A Spineless Column by Ronald L. Shimek, Ph.D.

Nano-Animals, Part I: Rotifers


Introduction:


Regular readers of this column will be aware that I have a fascination with the bizarre and weird. I guess this comes from growing up in central Montana. If one knows where and what to look for, some of this area may be seen to be badly disturbed ecologically; however, in comparison with most of the United States, much of it looks absolutely pristine. In such an area wildlife abounds, and when I was but a mere lad I became familiar with all of the common sorts of the animals found in the area, from horses to horsehair worms to toads, horned and otherwise, various bugs, Black Widow spiders, and fishes of all sorts. Once I become really familiar with an animal or animal type, I "file it away" and go on to something different. Lacking a microscope when I grew up, I didn't learn about microscopic animals until I was in high school, and then only cursorily. It wasn't until I reached college that I began to explore the wonderful world of the miniature animals and other organisms found all around us (and in us, as well, but that is another story).

Although my introduction to these, the smallest animals, came with the examination of freshwater microscopic organisms, similar organisms are common in marine systems. In fact, with few exceptions, most of the groups containing small animals are more common in marine ecosystems than in freshwater ones. That having been said, the animal group of focus in this column, the rotifers, is somewhat abnormal in that it is more diverse in freshwater environments than in salty ones. Subsequent columns in this short series on small animals will discuss animals that reach their acme of diversity in marine ecosystems, but that is not the case with the rotifers. Although rotifers may be easily found in salty water, they are both more diverse and often more abundant in freshwater habitats.

When discussing small organisms, the discrimination of what is, and what isn't, an animal becomes rather blurred. Some of this blurriness has come about in the last half century or so, directly as a result of research that increased our knowledge base. When you have only a couple of data types, it is easy to use them to categorize things and put them into some sort of order. When the amount and kinds of data become diverse, such categorization may become very difficult. Virtually all of the early examination of microscopic organisms used light microscopy as its primary tool. This is an excellent way to examine these creatures, but as with all ways of observing something, it imparts a bias. Basically, the bias is a simple one: "If the creatures look alike, they probably are alike." This seems simple and intuitively correct. For many large organisms this bias works pretty well. However, when examining the "teeny-tiny," such an approach becomes unworkable. Its major problem is one of structural complexity, or more correctly the lack of structural complexity. For example, while large animals are almost always structurally complex, having organ-systems, many types of tissues and cells, literally by the trillion, smaller organisms are often much simpler in structure. If aspects of that structural complexity are used to categorize or "pigeon-hole" organisms, then what happens to the ability to categorize when the complexity isn't there? When observing the apparent simplicity of small organisms, the question arises, "Are these organisms truly fundamentally simple or have they become simple while evolving from much more complicated ancestors?"

Life in the small lane is constrained by all sorts of odd factors, and as natural selection has worked in these environments just as well as in the realm of the large and ponderous, the question becomes one of convergences. In other words, "Do the structures we see in two different organisms reflect a common ancestry (divergent evolution), or do the organisms only seem similar due to superficial similarities caused by convergent evolution resulting in the loss of complex structures that yields similar simple appearances?" This fundamental question remained largely unresolved until the mid-1980s when examination of the actual genome started to become commonplace.

As a result of such studies, the whole "world-view" of how living organisms are related - and grouped together - has changed, and those changes continue unabated. The classification schemes and presumed relationships between groups that I learned and taught as recently as five years ago have been shown to be based upon inadequate data and have had to be revised. The species haven't evolved much in that period, but our understanding of their relationships has been altered by the inclusion of a lot of new, supportive data. This has allowed fine-tuning and precision within the "Tree of Life" that hitherto had been only surmised.

If we accept the premise that organisms with similar genomes are related, then their degree of relatedness can be determined by measuring the degree of difference between the genomes. Degrees of relatedness determined by genetic testing can then be compared with degrees of relatedness determined by other, more traditional methods, such as skeletal or structural analyses. With most of the larger animals, there have been few surprises; generally, the data sets concur pretty well, and although there have been a number of "revolutions" in the relatedness of major animal lineages, generally what had been thought to be closely-related groups usually turn out to be so. This hasn't been the case with many of the smaller organisms. Because of all these changes, the old dichotomy of life as belonging to either the animal or plant kingdoms has been discarded.

