significant sense a composite organism; we could not survive without these beings supplementing and supporting many of the functions which are essential to our existence.
The startling thing about all of this to us, is that such tiny forms of life could have some kind of recognition of other organisms that belong to their “tribe” and a recognition of those that are “alien”. Fascinating experiments have been done with sponges on this issue. The cells of the sponges of more than one species have been mechanically disassociated and then put into an aquarium. It was found that the cells did indeed reaggregate according to species and there are almost assuredly biochemical markers that produce a recognition of “like” or “other” just as cells in our immune systems “recognize” “alien” cells. Experiments have also been conducted in which 2 sponges of the same species have been disassociated and one of the sponge’s cells stained with a vital stain. Here the results have been less clear. It is likely that the cells of the same species, even though from 2 different “individuals” have the same or very similar biochemical signatures. A student, under my supervision, carried out some similar experiments with the fascinatingly odd Trichoplax adhaerens which is sufficiently strange that it is the only organism in the entire phylum Placozoa. You can read a bit more about it here.
The student stained one specimen with a red vital stain and the other with a blue vital stain, disassociated the cells, and mixed them together. As you can see from his photograph in the article mentioned above, Trichoplax does not demonstrate individual identity recognition, since in the reaggregation you can see both red and blue cells. For a few sponge cell studies, however, there were some intriguing hints that in some species there might be individual “self” recognition.
Back to bacteria. There are a number of protozoa that spend at least part of their time scouring the substrate for bacterial masses. Often when I have been browsing in a culture dish with my stereo-dissecting microscope, I have come across such clumps and have generally ignored them. However, in the last few years, I began to take more notice of them, because it struck me that some of these clumps had quite distinctive forms. For example, one type I encounter fairly frequently has a series of finger-like projections and, in fact, bears a sort of outline resemblance to certain kinds of “finger” sponges. We should not be surprised to find certain kinds of pasterns recurring in nature over and over. D’Arcy Thompson in his class work On Growth and Form marvelously demonstrates this profound and basic fact.
Other aquatic bacterial colonies take on a dome-shaped appearance and still others are nearly spherical in character. Generally, such colonies are relative easily disassociated by a strong jet of water from a pipet. However, there are some, certain of the finger-like ones for example, that seem to possess a relatively resistant membrane enclosing the colony. A significant number of micro-organisms secrete gelatinous, chitinous, or other membranes, loricas, tests composed of complex organic compounds. Others, like foraminifera, form complex shells composed primarily of calcium carbonate.
Still others, such as radiolaria, extract silica from the water to create elegant floating domiciles which are essentially glass.
And there is another group of marine amoebae that construct shells primarily composed of Strontium sulfate, the acantheriads.
One can almost image a committee of radiolaria having a meeting and deciding to build a glass Trump Tower (but, much more tasteful)–a glass sponge, such as, Euplectella aspergillus (Venus flower basket).
We know that ants, bees, and wasps cooperate to build suitable residences for the various needs of the colony, so why shouldn’t cells be able to band together to produce elegant sponges?
Interestingly, it has been recently discovered that termites can build absolute enormous colonies. In Brazil, hundreds of millions of mounds spreading over the area the size of Great Britain have been discovered and they are estimated to date back as far as 4,000 years. The mounds are essentially dumping grounds for soil that has been excavated creating a gigantic complex of underground tunnels which are still inhabited! Nature still has lots of surprises for us when we look carefully.
In an intriguing way, sponges are an evolutionary dead end, in that nothing seems to have descended from them. However, one should not sell them short as a phylum (Porifera). There diversity of shape and color as absolutely astonishing. Don’t take my word for it, however, just go browse on this site.
If I could afford a coral reef aquarium, I would, without question, include some of these gorgeously alien creatures. In the first place, who would ever think that they are animals, and in the second place, who would ever think that they are colonial?
However, before we get too far ahead of ourselves, there are some fascinating problems presented by certain protists. Let’s start with a few examples from the wonderful world of diatoms. The casual observer may be inclined to think of diatoms as separate and individual organisms which many of them are. However, consider Astrionella, a stellate form with from 4 to about 20 “individual” rods connect to a small central structure, although in some instances, that central area seems to be empty.
Are these colonies? Can the individuals if separated survive on their own? Do they seek out others to form a new “colonial” aggregation? Another example is Fragilaria where the “individuals” are stacked and “connected” at the center of the organisms.
Tabellaria, however, has “individuals” which are rectangular and have connections at the corners forming a strange sort of chain. The same questions apply: Can the individuals survive separately? What benefit do they derive from being connected? And these questions are just beginning? And if you looked carefully at all of those images of Fragilaria and Tabellaria, you will have noticed that there are variations which are departures from those simple descriptions which I gave.
If we look at the desmid Scenedesmus, we see four cells (usually) which are connected, but the 2 outside cells have “horns”, thin, elegantly curved projections on each side. I think it’s unlikely that these cells would survive individually, but perhaps any one of them could divide and create a new complement.
