to the Wonderfully Weird
World of Rotifers
illustrated by Wim van Egmond
The discovery of rotifers or "wheel animalcules" dates back nearly three centuries to their first description in 1703 by Leeuwenhoek. They are rather complex, multicellular animals with a size range from roughly 40 microns to over 2,000 microns, so many of them are smaller than the larger members of the protozoa. An excellent reason for studying rotifers is that they are found in such a wide variety of environments, so that if you poke around a bit you're almost certain to have an encounter of the rotifer kind. They are in virtually every lake and stream, every pond, on bits of moss in moist forest, in the tropics and even in icy arctic terrains. A virtually guaranteed place to find some is in sphagnum moss at the edge of a pond or lake. There are marine rotifers, but the vast majority occur in fresh water.
Rotifers come in such an astonishing variety of sizes, shapes, and types that it is difficult to describe them. The characteristic that gave them their name is, in fact an illusion. Some species—but, certainly not all—appear to have two lobes that look like tiny wheels as a result of the crown of cilia. Some look like tiny transparent inch worms as they make their way across the substrate and then suddenly surprise you by swimming off, albeit, in a rather lumbering fashion. These are the Bdelloid rotifers, such as Rotaria and Philodina.
Others, like Keratella, have a "shell" or lorica which is essentially rigid, with hooks, spines, and plates and some have the appearance of a heavily armored Klingon warship. Keratella, like many planktonic rotifers, shows striking variation in the form and size of the lorica and, once again, nature shows little concern for our effort to classify everything in neat categories. In one species, Keratella quadrata, there are specimens in which the posterior spines are absent and others in which the posterior spines equal the body length and these latter, when actively swimming, look like miniature manta rays swooping through the water scooping up great quantities of micro-plankton. Keratella belongs to the order Ploima which consists of free-swimming forms that usually have a posterior project with two "toes". Brachionus is also a common species in this order as are three other genera of particular interest.
Polyarthra is the "jumping" rotifer. It will be swimming around, busily feeding, and then suddenly jump in an astonishing way. This is accomplished by clusters of appendages that hang down from an area just below the corona or crow of cilia and extend down beyond the posterior rim of the organism. The appendages, look in some respects, remarkably like feathers! The first time I encountered these marvelous creatures, I felt like Leeuwenhoek must have when her first saw a Vorticella contract its stalk. One of the delights of exploring the micro-world is that one can repeatedly encounter creatures that defy imagination.
The second genus I want to mention here is Kellicottia and the species which is common here is K. longispinus. The body is shaped like a slender vase which come to a point at the posterior end, but what is striking in this organism is the spines—the anterior end has three very short spines and three that are longer than the body, the central spine being the longest and then a posterior spine that is even longer than the central anterior spine. At first glance, I didn't even realize that this was a rotifer. It was resting on the bottom and the spines drew my attention and while I was trying to get a better look at it, I bumped the culture dish slightly and slowly the Kellicottia began to unfold the coronal cilia and swim upward like some leviathan rising from the deep. It is an exhilarating experience to see these splendid creatures, like beings from another planet, glide through a world that we are just beginning to understand after nearly three centuries of investigation.
The third genus is the Steuben
crystal vase of the rotifer world—Asplanchna.
This is an organism which one needs to observe directly, for even the best
photomicrographs cannot do it justice. Asplanchna is a wonderful,
but not very cooperative subject. Because it is so transparent, its internal
structure is clearly visible and phase contrast, darkfield, Rheinberg illuminations,
and Nomarski differential interference contrast can all produce magnificently
detailed images, assuming you are successful in coaxing Asplanchna
to pose properly for you.