Presently, the classification scheme used to categorize organisms, based on genetic and cellular features, is one whose largest, most inclusive groupings are called Domains of Life. Generally, there are considered to be three Domains. Within, or under, each Domain are subdivisions called Kingdoms. The animal and plant kingdoms have been redefined, and both are now regarded as containing only multicellular organisms. Under the present classification scheme may be separate kingdoms for many of the major algal groups as well as for many unicellular organisms previously categorized as protozoans; however, that sort of standardization is in the future. For the purposes of this essay, though, the important point to remember is that all animals are composed of more than one cell.

Multicellularity


Animals, then, by definition, have more than one cell, which fundamentally limits organisms' possible "smallness;" animals can get very large, but they can't become infinitely minute. If a body is to be made of functional components called "cells" and that body is constrained to be small, then there is an absolute limit below which an organism made of multiple cells cannot be made. Below that limit, each cell would simply not have enough volume to contain what it needs to function in the context of a multicellular organism. Organisms constructed without cells, or organisms constructed of only a single cell, may be much smaller, but for organisms made of multiple cells, there is a lower size limit. The smallest common animals are the subject of this essay; these are the strange animals called "rotifers." Only the very odd animals called "Loriciferates" are smaller than rotifers, but loriciferates are so poorly known - no living specimen has ever been studied - and so seldom seen that it is hard to discuss them. Rotifers, on the other hand, are exceptionally common and very well known. Although small (large rotifers seldom exceed a fiftieth of an inch in length and some of the smaller ones are only about 40 µm, or about 0.0016 inch, long), they are among the most ubiquitous freshwater creatures, and many species are common in marine areas as well. When examining rotifers, it is astounding to realize that these complex animals, containing, for example, several organ systems, are smaller even than many single-celled organisms such as Paramecium and Amoeba that used to be called protozoans.

The animals normally referred to as rotifers share a single unified body plan and are readily recognized by anybody who has had the opportunity to see one. Even so, they are relatively diverse; about 2200 species are described. Taxonomically, they are placed into the Phylum Rotifera, which is subdivided into several major subgroupings and a couple of smaller groups. The recent "fly in the taxonomic ointment" is, however, that what had hitherto been considered a discrete phylum, or major group, of parasitic organisms, the Acanthocephala, are descended from, and should be considered as, highly modified rotifers. Genetic research has determined this is unequivocal; the Acanthocephalans are modified rotifers, but just how, exactly, they fit within the so-called phylum Rotifera is open to question and has not been resolved. The resolution of that question, among a number of other questions, renders meaningless the discussion of the relationships between major subgroups in rotifer taxonomy. However, the purely "classical" rotifer groups themselves haven't changed much, and so may be discussed without loss of information.

Adult and juvenile rotifers have no way to avoid desiccation and must therefore live in water. Freshwater rotifers often seem to be specialized to live in specific transitory environments, and part of that specialization has been the development of some water-resistant reproductive stages. These are basically eggs with highly resistant shells. Rotifers may make their homes in some habitats, such as the fluid captured deep inside a tubular flower blossom, or the muck of a simple mud puddle, that would seem unlikely places to be dignified with the name "ecosystem"; nevertheless, these small transient bodies of water have all the attributes of much larger ecosystems: a physical environment, biota and food webs distributing and transferring energy and materials. And in them, rotifers thrive. In point of fact, these small "here today, gone tomorrow" habitats are specifically the places where rotifers are most abundant and diverse, and many of their attributes appear to have been adapted for maximal utilization of such environments. Their small size notwithstanding, some species of rotifers are herbivores, others are carnivores, still others are suspension-feeders and yet others are parasites.

Morphology


Rotifers are sometimes described as elongated worms, but they really don't look very wormy. Their body is often divided into three regions, which often "telescope" in and out of one another. Their body is covered in a rigid cuticle that holds its shape very well, so unlike worms which twist and turn and appear to be almost infinitely flexible, rotifers' shapes are relatively constrained and consistent. Many of them look like self-mobile miniature torpedoes. One of the rotifers' best identifying characteristics is that the front of the animal bears a crown of large, evident, beating cilia. These cilia whip, flex and bend, and in the process they move water past themselves or they move the animal through the water (see some rotifer movies: 1, 2, 3). Rotiferan cilia function to move the animals through water and additionally, they bring food particles to the animal. Rotiferan cilia are arranged in circular patterns to form the "corona," or ciliary crown, on the front of the animal. When these distinctive cilia are actively beating, they tend to look like moving wheels, so much so that for many years, rotifers had, among biologists, the peculiar name of "wheel animacules." In fact, the name "rotifer" is derived from a combination of the Latin roots "rota" or "rotula," meaning "wheel," and the Greek root "phoreus," meaning "a bearer." In other words, they are animals that bear wheels.

