Rotifers are microscopic animals of varying sizes, shapes and colours. They measure up to 2mm in length, though few exceed 0.5mm, and are recognisable by their single, double or sometimes multiple crown-like corona of cilia which appear to rotate, and which is how they derive their name, i.e. from Rota, Latin for wheel. Though they are generally the size of the protozoa, the single celled animals, they are multi- celled and so form part of the metazoa, being one of the five main classes of round worms. Worldwide there are around 2,000 species, of which there are over 500 in Britain, with a great variety of body forms.
They are found in freshwater lakes and ponds (where they are particularly abundant), in puddles, in brackish water and, to a lesser extent, in salt water and can be free swimming or sessile, that is attached by the foot to plant stems, debris, etc. They are also found in damp soil, in damp moss, and in fact in any place where there is even only a little water, particularly in house or garage rainwater gutters that tend to hold some water for a time before it evaporates, where in their dried-up state, like most water living micro organisms, they can survive being baked during long hot and dry summers, quickly resuming their full lives when damp or wet conditions return. The bodies of many species of rotifers are transparent and in their living state you can see their internal organs. After death the bodies tend to cloud over. Most rotifers are females. Males are few and not able to feed, living not more than a few days.
Rotifers were first discovered by Leeuwenhoek some 200 years ago and he explained how wonderfully nature has provided for the preservation of the species by their resistance to the drying up of their abodes and their resulting desiccation until moisture returns again. He wrote We can easily conceive that in all rainwater which is collected from gutters and in all waters exposed to the air, animals may be found, for they may be carried thither by the particles of dust blown about by the winds.
These fascinating animals are ever so easy to find. Try taking a little of the dried mud or leaf litter found in house, garage and outhouse gutters, put it in a little water and leave for 24 hours. Place a little on a slide, cover with a cover slip, and examine with patience. A magnification of x20 is ideal to start, changing up to around x60 to x100 when you have found a suitable specimen. You will find lots of nematode worms, paramecium, tardigrades and other minute invertebrate life. On your slide you will be sure to have bits of debris from the mud in the water and whilst it is easier to find the swimming rotifers because of their greater numbers I found it more interesting to search for and watch at leisure the rotifers adhering to this debris.
Now how to spot the rotifers when they are swimming, because when they are swimming they do not always use their cilia, which are then retracted. Also a lot of the life you will see swimming around will also have transparent bodies. The really fast movers will be paramecium. The rotifers swim in two ways. Those from the larger areas of water swim with their coronas of cilia extruded, rotating as they pull themselves through the water by means of their cilia. Those that inhabit damp earth and waterlogged moss, or in very shallow water and where there are lots of bits of debris, such as water from the gutter, swim in jerky movements called looping, momentarily fixing their foot as a temporary anchorage whilst they bend from head to foot. They straighten out before loosening their anchorage, repeating the process so that they move along at a fair speed in these jerking movements. Then suddenly they straighten out, extruding their cilia and pull themselves along. It is hard to keep them not only in the field of view but also in focus, moving rapidly as they do. You are continually trying to follow them by manipulation of the mechanical stage in both up and down and sideways directions, as well as continually adjusting the focus as they change their level in the minute film of water. A drop of 10% alcohol solution placed at the edge of the cover slip will gradually be drawn in and absorbed by the water, slowing down the animals in the water, eventually killing them. A solution of gum or cellulose such as wallpaper paste can be put in the water instead of using alcohol.
This is why I find the sessile ones much more interesting. Their coronas of cilia are continually beating, are so easy to see and no slowing down is needed. Dark ground illumination is recommended. It shows all the colours and makes everything much more clear by bringing out the fine detail which you would not see under normal bright field illumination. For my x60 to x100 observation I found a disc with a half-inch patch-stop ideal, with good lighting, the iris diaphragm fully open and the condenser racked at its highest. This is all very simple and if you are a beginner and not familiar with dark ground illumination, ask me for a copy of the dark ground article which appeared in the Society's Newsletter No. 12.
By means of a sticky substance exuded from their single foot (which in many species ends in two toes, sometimes in one long thin toe), rotifers can anchor themselves to any solid object, plant stem, stone or stick, and whilst some species are sessile and spend their adult life attached to the one position, with others it is not always a permanent anchorage because they can take up a swimming existence again. Some can do this temporarily, spinning a fine thread as does a spider, so they can pull themselves back to the anchorage again. So if you see a swimming rotifer trailing a long thin thread you will understand what it is. You will see the anchored ones wafting sideways to and fro in the water as they search for the minute life that is their food. The continually moving cilia produce circular currents in the water, and this you will also see as the currents move the food supply in reach of the mouth. Between the mouth and the stomach is the mastax, the most conspicuous organ seen in the upper part of the transparent body, containing hard pointed jaws to chop the food. An inner set of cilia surrounds the entrance to the mouth and the rotifers are able to reject anything that is not in the nature of suitable food.
The different species feed in three ways, known as the filterers, the graspers and the capturers. The filterers have already been described, their cilia creating currents in the water to bring the food in the region of the mouth. The graspers extend the mastax forward into the mouth, whilst the capturers form a trap by the closure of the corona cilia around the prey. These capturers are always sessile.
The species are tremendously varied. Most rotifers have one eye, some have two and some sessile ones are eyeless. The bodies of some species have telescopic segments and they can expand or retract like a telescope, whilst others have slight segmentation. Some have a hardened skin, the lorica, and others build themselves a tube to live in.
It was reported in the May 1990 Journal of the Worshipful Company of Spectacle Makers that their member Mr. H.J.A. Dartnell was taking part in the 1989-90 Australian National Antarctic Research expedition during which he would be making a study of the rotifers found in Antarctic waters. Mr.Dartnell points out rotifers are survivors par-excellence as some species can survive being frozen solid in the Antartic. Also that as there are no fish in the freshwater lakes and ponds of the Antartic there are no predators to eat the rotifers and they therefore proliferate in their billions to such an extent that they colour the water red.
Acknowledgements
The Micscape Editor would like to thank Roy Winsby for
generously permitting his article to be published on Micscape.
The image by Ron Neumeyer is a rotifer of the genus Philodina
which can be commonly found in bird-baths. This was the July
'Image of the Month', but it deserved to be permanently on the
site, rather appropriately we thought in this article published
the following month.
Please report any Web problems
or offer general comments to the Micscape Editor,
via the contact on current Micscape Index.
Micscape is the on-line monthly
magazine of the Microscopy UK web
site at Microscopy-UK