Rotifer

Rotifers are also known as wheel animals or wheel animalcules and they belong to the phylum Rotifera. The Rotifera phylum is a small phylum that consists of minute multicellular aquatic animals which tend to have a typical wheel-like ciliated organ which they generally use for swimming and feeding. Rev. John Harris in 1696 first described them. Later, in 1703 Antonie van Leeuwenhoek described their other forms. Mostly, rotifers are approximately around 0.1 mm to 0.5 mm in length but some of them can range from around 50 μm to 2 mm. 

These species are common in freshwater environments all across the globe but a few of them can be found in saltwater environments too. Some rotifers can be free swimming and truly planktonic species, some of them can move by inch worming along a substrate and some can be sessile which can live inside tubes.

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More About Rotifers

There are about 25% of rotifers that are colonial, either sessile or planktonic. One such example is Sinantherina semibullata. Rotifers form a major part of the freshwater zooplankton, being a chief source of food and with many species, it also contributes to the decomposition of soil organic matter. Rotifers are mostly cosmopolitan, but we can also find some endemic species such as Cephalodella vittata. However, recent barcoding evidence suggests that certain cosmopolitan species like Brachionus plicatilis, B. calyciflorus, Lecane bulla, out of the others, actually come under the category of species complexes. In some recent treatments, rotifers are said to be placed in a larger clade called Syndermata along with acanthocephalans, a phylum of parasitic worms.  

Taxonomy

Rotifers were first defined by Rev. John Harris in 1696. In his words, he said that a rotifer is an animal that is like a large maggot that could contract itself into a spherical figure and then stretch itself out again. He then further added that the end of the rotifer’s tail appeared with a forceps like that of an earwig. It was later found out that he was describing a bdelloid rotifer. In 1702 however, Antonie van Leeuwenhoek gave his own extremely detailed account of Rotifer vulgaris and then went on to define Melicerta ringens and some other species as well. He was also the first scientist to print the scrutinized observations of rejuvenation of some particular species after they got dried. Breakthrough then happened some 136 years later when the publication Infusionsthierchen als vollkommene Organismen by Christian Gottfried Ehrenberg classified rotifers as being multicellular organisms in 1838. 

To date, around 2200 species of rotifers have been discovered and enlisted. Their naming is in a state of disagreement as of now. According to one treatment, they belong in the phylum Rotifera and have three subclasses: 

1. Seisonidea: These are the least common groups. These have only 2 known genera with 3 species each. 

2. Bdelloidea: There are around 350 known species of this group.

3. Monogononta: This is the largest group of them all with about 1500 known species in existence. 

Anatomy

Rotifers are bilaterally symmetric animals. They are of vastly different shapes as well. In general terms, their body is primarily divided into three parts - the head, the trunk, and the foot. Their shape is believed to be cylindrical to a certain extent. There is the presence of a cuticle which is believed to be extremely well developed. This cuticle, however, differs in shape. It may either be thick and rigid, giving these animals a very boxy shape. These rotifers are called loricate rotifers. On the contrary, illoricate rotifers have a malleable cuticle that makes them worm-like. 

There are two distinguishing characteristics in females of all species of rotifers. These are: 

1. The existence of a Corona in their head. This is a structure that ciliates in all genera other than Cupelopagis.

2. The presence of a Mastax.

In the earliest species, the Corona is believed to form a very simple layer of cilia around the mouth region, and from this region, another band of cilia originates and extends over to the back of the head. In most rotifers, however, the presence of a more complicated structure has evolved. The trunk of the rotifer is known to construct a considerably large part of the body and it surrounds all the internal organs of the rotifer. The foot usually begins from the back of the trunk and is much narrower compared to the trunk which is why it looks like a tail. There are many examples of rotifers that can withdraw their foot either partially or wholly into their trunks. The foot might contain between one to four toes. However, in species that can swim, the foot can be much reduced in size and is even known to be absent at times.