Figure 1. A couple of marine rotifers. See Figure 3 for anatomical structures. Both animals are between 0.05 and 0.1 mm (0.002 and 0.004 in.) long.

Figure 2. Not all rotifers are mobile swimmers. This individual of a sessile species was living fastened to a hydroid stalk (the brown structure running vertically through the image). Sessile rotifers often use their lorica as a "house" to withdraw. The corona's cilia are visible as thin, straight lines in the image. This animal was about 0.05 mm (0.002 in.) long.

Behind the crown of cilia, the majority of the body is called "the trunk" and is covered with a hard shell, often called a "lorica." This lorica is transparent and appears to be the body wall; it is not distinguishable by color or shape. It is often composed of cylinders or rings, and may give the appearance of being segmented. Many rotifers' bodies are "telescoped" with one section of the trunk sliding into another. The lorica is a peculiar and complex structure; its external part is formed outside the animal by secretions of the body surface. This part is non-living and made of proteins. There also is an internal portion of the shell formed of cellulose-like materials. Consequently, in some regards, a rotifer's shell is somewhat like, but on a much smaller scale than, a turtle's bony shell, the inside of which is living, and the outside, which may be dead.

The last part of the rotifer's body consists of the foot, which often has two lateral branches called toes. This foot is used in locomotion, but not as an ambulatory organ. Rotifers usually fasten to the substrate using their foot to provide adhesive purchase. The foot contains duo-gland adhesive structures, so when the animal wants to stay in one place, it touches its toes to the substrate and secretes the adhesive. When it wants to move, the toes secrete a second substance that releases the toes from the glue, and the animal swims off. Many rotifers creep along the substrate almost in an inch-worm manner using their toes and adhesive glands to fasten to the substrate as they crawl along.

Figure 3. The basic structures found in a rotifer, using the common aquarium food species, Brachionus, as an example.

Rotifers are adapted to many different modes of life and, not surprisingly, these are reflected in their bodies' different shapes and functionality. The swimming forms generally are rigid and many have spines to protect them from predators. Often, these spines are variable in size and extent. In some species of Brachionus spines appear only when its major predator, another rotifer called Asplanchna, is present. For its size, Asplanchna has a huge mouth and eats Brachionus individuals by engulfing them whole. When Asplanchna are absent, Brachionus is spineless or has, at best, very short spines. If Asplanchna appears, the longer spines are found in the NEXT generation of Brachionus. The presence in the water of specific chemicals, small protein fragments, released by Asplanchna as it swims around, causes the production of spines in Brachionus embryos. These spines can be large enough to make the prey inedible; the predator simply can't fit the prey into its mouth.

Rotifers are very peculiar animals, however, much more so than people who have examined them only in passing ever realize. Most people who have taken any biology or even general science courses know that "ALL" living things are composed of cells. Unfortunately, I suppose, no rotifer has been to these classes, and consequently they don't know that they are supposed to be made of cells. So... they aren't. The majority of rotifer structures and tissues appear to be made of standard cellular tissues, but closer examination shows that most of them lack cell membranes that completely delineate those cells. In effect, each "tissue" or "organ" is a blob of protoplasm containing several nuclei or cellular control centers. Unlike most animals, however, these nuclei are not found in cells, but float together in their little "blobs of goo." These protoplasmic masses, which have many nuclei but lack cellular membranes separating them, are called syncytia. Syncytia seem to be characteristic of many smaller animals such as the rotifers and nematodes, in addition to some other odd groups such as the glass sponges (Hexactinellids). In the smaller animals where they are found, they may simply be a biological solution to microminiaturization. On this scale of animal dynamics, expending the energy and materials necessary to make membranes to delineate cells may not be cost efficient. In effect, the animal is so small that the benefits of being cellular, presumably a compartmentalization of structure and function, don't outweigh the costs of producing the cells or the costs of moving materials across cell membranes.

Rotifers and a few other groups of animals, including nematodes, share another odd property. It is called "Eutely," or "cellular constancy." Eutely means that all adult members of a given species of animal have exactly the same number of cellular nuclei in exactly the same place. For example, in the common freshwater rotifer Epiphanes, their 959 nuclei are arranged so that 172 are in the corona, 108 in the trunk and foot epithelium, 19 in the pedal glands, 22 in the circular muscles of the body wall, 40 in the coronal retractor muscles, 183 in the brain, 68 in the peripheral nervous system, 167 in the jaw apparatus called the mastax, 15 in the esophagus, 39 in the stomach, 12 in the gastric glands, 14 in the intestine and 25 others distributed in other places. Every Epiphanes will always have this number of nuclei in exactly the same place in its body.

