General characteristics of rotifers

Rotifers are very small animals that possess numerous unusual characteristics, a very diverse morphology, and a complex anatomy. As they occur in so many forms and shapes, it is justifiable to call them “Nature's gems” and to compare them to butterflies and birds (Donner 1973).Pourriot and Francez (1986) claim that rotifers are beautiful, occasionally whimsical, and always fascinating. They most often range in size from 0.1 to 0.6 mm, growing occasionally to 3.00 mm. Their body is not segmented; it is bilateral and has the pseudocoel, i.e., the body cavity lacking its own epithelium and corresponding to the schizocoel. Rotifer growth involves the growth of individual cells or dorsal separation of cells; it is allometric (non-uniform); and rotifers grow very fast, occasionally doubling in size during the initial 24 h of their life (Ejsmont-Karabin et al. 1993). The diversity of rotifer shapes and forms is still more outstanding when one realises that, like in nematodes, the number of rotifer cells is low and constant (eutely) throughout the life span.

Although a relatively small group of animals, their very high production renders them extremely important players in inland water environments. Thanks to their adaptive abilities, rotifers have colonised almost all aquatic habitats: they are present in coastal waters of the seas (with up to 50 species); in lakes, ponds, and episodic pools; among wet plants (e.g., mosses), and in moist soil; they may also parasitise various invertebrates. Rotifers may occur at enormous abundances. Polluted ponds and lakes may support up to 20,000 individuals/dm3 water (Bielańska‑Grajner and Majewska 1994), an Upper Silesian bell pit supporting >> 25,000 individuals/dm3 (Bielańska-Grajner and Niesler 2002). The highest rotifer abundances (in excess of 100,000 individuals/dm3) have been so far reported from African lakes (Nogrady 1983), while commercial cultures in Israel produce abundances from 50,000 to 500,000 individuals/dm3 (Lubzens 1987; Lubzens et al. 1989).

The origin of rotifers

All early theories concerning the origin of Rotifera, found in textbooks written before the introduction of modern phylogeny based on the cladistic system of Hennig (1953, 1965), are of historical significance. According to the cladistic theory, the reconstruction of phylogenetic trees should be based on apomorphies and should only take monophyletic groups into account, i.e. those that have a common ancestor.