Janice Robson, age 14, of St. Boniface, Manitoba, Canada, for her question:
How advanced is the rotifer?
In everyday language we tend to overwork the word advanced. Too often we think that things are advanced just because they are the most up to date, extra large or extra complicated. And too often we judge such things to be superior. Biologists use the word more sparingly. To them the more advanced animals are more complicated than the simpler life forms. But this does not necessarily make them superior creatures.
The amoeba is one of the simplest living organisms, and a giant amoeba is just big enough to be seen without a microscope. It is at least ten times larger than a average sized rotifer. But its jelly like body is merely one living cell. The tiny rotifer is a miraculous organization of 1,000 or more cells. fie has a head and trunk, a tail, feet and even a pair of clawed toes. He has a mouth and a digestive system, muscles and nerves and even sensory organs. These complex, well organized characteristics place the rotifer very high on the scale of animal classification.
Biologists rate him as more advanced than the single celled protozoa and the sponges, the hydras, the flatworms and the roundworms. Most of these creatures are far superior to him in size which makes the little rotifer a very remarkable creature indeed. In fact, biologists give him ¬or rather her a class of her own. The group is called Rotifera and we know of 1,200 amazingly different rotifer species.
The name rotifer means the "wheel bearers," and it refers to features shared by all rotifers. The head has a circular crown of thread like celia which constantly spin like whirling wheels. Some rotifers have a double crown. All of them live aquatic lives, and there are sure to be a few rotifers in every drop of pond water. Some live in lakes or streams, others in gutters and muddy puddles that come and go. A few live in salt water.
One needs a microscope to appreciate their various forms, their antics and their amazing life stories. All of them are encased inside lovely little transparent shells. Some are fixed in hard, ornate cases, but most of them move about, performing all. sorts of frisky antics. They may be shaped like cylinders, flasks or spheres but all of them very colorful and wondrously beautiful.
The two toes have adhesive glands, and when a rotifer feeds on algae or other aquatic food, she often glues herself down to dine. Her wheel of celia spins water into her mouth, thus acquiring food and also oxygen. I?hat's more, she even has teeth to chew the crusts.
The rotifer produces a batch of eggs all females. These daughters are unfertilized. But they go on producing batches and batches of eggs all females. This type of all female reproduction is called parthenogenesis and it explains why we refer to the average rotifer as she.
The parthenogenetic female eggs have soft shells, suitable for life in the water. The rare breed of fertilized eggs have hard shells, strong, enough to withstand cold and drought. They can survive when a puddle dries up. Many get carried by breezes and birds to better locations. This explains why rotifers are present in every lake and pond.