Every aquatic biologist is familiar with the lovely Leprechaun hair of the alga Spirogyra. These long strands have distinct cell walls and with each one is a green coil which contains chloroplasts. It has sexual stages and, if you’re lucky and patient enough, you can sometimes find strands that are conjugating.
Spirogyra often grows in large clumps and one can cut pieces into sections and the sections will continue to grow given the right conditions. So, is a strand of Spirogyra a colony?
There are some algae which seem to be distinctly colonial. such as, Eudorina and Pandorina. Each species has a more or less constant number of cells which are in a spherical gelatinous matrix and each cell has 2 flagella which extend through and out beyond the matrix. If these colonies were to be chopped up, again, it’s possible that a single cell could reproduce and reconstitute a new colony, but I don’t know that anyone has ever demonstrated that.
Here a different sort of question occurs to me. Admittedly, there are variations and anomalies, but given that: Why do some colonial flagellates have 4 cells, and others, 8, 16, or 32 on a fairly constant basis? Is it always an even number? How do cells coordinate their motion so that they are able to move the colony in a given direction?
There is also the puzzle of the alga Hydrodictyon, the “water net”. It has a sufficiently distinctive shape to make it readily identifiable and yet it forms a lattice to which it continues to add. Colony or individual organism?
Flip a coin. So, let’s move to some more difficult kinds of cases. Nostoc is clearly a colony. It is a filamentous cyanobacterium which looks like a string of beads, but these filaments are embedded in a greenish-white to greenish-brown matrix which can vary in size from a fraction of an inch to over 8 inches. In a shallow lake at an altitude of about 10,000 feet, I have found colonies about 3 inches in “diameter”. The reason for the quotation marks is that these colonies are very often not spherical, but rather lumpy in character. They are somewhat elastic and firm; it takes considerable pressure to cause one to rupture. Nostoc must secrete this gelatinous material abundantly all along the lengths of the filaments because you can trace individual filaments with the matrix and they are not in direct physical contact with one another. An interesting aside, Nostoc was first described and named in the 16th Century by the fascinating medical eccentric Paracelsus (Full name: Phillipus Aureolus Theophrastus Bombastus von Hohenheim).
There are cells along the filament that are morphologically different from one another and it may be that certain ones specialize in producing the secretions. Does that mean that we might have 2 types of colonies here?–one, the filament itself, and two, the aggregate within the quasi-spherical gelatinous matrix? (I told you in the subtitle that there would be more questions than answers. Even in grade school, I used to get into trouble with the teachers for asking too many questions.)
Another set of puzzles is present by large brown algae like Fucus, frequently found attached to rocks along the seashore and Macrocystis, the giant kelp. Both have special structures called bladders that are filled with air and play a crucial role in their survival.
Giant kelp have massive holdfasts that they use to anchor themselves. (If you ever get a chance to study one of these, you will find that an enormous number of small organisms have taken refuge in the tough root-like structures constituting the holdfast.) So, are these large algae colonial? It seems a bit odd to talk about them as having organs (air bladders , the holdfast) just as it would seem odd to speak of a tree having organs. As you can see, there is a great deal of confusion about how to talk about biological individuals and biological colonies and not all of that confusion is mine. In the last few years, I have read a considerable number of discussions on this topic by scientists and their comments too express a good deal of confusion, vagueness, and a lack of clarity. Part of this problem is ours; we have not yet evolved sufficiently in terms of conceptual and linguistic discrimination. However, the other part of the problem is that nature is a trickster and has no respect for your Herculean efforts to get things neatly ordered and classified.
Let me give you a case of a “micro-tree”. It is a colonial freshwater flagellate called Anthophysis vegetans. There is a long brown, branching structure which has an aggregate of flagellates at the tips of the “branches”. These tips are rather fragile and when you mount a nice drop of brown goo that happens to contain Anthophysis, the pressure of the cover glass tends to break some of the tips, thus “liberating” the little aggregates of flagellates that go swimming off, spinning through the water as colonial flagellates are wont to do. Then what happens? I don’t know and I don’t think anyone does but, if the government would quite wasting money on wars and give me a billion dollar grant, I’d find out for them. Can this one little colony go off and start a whole new trees with tips, each of which has a new colony? or does it just whirls through the aqua-sphere into oblivion? People use to ask me what I was going to do in my retirement and I would jokingly answer that I had enough projects of 1,000 years–not nearly enough time, I have discovered.
Branching structures are, as D’Arcy Thompson so graphically reminds us, common in nature. We find wonderful examples of branching in the colonial invertebrates known as bryozoa or “moss animals”. Some bryozoans form the familiar white flat colonial mats that are frequently found on the larger algal forms washed up on shores. However, many, like Bugula, form delicate branching colonies that are sometimes difficult, at first glance, to distinguish from hydroid colonies.