Such a stereotypical body plan is unlike what is found throughout most of the animal kingdom, and it has some profound consequences for all animals that possess it. On the down side, because no cell, or, more correctly in this case, any nucleus, has any latitude in its genetically determined position or function, there can be NO repair of injuries. Simply put, a damaged rotifer dies. The trade-off is that such a deterministic body plan apparently allows very rapid growth and development. Many rotifers live fast, die young and leave a little blobby corpse. Many of them have a life span of only about a week. Numerous species may reach adult size less than a day after hatching from the egg. Such rapid development may be possible only with a "slimmed-down" genetic complement. In effect, many of them are lean, mean, reproductive machines. Once an egg reaches a puddle it can hatch and be ready to reproduce within a day, and several generations may be found in a mud puddle that lasts a week. All of this appears to be the result of countless generations of rotifers adapting to transient habitats by reducing their genetic code to the barest necessity.

All of these genetic reductions might tempt an observer to think that they have reduced other things, such as organ systems, as well. This definitely does NOT appear be the case. For example, their nervous system is remarkably well developed. Consider that the Epiphanes mentioned above has 183 nuclei in its brain. This means that 183/959, or 19.1%, of the animal's nuclei and protoplasmic material is devoted to the brain. In comparison, an adult human's brain contains no more than about 2-3% of the body's mass and cellular structures. On this scale, at least, rotifers are proportionally and significantly more "brainy" than most politicians, if not the rest of humanity. Rotifers have a well-developed, bilobed brain. Additional components of their nervous system include several sensory tentacles, used to detect chemical and tactile stimuli. They generally have at least one photoreceptor, commonly called an "eye," and possess tactile cilia as well.

The rotifers' muscular system is well-developed, with individual cells working as independent, separate muscles. Probably as a result of their small size, they lack the muscles comprised of many cells and the muscle bands found in other, larger animals. Nonetheless, they are capable of a wide array of muscular movement and precision.

Figure 4. A marine rotifer, with the position of its muscles indicated by the red bands on the right image. The animal was about 0.1 mm (0.004 in.) long.

Their digestive system is complex, consisting of three major regions: a foregut, a midgut and a hindgut. The foregut consists of the mouth and the food grinding apparatus, or mastax. The mastax is a large (for a rotifer) muscular mass surrounding an inner mouth region. In it are three grinding jaws arranged so that its gut in cross-section has the appearance of a pie made of three slices, but with each slice being a massive grinding plate complete with ridges and teeth. The short esophagus passes the food to the large stomach, which is surrounded by a digestive gland. Digestion, unlike the condition found in many invertebrates, is extracellular and occurs in the stomach cavity. Nutrients are absorbed through the stomach lining and transferred directly to the digestive gland by syncytium-to-syncytium transport. Food is further processed in the digestive gland, in a manner probably analogous to what occurs in a vertebrate's liver. The short intestine also absorbs some nutrients but functions mostly to transfer food remains to the anus.

Nitrogenous waste, mostly ammonia, is excreted by the kidneys through a flame bulb, or protonephridial construction. Excess water is also moved out of the body though the same tubules, flushing the system. Rotifers are so small that they have no need for a circulatory system. Nutrients are simply passed into the fluids filling the body, and moved from place to place by the movement of the whole animal.

As with many of the rotifers' other characteristics, their reproduction is an exercise in strangeness. In one of the groups, the "Bdelloid rotifers" commonly found in freshwater, no males have ever been seen and likely do not exist. The females in this group produce parthenogenic eggs. These eggs develop into small juveniles before they are released from their parent, and actually are clones of their mother. To further maximize reproductive potential in some species, when the juvenile exits the mother, it already contains developing embryos. In a very real sense, they are truly born pregnant.