In the order Flosculariacea, there are two genera which I have collected which I think are worthy of special mention. At first sight, one might mistake a colony of Conochilus unicornis for a colony of large peritrichs. The colonies are rather fragile, so it is virtually impossible to determine the average number of individuals per colony. In the process of collecting, some colonies are damaged and in transferring colonies from a sample to a watch glass or slide, further damage may result. I have observed colonies of 15 to 20 rotifers, others with only 3 or 4, and have also seen solitary organisms swimming through a sample. This last situation is, I am fairly certain, a result of damage, since the remnants of a gelatinous tube can invariably be seen around the posterior end of the rotifer. When you observe the colony closely, you will notice that the central area consists of a gelatinous sphere and the posterior of each rotifer occupies a distinct tube within this mass. The species name of this organism, unicornis, derives from the fact that each rotifer has a single spike-like antenna arising from the corona.
The second genus I want to mention is Floscularia. The members of this genus, are tube builders and the particular species which I observed was one which should be given medals for recycling and for environmentally compatible architecture. This rotifer builds an elegant tube of what looks like tiny beads which are, in reality, fecal pellets formed by a concavity on the head in which a gland produces a gelatinous secretion which is mixed with its waste material by ciliary action to form pellets which are then added to the tube as the rotifer grows. An amazing process to observe. In the Gallery of Rotifers you can find an example of another genus (Ptygura) that builds a similar tube.
There are over 2,000 species of rotifers and virtually every genus contains species with strange and surprising characteristics. In some genera, such as Keratella, a single species can exhibit variations in the size, length, and shape of the spines and projections of the lorica to such an extent that earlier microscopists gave many of these variants distinct species status. This remarkable phenomenon is know in a few other groups of organisms, such as the Cladocera or "water fleas" like Daphnia and it is a maddening phenomenon for the taxonomist and a nightmare for the amateur.
But the weirdness of the world of rotifers is only beginning. Rotifers possess a structure called a mastax which is unique in the animal kingdom. The mastax is a bulb-like structure consisting of many muscles which control a complex set of jaws which consists of hardened proteins; that is, the jaws are sclerotized and from tough, but slightly flexible structures rather like cuticle.
The jaw apparatus is called the trophi and it grinds the food brought down to it by the ciliary action. The trophi consists of a number of structures which show considerable variation. These structures can become quite complicated and there are minute ridges, crossbars, and protrusions on the small projections from the component parts that can be used to distinguish species, but in many cases are distinctive only at 1000 magnifications! So the taxonomy of rotifers, is as you can imagine, quite taxing. Every rotiferologist is hoping to win a trophy for finding a new kind of trophi. There are eight basic types of mastax and a good textbook on invertebrates will show you their morphology. I won't bother you with the specifics here as they are readily available elsewhere and are really of interest only to those who develop a passionate interest in the fascinating details of rotifer morphology.
And now, for a bit of sexual gossip—a tidbit to delight female microscopists and dismay male ones—a tabloid headline which happens to be true:
FOR MOST SPECIES OF ROTIFERS
NO MALES ARE KNOWN!
In fact, remember the bdelloid rotifers, the inch worm type,—well, in that group, males are completely unknown. "How can this be?" you protest—"it's unnatural!" Well, it's not for rotifers and they are a very cosmopolitan and successful group. In the bdelloids, reproduction is exclusively parthenogenetic and since there is no fertilization, all offspring are females. In orders other than the bdelloids in which males are known to occur, they are significantly smaller than the females and have a very short life span of just a day or two. As you might suspect, rotifers produce several different types of eggs. One type of females produces only eggs that hatch into females; another type of the same species which occurs only when there are significant environmental changes, produces eggs that quickly hatch into males which can then fertilize a certain type of egg that becomes a "winter" egg. These remarkable eggs can survive for long periods, can be dried, frozen, baked in the hot sun, and generally resist adverse environmental changes.
A series of very interesting experiments can be conducted with such eggs to test their resistance to a variety of conditions and chemicals.
The investigation of the morphology
and behavior of rotifers can be absorbing and rewarding. They possess an
intricate musculature which contains both smooth an striated muscle fibers,
they have a primitive nervous system, possess and eyespot consisting of
red granules, some have adhesive glands which occur in the species with
"toes" and they use this glue to anchor themselves temporarily while feeding.
Rotifers are full of surprises. The deeper one probes into the wonderfully
weird world of rotifers, the more it seems that one is truly exploring
an alien world.
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