A bryozoan lives in each of the little capsules along the stalk and presumably each individual contributes to the constitution of this stalk. No one know why one will suddenly “decide” to go off at an angle and start a new branch. However, even more amazing are structures that occur at various places along the stalk and which are called avicularia because they look like tiny birds’ beaks. (COULD THERE BE SOME KIND OF PARALLEL TO THE DEVELOPMENT OF PEDICELLARIAE?)
Bryozoa are multicellular organisms and are mostly marine. There are, however, some freshwater species and one that occurs in lakes and pond in the area around Laramie. One summer I found some lovely, lacy colonies of Plumatella repens on the underside of some logs in a wonderful spring-fed beaver pond located at about 8.000 feet. It is a somewhat difficult place to get to and requires a high-profile, 4-wheal drive vehicle–otherwise you’ll damage the undercarriage of your vehicle on the rocks on the trail, which goes down at a steep angle, makes a sharp left, goes up and the stops in a splendid meadow populated with sage, cacti, and wildflowers and to the right below this is the beaver pond. Since I sold my old, reliable 1973 truck several years ago–unfortunately, after 30 years, it was not so reliable–I can’t get into that pond anymore, so one summer I decided to try hatching and growing my own Plumatella colonies from statoblasts which are a sort of cross between and egg and a cyst.
I was aware that this was likely to be a frustrating undertaking, since over the years I had found statoblasts in dozens of lakes and large pond, but had found actual colonies only in the beaver pond I mentioned above. I collected a fair number of statoblasts from the shore-side detritus of large lake. I selected out ones that appeared to be in good condition and added about 20 to each of five 4 ½ inch culture dishes along with some Colpidium from cultures in other dishes which had them in large numbers. Bryozoa like a plentiful supply of food, but they also like clear, clean water without much bacterial contamination. Everyday I would check to see if any of the statoblasts had settled to the bottom and begun to swell. In their dormant stage, they are designed to float and while there is a bit of a bulge in the middle, the edges are very thin. When one begins to develop, however, the edges thicken and it look like a miniature oval pastry that is rising. After 2 weeks, 2 of the dishes had a statoblast that had settled to the bottom , begun to swell, and were beginning to extend themselves beyond the edges of the statoblast. When the first Plumatella “polyp” extended itself, I was enormous pleased and excited. It is a remarkable experience to see the organism extend its white, feathery tentacles, covered with cilia, come out of its tube and begin to feed by creating currents in the surrounding water. The other 3 dishes hadn’t produced anything, so I discarded them and concentrated on the 2 that were developing colonies. I fed them every day with Colpidium which was apparently the bryozoan equivalent of filet mignon. Every few days, I would also remove any accumulated debris and remove a portion of the old water and add some fresh artesian water. Although this process was rather tedious, I was able to get the colonies to spread across the bottom of the dish and they flourished for over 3 months. It was quite marvelous to see a new extension begin and then a day or two later to watch a new polyp extend its tentacles and begin to feed. Bryozoa are among those remarkable animals which can reproduce both sexually and asexually and the asexual type is accomplished by the extension of tubes or stolons creating new members of the colony. So, from this one, brown, papery-looking, oval statoblast, a lovely, lacy colony several inches long, branching in multiple directions, can be produced. Some species are so rampantly reproductive that they can become a major and costly nuisance. J.S. Ryland, in his wonderful little volume on bryozoans reports an instance of an enormous bryozoan mass clogging water outlet pipes from a reservoir near Manchester and the recounts that the unclogging involved the removal of tons of these organisms!
At this point, I am going to make a tentative suggestion which may be going in the wrong direction, but will perhaps give us a bit of a general framework. It seems to me to be necessary to make some distinctions between certain usages of the term “colony” in order to avoid serious confusion and equivocation. Right off, I can think of 6 or 7 different senses which are distinguishable if not completely distinct. However, for the moment, let’s focus on just 3 aspects to begin to try to orient ourselves.
It occurs to me that it might be helpful if we were to emphasize that the term “colony” is applied differently in relation to 1) Plants, 2) Animals, and 3) Protists (and, for the time being, throwing Fungi, Bacteria, etc. into this general heading.) This will make a good point of departure for the next part of this article in the future. This has already gotten rather long and so is best continued later.
NOTE: Over the years, my friend Mike Shappell has encouraged me (even prodded me) to write on this topic of colonies. It is enormously complex and a huge undertaking. My health is such that I may not be able to continue it and so I am passing the torch to Mike and I am sure that he can provide some future reflections that will be valuable in furthering the discussion of this fascinating set of problems.
All comments to the author Richard Howey are welcomed.
Editor's note: Visit Richard Howey's new website at http://rhowey.googlepages.com/home where he plans to share aspects of his wide interests.
Published in the February 2021 edition of Micscape Magazine.
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