One of the tenets of biological science over the last 60 or so years is the concept of the biological species. Proposed by Ernst Mayr in 1942, a "biological" species is supposed to be a group of animals, all of which can interbreed with each other. Mayr erected the idea of the biological species in response to the "morphological" species, or "morphospecies," concept, wherein a species were all of the animals that looked alike. Of course, the problem with the morphological species is that "looking alike" is a lot like the concept of beauty. How "alike" animals are varies with their beholder. The morphospecies concept can create all sorts of problems depending upon who is deciding what is "alike." One biologist, C. Hart Merriam, over a period of years described more 90 species of different Grizzly bears. We now recognize that all of these so-called species were, in fact, just habitat or even individual variations. Such an absurd subdivision of any group creates a nomenclatural fog that obscures and retards actual research. Consequently, Mayr's attempt to create biological species was considered to be a great leap forward in biological thinking. However, it didn't take into account such organisms as bdelloid rotifers. In the biological species concept as used by Mayr, either no bdelloid rotifer group constitutes a species, or each bdelloid lineage is a species unto itself. To some extent, such a situation is simply a matter of nomenclature. On the other hand, it also indicates that these animals are very different from sexually reproducing animals, including other rotifers. Having no sexual reproduction and no means of expressing variability, bdelloid rotifers may have, in effect, opted out of evolution. They don't appear to change or to have any adaptive capability. On the other hand, they are widespread and perfectly adapted to habitats that will probably always be present, at least for a long time, evolutionarily speaking. Other rotifer types do have males, at least part of the time, and by examining their life history patterns, it is easy to see how the parthenogenesis of bdelloids got started. Once it occurred, that lineage was locked into a form that never can vary.

The sexually reproducing rotifers constitute the other, and largest, group of rotifers. These animals, including the aquarium food genus, Brachionus and other cultured rotifers, consist of separate sexes that do not look alike. The small males inject sperm through the female's cuticle. The female typically has a small ovary which contains about 20 nuclei (eggs). These particular species' life cycle is cued to environmental conditions. As long as the animals are growing in good conditions, thin-shelled eggs are produced that have two sets of chromosomes. These eggs are produced parthenogenically. When the environmental conditions in the habitat turn bad, if the animals are overly crowded for example, a different kind of egg is produced, with only one set of chromosomes. If the female doesn't mate, the egg is unfertilized and develops into a male. If she mates, it gets fertilized and develops a thick, resistant shell. When this occurs, it is called a "resting egg." It will not hatch until environmental conditions improve.

This type of life cycle is tailor made for aquarist cultures. The resting eggs may be sold as starter "cysts" or "eggs." Once rehydrated in a good culture medium, the animals rapidly grow and produce a lot of offspring. As long as the culture conditions remain good, that is, as long as there is no accumulation of waste materials, the oxygen concentration remains high, and steady, carbon dioxide remains low, and there is plenty of food, the animals continually reproduce, and can be harvested and fed to the animals in the reef tank. Once the culture conditions deteriorate and the population starts to crash, resting eggs are produced and , with care, may be collected, dried and used to start a new culture.

Rotifers make a good food for many small fishes, fish larvae, or invertebrates specialized on small planktonic animals. The major drawback of these cultures is that although the rotifers used in them, mostly one species or another of Brachionus, may tolerate saltwater for short periods, they really don't do well in it, and therefore don't last in the tank for any appreciable period of time. Nonetheless, they are easy to grow and do make a good transient food source that mimics the small gelatinous zooplankton that many animals feed upon. Additionally, they are harmless to anything in the tank and will not spread diseases.

Truly marine rotifers do exist, but they are somewhat uncommon and hard to culture, and most of them are not planktonic. One group is even wholly parasitic on crustaceans. Nevertheless, the available rotifer cultures do make a good substitute for the small gelatinous zooplankton lacking in many of our systems.



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References:


Mayr, E. (1942) Systematics and the Origin of Species. Columbia Univ. Press. New York, New York. 567 pp.

Standard invertebrate zoology texts with good treatments of the rotifers are:

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Ruppert, E. E, R. S. Fox and R. D. Barnes. 2003. Invertebrate Zoology, A Functional Evolutionary Approach. 7th Ed. Brooks/Cole-Thomson Learning. Belmont, CA. xvii +963 pp.+ I1-I26 pp.

Much more in-depth information about rotifers is found in the classical treatment in this volume:

Hyman, L. H., 1951. The invertebrates. Volume 3. Acanthocephala, Aschelminthes, and Entoprocta, the Pseudocoelomate Bilateria. McGraw-Hill Book Company. N. Y. 572 pp.

And a LOT of modern information on their anatomy and biology is found in this volume:
(if you really like invertebrate anatomy, you may purchase the entire series for only $6250)!

Clément, P. and E. Wurdak. 1991. Rotifera. In: Harrison, F. W. and E. E. Ruppert, eds. Microscopic Anatomy of Invertebrates, Vol. 4. Pages 219-297. Wiley-Liss, New York. 424 pp.




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Nano-Animals, Part I: Rotifers by Ronald L. Shimek, Ph.D. - Reefkeeping.com