A General Introduction.
Bdelloid Rotifer Gallery 1.
1 of 2
In these galleries, rotifers have been divided into four categories which relate to their appearance and habitat preferences rather than to their taxonomic classification.
What Are Rotifers?
Rotifers are small, mostly freshwater animals, and are amongst the smallest members of the Metazoa -- that group of multicellular animals which includes humans, and whose bodies are organized into systems of organs.
Most rotifers are about 0.5mm in length or less, and their bodies have a total of around a thousand cells. This means that their organ systems are a greatly simplified distillation of the organ systems found in the bodies of the higher animals.
A typical rotifer might have a brain of perhaps fifteen cells with associated nerves and ganglia, a stomach of much the same number, an excretory system of only a dozen or so cells, and a similarly fundamental reproductive system. They have no circulatory system.
It is an anomaly that despite their complexity, many rotifers are much smaller than common single-celled organisms whose world they share.
Their most salient feature, and the one which caused them to be named wheel animals by early microscopists, is the corona -- usually in the form of two lobes surrounded by beating cilia, which give a vivid impression of rapidly rotating wheels. When the rotifer is attatched, the current created by the corona brings food particles to the mouth, and when the rotifer releases the grip of its foot, they act as twin propellers, transporting the rotifer rapidly from one place to another.
Rotifers have at least two other remarkable qualities. Firstly, they are able to survive long periods -- even perhaps hundreds of years -- in a dried or frozen state, and will resume normal behaviour when rehydrated or thawed.
Secondly, they exhibit what biologists call cell constancy -- they grow in size not by cell division, but by increase in the size of the cells which they already have. In some species, the cells of a particular organ will merge together, forming a syncytium -- a cytoplasmic mass with interspersed nuclei. In any case, a rotifer ends its life with the same number of cell nuclei with which it was born.
Ecologically speaking, they play an important contributory role in the natural water purification process, since they feed for the most part on suspended organic particles and free-swimming algae, and are often present in large numbers.
With their highly transparent bodies, complex movements and varied lifestyles, rotifers are amongst the most fascinating creatures the microscopist can encounter.
Bdelloid rotifers are the commonest variety found in fresh water. The term describes their habit of moving about by extending the body and attatching the head, then catching up with the foot and again extending the body -- bdelloid, like a leech.
No males have ever been observed among them. They are all females and produce eggs requiring no fertilization. In some bdelloids, the young develop to maturity within the body cavity and are born live. In others, eggs are produced and deposited in the vegetation of the pond's edge. In either case, the invariably female offspring in turn produce eggs requiring no fertilization and so on -- a process called parthenogenesis.
Since there is no sexual stage to introduce change and variety to the genome, it is something of a mystery as to how bdelloid rotifers have survived for so long as a thriving and stable animal group.
Header Illustration: A digitally elongated bdelloid rotifer in looping mode. See No. 2 of Bdelloid Rotifers for the undistorted specimen.