Article and Photomicrographs by Bruce J. Russell
But, What Are They?
Phylum Rotifera includes a bizarre array of some 2500 species of microscopic creatures of which, about 2300 live only in fresh water habitats. Why are there so relatively few marine rotifers? The probable answer is that this is a line of animal evolution that got its start in fresh water. Most rotifers are in the size range of protozoans, between 40 and 200 micrometers. Despite their small size, these are true animals, although their exact position on the tree of life is still being argued. Most biologists see enough similarities with nematodes to imagine that these two groups share a common ancestor.
They May Be A Little Different, But They Come Fully Equipped
Rotifer tissue can be syncytial--a patch of tissue without cell boundaries, but containing several to many nuclei. All contain powerful striated muscles (a good place to see muscle cells in action through the microscope). Their bodies are laced with tubes associated with little pockets that flicker like flames. The action is produced by tufts of cilia that move water and waste materials through the tubes, in most cases to a bladder that emptied to the outside. But the key feature, obvious to anyone who has seen Floscularia or Philodina , both common rotifers of pond weeds, is their "wheel organs." These bands of cilia give a powerful illusion of rotation due to the progressive waves of ciliary motion. Food brought in by the cilia-generated feeding currents is passed through a set of grinders (the mastax) and on down the digestive track.
Let's Go Fishin
Many rotifer species are encased in a shell called a lorica. Check out Keratella and Kellicottia , both from lake plankton. In each case the lorica is drawn out into sharp spines. Other planktonic species resemble clear sacs adrift in the open water--until they sense prey. Then powerful muscles contract, sucking in dinner. Asplanchna is such a rotifer. Go after all of these plus Conochilus , a colonial rotifer and Hexarthra (definitely from outer space) with a weighted plankton net. In lakes and reservoirs the best daytime rotifer netting may be at a depth of several meters.
Can You Dig It
For sheer numbers of rotifers, dig down and take a sample of wet sand, a few centimeters down, just above the water line. A cubic centimeter of damp sand can hold hundreds of individual rotifers. And while your out and about, remember that moss is a rotifer stronghold. Wring a pinch of moss out in a petri dish and look for rotifers, nematodes, and waterbears. Dry moss is no problem. Just sponge up some water and squeeze it out. Out will rain dormant rotifers that in a few minutes will rehydrate and wheel into action. But be careful. You could get hooked and spend your life studying these bizarre little animals.
For more about rotifers see Guide to Microlife , published by Franklin Watts, a division of Grolier Publishing, ISBN 0-531-11266-7. This informative text includes color photomicrographs by Bruce J. Russell.
Also see our video/DVD program The Biology of Nematodes, Rotifers, Bryozoans and some Minor Phyla in the BRANCHES ON THE TREE OF LIFE section.
Microscopy-UK collection of pages about rotifers . This site contains some interesting articles including a series of stills showing a rotifer emerging from a dormant 'egg'. This site is a good place to begin exploring for more information on rotifer biology.