Digestive System

There is a current created by the coronal cilia that sweep food into the mouth of these species. The mouth may open into a characteristic chewing pharynx sometimes via a ciliated tube, and sometimes directly. The pharynx constitutes a powerful muscular wall and also consists of tiny, calcified, jaw-like structures known as trophi, which are said to be the only fossilizable structures in rotifers. 

The shape of the trophi which depends partially on the nature of their diet differs in different species. The trophi are covered in grinding ridges in suspension feeders whereas, in more active species which are carnivorous, the trophi may be shaped in the shape of forceps which will help them to bite into their prey. In some ectoparasitic rotifers, the mastax may be adapted to grip onto the host whereas, in the rest, this function is performed by their foot.

The oesophagus lies behind the mastax where it opens into the stomach. Here, most of the digestion and absorption takes place. It is noticed that the stomach further opens into a short intestine that ends with the cloaca on the posterior dorsal surface of the rotifers. Some species may have up to seven salivary glands which empty to the mouth in front of the oesophagus, while there are two gastric glands associated with the stomach where digestive enzymes are produced. 

A pair of protonephridia is said to open into a bladder where it drains into the cloaca. Water is expelled from the body with the help of these organs, which help these species to maintain a proper osmotic balance.

Nervous System

There is a presence of a small brain in rotifers which is located right above the mastax, from where several nerves extend throughout the body. The actual number of nerves varies from one species to another, though the nervous system generally has a simple layout. A retrocerebral organ lies near the brain which consists of two glands on either side of a medial sac. The sac drains into a duct that is divided into two before it opens through pores on the uppermost part of the head. 

About 25% of the total 1000 cells in a rotifer are said to be a part of the nervous system.

Rotifers generally have one or two pairs of short antennae along with up to five eyes. The structure of the eyes is simple, sometimes with only a single photoreceptor cell. The bristles of the corona are touch-sensitive. A pair of tiny sensory pits is also noticed to be present that are lined by cilia in the head region. 

Reproduction in Rotifers

Reproduction in rotifers occurs either sexually or by parthenogenesis. We see sexual dimorphism in rotifers and the females are always larger when compared to their male counterparts. The difference is not so much in some species, while in others the female can grow almost 10 times the size of the male. Whereas, males are present during a particular time of the year or just are absent at all times when it comes to parthenogenetic species. 

The female reproductive system is usually made up of one or two ovaries that have a vitellarium gland each. These glands provide yolk to the eggs. Each vitellarium gland and ovary as a unit form a solitary syncytial composition in the foregoing part of the animal, cracking through the oviduct into the cloaca. 

Male rotifers usually have a digestive system that is not functional and henceforth have a very short life span. They are often deemed to be sexually fertile right at birth. They have a unitary testicle and sperm duct held together with a couple of glandular structures called prostates. This, however, does not bear any relation to the prostate found in vertebrates. 

The rotifer phylum Rotifera enlists within itself three subgroups. All these three subgroups allegedly reproduce by three different and unique processes. Sexual reproduction gets carried out in the group of Seisonidea while Bdelloidea reproduces only by parthenogenesis which is asexual in nature. On the other hand, Monogononta undergoes reproduction by exchanging between the above-mentioned processes. Their way of reproduction is often referred to as cyclical parthenogenesis or heterogony.

Conclusion

Rotifers are also known as wheel animals or wheel animalcules which belong to the phylum Rotifera, in the kingdom Animalia. They have no circulatory system but the exchange of gases takes place through their body surface. Reproduction in these species occurs either sexually or by parthenogenesis. Male rotifers usually have a digestive system that is not functional. Hence, they tend to have a short life span. 

Rotifers are usually animals that have a diet of particulate organic detritus, dead bacteria, algae, and protozoans. Due to their nutrient recycling nature, they are also used in fish tanks to help clean the water and prevent the formation of clouds of waste matter. 

Did You Know?

1. Rotifers can fall prey to several animals like copepods, jellyfish, starfish, and comb jellies.

2. It is noticed that the genome size of a bdelloid rotifer is around 244 Mb. The genome size of monogononta rotifers is said to be significantly smaller than the bdelloid rotifers.

3. Rotifers have no circulatory system. The exchange of gases takes place across their body surface.