Euplectella aspergillum: Part I I.
Some Reflections on Sponges, Colonies, and Individuals
by Richard L. Howey, Wyoming, USA
In Part I, we took a somewhat detailed look at some of the types of spicules to be found in sponges and glass sponges, in particular. Here I want to discuss another quite exciting aspect of observing and thinking about these sponges. They raise some very basic issues in the foundations of the biological sciences and interestingly, some "Creationists" have tried to use Euplectella as an argument to support their position, so, in an important respect, this modest, but beautiful little sponge, rests at the heart of a major controversy. But, before we look at that controversy, let's first briefly examine some of the basic biological issues which sponges raise.
1) In most respects, sponges seem almost like an evolutionary aside. Although they are extremely ancient life forms, their evolutionary development seems to have been focused largely on perfecting their structures as water transport and filtering systems. In the most reductive view, they seem little more than aggregates of choanocytes with a few specialized cells and a few variable cells and a bit of jelly thrown in. The ancients, unsurprisingly, thought that they were primitive marine plants. Yet, for all that, there are surprising gaps in our knowledge about sponges. This is, I suspect, a consequence of three sets of circumstances:
a) Most sponges readily observable along a coast are rather drab, although there are exceptions. In fact, many reef sponges have startlingly vivid colors and bizarre shapes, but were visible only to divers and before the development of snorkels and SCUBA, this was a realm largely investigated only by native populations, usually in search of food (this being before the days of the Big Mac and fries). Sponges do not taste good and, in fact, have very few predators. Add to this the fact that sponges are not motile and don't manifest any exciting behavior patterns, whereas all kinds of other reef organisms are engaged in doing astonishing and inventive things and so, it's not surprising that sponges got neglected.
b) A further consequence of this was, that as science got bureaucratized and research was increasingly expected to produce short-term "practical" results, funding to study sponges (and an enormous number of other organisms) became more and more difficult to obtain. This is extremely unfortunate, since there are clear indications that many marine organisms produce complex biochemicals which could someday provide us with the means of producing important medicines for ourselves. In fact, the marine sciences, and especially marine pharmacology, should, in my humble, but very correct view, be funded at levels higher than the space sciences and perhaps even at their expense.
c) But perhaps the major obstacle to unlocking certain secrets is the nature of sponges themselves. They are extremely difficult to study in their natural habitats and very few species survive long in aquaria. Simply maintaining a proper balance of food in the water is no small undertaking and choanocytes remove large numbers of particles of a size of 1 micron or less and the amoebocytes capture and digest larger (but still quite small) organisms and transport metabolic products to other cells. To accomplish this an enormous quantity of water must be filtered and one classic study reported that a small leuconoid sponge about 4 inches high and A inch in diameter, would filter over 20 liters of water in a 24 hour period! Not being able to grow sponges in a controlled environment over long periods means that it is exceptionally challenging to acquire information about their physiology, reproduction, and regeneration.
2) The evolutionary and taxonomic status of sponges presents intriguing problems. It seem likely that they developed from a choanocyte ancestor, but whatever its character, it was quite different from the primordial ancestor of the other animal phyla (the Metazoa). Jn the 19th Century, T.H. Huxley, Darwin's defender and spokesman, and a biologist named Sollas suggested that sponges be regarded as quite separate and distinct from the other Metazoa and Sollas proposed the name of Parazoa for this group. Although, in certain respects, sponges have remained primitive, ironically, it makes them especially good at surviving by having developed a remarkable strategy—the formation of reduction bodies. When conditions become adverse, the sponge reorganizes itself. The choanocytes dedifferentiate, a protective covering is formed and various kinds of amoebocytes are encased. When better conditions return, the sponge begins reconstituting itself and all of this takes place without any nervous system as a coordinating mechanism. One should also recall that sponges are filtering (processing) enormous quantities of water on a continual basis and that brings us to the next point.
3) Many filter feeders have found ways of utilizing substances which are metabolically unusual or even toxic. Certain tunicates, for example extract the rare element vanadium from sea water providing them with green-colored blood—so take that all you Vulcans and Martians! As mentioned, above, very few organisms prey on sponges and the reason seems to be that most of them have developed ways of tasting bad in addition to their generally obnoxiously crunchy texture—which would make eating them like chewing a mouthful of sand or glass in the cases of calcareous or siliceous sponges or like dining on foam rubber in the case of keratose sponges. Some sponges have a strong and unpleasant odor and have been popularly named "garlic" sponges. Other sponges can secrete large quantities of mucus which is repellent to most creatures and toxic to some. The trigger for this behavior must be biochemical, since there is no nerve network to coordinate this response. Another interesting and puzzling behavior is exhibited by the boring sponges (Yawn!—oh, sorry, it's the other sense of boring). Cliona is a typical example and apparently certain specialized amoeboctyes surround minute chips of calcium carbonate and remove and discard them into the excurrent water streams moving through the sponge. What on earth (or water) am I talking about? Well, when you have been sorting through shells that you collected, you have undoubtedly noticed some that had nice, neat round holes in them and, believe it or not, this was the result of the activity of a lowly sponge. It seems that this boring process is not (at least, usually) a result of an acidic secretion and the details of this complex behavior are not very well known.
