Small wonders in freshwater systems

A rotifer is a microscopic creature which has an interesting morphology. Rev. John Harris, who first described the species in the 17th century, said it was like a large maggot which could contract itself into a spherical figure and then stretch itself.

A rotifer, a microscopic aquatic animal, can be found in freshwater environments apart from moist soil, where it inhabits the thin films of water that are formed around the soil particle. Other habitats include still water environments like the bottom of a lake or flowing water. It can also dwell in mosses and lichens growing on tree trunks and rocks, storm water drains, puddles, leaf litter, mushroom, dead tree trunks apart from sewage treatment plants.

The creature’s body structure consists of head, neck, trunk and foot. It is typically cylindrical in shape. A thick, rigid and well developed cuticle lends the organism the shape of a box, while a flexible one could give it the shape of a worm. It is categorised as a loricate or a illoricate, respectively. The softness of the species means that it is not fossilised.

Only the hard part, i.e. the jaw, may be present in fossil records. The miniature size of this creature makes the identification of the same a challenge. However, fossils of the species, habrotrocha angusticollis, were found about 6,000 years ago in peat deposits at Ontario, Canada. The oldest reported fossil of rotifer is found in Dominican amber, dating to the Eocene epoch.

The rigid cuticle has multiple plates and later becomes spines, ridges and other ornamentation. The trunk forms a major part of the body which encloses most internal organs of the animal. This is a multicellular animal with body cavities. These organisms have a specialised organ system and a complete digestive tract. The size of a rotifer is normally 200 to 500 micrometers and a very few species such as rotaria neptunia are longer than a millimeter. These multicellular creatures make their living at the scale of unicellular protists.

The head of the rotifer carries a corona or a crown of cilia that draws a vortex of water into the mouth which is sifted for food. The food is ground by trophi (jaws) located just behind the mouth in pharynx (throat). Trophi is found in almost all rotifer varieties.

The body of the organism is externally segmented. It is telescopic with a semi flexible extendable covering of transparent cuticle.

It is the cuticle that suggests its relation to round worms and arthropods. Within the body are the stomach and reproduction organs. Lastly, it has a ‘foot’ which ends in a toe containing a gland with which the rotifer may attach itself to objects in the water and sift food. 

Because rotifers are microscopic animals, their diet too is very small which can fit through their miniature mouths during filter feeding. Primarily these animals are omnivorous. Their food includes dead or decomposing organic matter, unicellular algae and other phytoplankton primarily produced in aquatic communities. Rotifers in turn become prey to carnivorous secondary consumers like shrimp and crabs.

Why they are unique

Their morphology and feeding habits apart, rotifers are considered special because their reproduction system is unusual. Some consist only of females that produce their daughters from unfertilised eggs, a type of reproduction called parthenogenesis (developing asexually). Other species produce two kinds of eggs that develop by parthenogenesis. One kind forms females and other develops into degenerate males which cannot even feed themselves.

Some of the rotifer species are free floating and truly planktonic. Others are of the inchworm type along a substrate and some others are sessile, primarly attached to a substrate. About 25 species are colonial (sinantheronia semibullata) which are either sessile or planktonic. They are an important part of the fresh water ecosystem.

Rev. John Harris, who first described the species in the 17th century, called it an animal and said it was like a large maggot which could contract itself into a spherical figure and then stretch itself. A Rotifer has three classes — the largest grown are monogonota (about 1,500 species) and bdelloidea (about 350 species), and the smallest being seisonidee.

Acanthocephala (parasitic) is considered a separate phylum and is also a rotifer. A rotifer literally means a ‘wheel bearer’. The corona ruthlessly crushes its prey which is microbial. According to biologists, it is equivalent an elephant lifting its tail in full view.