Apparently some other creatures, mostly crustaceans, and crabs in particular, have learned that sponges can be a valuable defensive ally. Certain species of crabs, some of which are known as decorator crabs, deposit all sorts of bits atop their shells—fronds of hydroids, bits ofsponge, anemones—but in some the sponge seems dominant and takes over, essentially engulfing the dorsal carapace of the crab, thus providing protection in exchange for free transport. Sponges have developed a considerable chemical arsenal for their own protection.
4) The spicules and skeletons of the hexactinellids are the most intricate and complex of those created by sponges which, in one respect, is rather surprising. Sea water contains only very small amounts of silica and in order to construct a 10 inch Venus Flower Basket an enormous amount of water has to be processed.
5) The issue of how we talk about sponges is not simply an academic matter. Are they colonies, simply aggregations of cells, or distinct individual organisms? As I indirectly suggested earlier, it may be that we don't yet have an adequate conceptual and linguistic framework for characterizing these enigmatic creatures.
a) Some provocative experiments have been carried out involving the disassociation of sponge cells. Cells from different species of sponges were pressed through fine sieves or pieces of cloth and mixed and the resulting material was mixed together. It was found that after a period of time, the cells were reaggregating by species. What this suggests is of extraordinary interest. Consider the variability of sponges. In the first place, it is remarkable that there is such a variety of forms in terms of spicules, overall skeleton, composition of the spicules within the skeleton, color, ecological niches, and oscula—all of which indicates that life is constantly experimenting with subtle survival strategies. Each particular species has its own programming to produce 1) a distinctive array of spicules often arranged in a definite pattern, 2) a particular sort of arrangement of the oscula, and 3) the information to determine whether the skeleton will be composed of spongin, calcium carbonate, or silica. Furthermore the amoebocytes seem to comprise a kind of Urstoff, "primordial stuff', from which all the other specialized cells, except choanocytes, can be generated; in other words, they play the sort of role for sponges that stem cells play in human beings. All of this suggests a kind of biochemical imprinting that has a remarkable specificity.
b) Some other equally intriguing experiments have been done which might extend the notion of specificity even further. Sponges of the same species were disassociated and the cells of each individual sponge were stained with a different vital stain and then the cells were mixed together in one container to see if they would reaggregate by individual or whether cells of the same species, hut from different individuals, can mix and form viable organisms. The results have been rather ambiguous and further research needs to be done. This turns out to be an exceptionally important issue, because the question being posed involves the notions of the identity of an organism and cellular recognition. This is not as bizarre as it might seem at first glance. Think for a moment about the human immune system—a series of extraordinarily complex interactions designed to distinguish between "self' (friendly) and "other" (enemy). This is a significant part of why organ transplants are so tricky and the body's natural defense and attack responses have to be artificially suppressed to prevent rejection of the transplanted organ which carries the biochemical signature of enemy.
However, the "New Biology" of genetic engineering, genome mapping, stem cell research, and their spinoffs have led to a kind of exponential explosion of knowledge much like the computer revolution of the past two decades and neither of these sets of innovations shows the slightest indication of slowing down. On the contrary, the developments of new biology are creating a whole new set of perspectives and forcing us to modify many of our old paradigms.
Let's return for a moment to the issue of what it means for an organism to be an individual. I have long been puzzled by this problem and the accompanying issues of colonies. The more I have thought about it, the more convinced I have become that it is very difficult to be clear about either of these concepts.
b.1) We speak of bacterial colonies which seem to have, in many instances, a more or less distinct shape, and sometimes a distinct color as well. However, when we observe such colonies under moderate to high magnifications, we see myriads of individuals actively moving, especially at the edges. So, is this a colony or simply a clustering of individuals? Do these accumulations share benefits and functions or do they just gather together because they are lonely? Humans may do that—consider rock concerts and political rallies—but not, I think bacteria. It is difficult to conjure what benefits might derive from these bacterial bacchanals. There is almost certainly some sort of biochemical attractor operative in these cases. The same is very likely true with Acanthocystis and the dense bacterial population that gathers around the gelatinous dome which it secretes; however, in this case, the tiny amoeba is tricking the bacteria into becoming its lunch.
b.2) How many cells does it take to make a colony? If there are only 2, are they just friends? Phytoflagellates consisting of 4, 8, 16 or more cells have traditionally been described as colonial. Volvox is a particularly interesting instance, because daughter colonies form with the main colony, and fascinatingly, one Volvox might have subcolonies that are all male, another might have only females, and a third might have both male and female subcolonies.
b.3) Accompanying this last issue is the consideration of the transition from a colonial organism to a distinctive, "solitary", "separate", "individual", "independently functioning organism". The reason for all these quotation marks is that these terms and phrases are also used in a variety of senses. Sometimes we speak of the individual cells and/or types of cells within a colonial organism and, at other times, as is the case with certain coelenterates, we also talk of individuals in a colony, but, in this case, we mean a collection of cells within the colony which usually have a specific function and may be morphologically very different from other individuals in the colony. This is particularly striking in the group of 'jellyfish" called Siphonophores in which different individuals within the colony have quite distinct functions which cannot be performed by the other types of individuals which make up the colony.
b.4) This brings us to a set of cases for which we use the term "colony" in yet another sense—the social insects. As in the case of the Siphonophores each type of individual has a specific role and set of functions, but with bees, for example, individuals are separate, but expendable and replaceable, since they are biologically programmed to serve the interest of the hive or colony.
Mammals also form social groupings which we refer to as colonies and here the associations are much looser. Some of these colonies may be very large—those of prairie dogs, for example, which are sometimes described as "towns
b.5) A few biologists have even gone so far as to propose that there is a sense in which an individual human being can be regarded as a sort of "colony". Human beings do, of course, form all sorts of social units and some of them are rather reminiscent of those of the social insects. We often tend to think of ourselves as individuals in terms of uniqueness and a distinct personality but, biologically speaking, the harsh fact is that we are as expendable and as replaceable as the drone bee in the hive. The belief in our own individual uniqueness has led to the positing of a "soul-principle", something which outlasts our rather poorly "designed" bodies. Most human beings seem incapable of accepting the idea that we like other animals are born, live a more or less brief time, die, and decay.
However, the biologists who take this colonial viewpoint have something quite different in mind from the model of the social insects. Some have proposed that the earliest sort of proto-organisms were a result of symbiotic relationships that resulted in a fusion and integration. For example, it has been suggested that a spirochaete-like organism attached itself to a primitive non-motile cell and fused in such a way as to form a flagellated organism. A further case has been made that, at one time, mitochondria were distinct protoorganisms which got incorporated into cells and derived additional protection while functioning as "power supplies" for the new fused organisms. The notion seems to be that individual humans can, in one sense, be regarded as a kind of accumulation of vast numbers of cells which form organs which constitute (at least, in some metaphorical sense) a colony of entities most of which function independently of reflective consciousness.
As scientists have begun, in the last decade or two, to unravel bits and pieces of the staggeringly complex biochemical and biophysical interactions that are the foundation of molecular genetics, they have learned surprising things which hint at many further new chains of insights that may become available to human beings, if we are persistent enough, careful enough, and yet wise enough to proceed with some caution. Some of the hints we've already gotten are exhilarating and also vaguely disturbing.
What we are learning about the complex molecules that are the basic constituents of life is that very minute variations can produce very large changes and for nearly a century, it was thought that all such changes required very large time frames. Barbara McClintock's work, which revealed the 'jumping genes", helped Darwinists resolve, in significant part, the temporal difficulties about adaptive mechanisms. Even Darwin himself had noticed instances in which the evolutionary "process" seemed to require an accelerated framework. Now, we know that adaptation, mutation, and evolution can occur very rapidly at a bacterial, fungal, or viral level. It has been observed that some coastal areas, for whatever reasons, have heavy concentrations, not only of bacteria, but of fragments of bacterial genetic material as well. Such phenomena produce a profound ambivalence in us, because we are discovering that these genetic bits and pieces can be incorporated by one species from quite a different species. From a biological perspective, these discoveries are enormously exciting; from a more self-interested human viewpoint, we recognize the potentiality that some of these genetic combinations may produce some lethal pathogens.
So, what does any of this have to do with Euplectella? Actually, from an evolutionary point of view, possibly quite a bit. When we look at its skeleton, we find it hard to believe that aggregates of "primitive" cells can produce such an amazing piece of architecture. In order to get some idea of how this transpires, we need to remind ourselves of several things.
Relatively simple inorganic chemical compounds can "behave" in surprisingly complex ways. If you make a series of slides of copper sulfate and let each one crystallize at a different temperature, you will find a very pleasing variation in the crystal forms. Impurities in the copper sulfate can also have an effect in this process. An ordinary vitamin C tablet can produce stunning crystals, but "non-active ingredients" used in making the table may affect the formations as well. As a result of careful observation and analysis, distinctive patterns can be mapped out for specific chemicals under known conditions and this knowledge can be quite useful and reliable in many circumstances. However, as with much of science, there is a certain abstractive idealization of conditions for most experimentation and it is at this point that scientists frequently take a pragmatic stance when confronted by nit-picking, philosophically-inclined critics. Both sides are, of course, right.
Let me try to be clearer about this. Take the simple case of the copper sulfate. Ideally we would have this chemical in a "pure" state (untouched by the Seven Deadly Sins). But what, in this case, counts as pure—95%?, 99.95%"?. It would be very difficult and very expensive to produce chemicals that are 99.999999999999999% pure and yet, in some circumstances, we talk about contaminants, especially potentially highly toxic ones, in terms of parts per billion, and fairly commonly, in parts per million. So, here's where the pragmatism comes in. We say 99.95%—that's good enough—that's"pure" stuff and the traces of the other stuff are insignificant. Or if we're really casual, we may use a technical or lab grade of a chemical that's only 90% or 95% "pure", but we always need to be aware that these impurities may affect the results. Remember that our Euplectella sponge cells are extracting silica to build the skeleton from an enormously complex chemical broth with currents mixing and stirring everything up, with temperature gradients, and with not only inorganic compounds, but complex organic molecules floating around as well. On top of all that Euplectella has been found at depths ranging from 500 feet to over 15,000 feet which strongly indicates that it can adapt to a wide variety of conditions. Think for a moment about the variation in pressure from 500 feet to 15,000 plus. This factor alone is why I am not inclined to regard its skeleton as fragile in any ordinary sense. Given all of this chemical and physical chaos, just how pure can the spicules in Euplectella be? In his wonderful book, The Biology of Marine Animals, Nicol has a chapter on "Skeletons, Shelters, and Special Defenses" in which there is a chart titled "Inorganic Constituents in the Skeleton of Some Marine Animals". The analysis which he presents, reports that the skeleton of Euplectella is 99% silica (Si02)!—so that's a rather efficient little laboratory functioning at a cellular level. We still have so much to learn from "primitive" organisms.
These spicule-forming cells are able to lay down an ultra-thin protein thread, extract silica from the surrounding soup du jour, and begin depositing layers bit by bit in characteristic patterns and, clearly, some cells are programmed" to produce one type of spicule and others quite another type. If for the Intelligent Design theorists (who are the "new" "Improved" "updated" Creationists), these kinds of examples are evidence for the existence of a deity—well, that deity must be the Ultimate Micro-Manager, fussing over all these little details, right down to which cells will produce spicules with tiny "thorns" all over them. However, what's even worse than an obsessive micro-manager is an incompetent one and we can find "production errors" in everything from spicules to "monster formations" in Stentor to two-headed calves.
Polytheism would be an explanatory system much more compatible with these biological anomalies than benevolent monotheism, but search as we will, we can't seem to find any traces of Zeus or Hera or Ra, let alone Quetzlcoatl. So, it seems that these sponge cells, much like the crystals are all on their own, busily and mindlessly performing their tasks, "communicating" and coordinating at some very basic, yet enormously complex biochemical and biophysical levels. These phenomena seem so mysterious and "miraculous" to us because we are just beginning to develop the techniques and technologies to understand these processes. One of the wonderful things about nature is that it constantly provides us with puzzles and challenges that force us to stretch our minds and imaginations in order to come up with viable explanatory models.
Science also must take great care not to become dogmatic. Science is, of course, only one of many ways in which human beings explore their world and themselves and a fundamental tenet of scientific enterprises is that one should not make "leaps of faith" in the face of unexplained phenomena merely because they so overwhelmingly complicated. Given our present, rather primitive state of knowledge (just consider the state of medicine). My own feeling is that we should devote much more money and time to researching the phenomena involved in regeneration, try to find ways to get our own organism to utilize its own resources to repair and heal itself, always keeping in mind the temporal limitations of all organisms. It is in these areas that creatures like sponges and echinoderms can provide us with valuable hints.
In the next part of this essay, I will be looking at some issues relating to design, natural order, and again, sponges.
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.
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