The Perfect Classification and Characteristics of Rotifers

Classification and Characteristics of Rotifers
Rotifers are named for their characteristic ring of hair-like structures. Let us learn more about these primitive creatures, using their classification and facts, through this BiologyWise post.
BiologyWise Staff
Last Updated: Feb 8, 2018
Did You Know?
In some species of rotifers, the males do not have digestive organs and cannot eat.
Think of animals, and you will most probably visualize images of creatures like dogs, cats, or cattle. However, for scientists, this term includes creatures like fish, insects, and even sponges. While grouping such beasts together may seem extreme to some, this is by no means the end of it. There are, in fact, animals so small that only the largest ones can be seen under a magnifying lens. To make things more weird, these creatures have rings of hair-like structures on their heads, which give the appearance of the wheels of a car. Let's check out the characteristics of these animals, called 'rotifers'.
Classification
Kingdom: Animalia
Since rotifers are made up of many cells (which show a structure called 'nucleus'), and need food to survive (unlike plants), they are included in the animal kingdom.

Sub-kingdom: Eumetazoa
As rotifers have the ectoderm, mesoderm, and endoderm layers as an embryo, and nerve cells as adults, they are included in this sub-kingdom.

Superphylum: Platyzoa
Animals that do not show a cavity between their body wall and the intestine (or show a small cavity), are included in this superphylum.
Phylum: Rotifera
As the name suggests, all animals in this phylum are called rotifers. They are divided into the following three classes.
  • Class Monogononta
    It is the largest group of rotifers, containing about 1,500 species.
  • Class Bdelloidea
    It is the second-largest group, and contains about 350 species of rotifers.
  • Class Seisonidea
    This group contains only 2 primitive species of rotifers.
Rotifer Characteristics
❍ Rotifers are considered to be the smallest animals on Earth. Despite being 0.1 to 0.5 mm in size, they are made up of about a thousand cells.

❍ The term 'rotifer' does not refer to a single animal or species; it is used for any of the 2,000 species that are included in the phylum Rotifera.

❍ Their body is divided into three sections - the head, trunk, and foot. Despite being microscopic creatures, they have jaws, a brain, stomach, intestines, testes/ovaries, and excretory organs.
Rotifer Structure
❍ The name 'rotifer' means 'wheel-bearer' in Latin. It is named so because it has two rings of cilia (hair-like structures) on its head, which, on moving, appear like two rotating wheels. They are also called 'wheeled animals'.

❍ Most of them are aquatic, living in freshwater bodies, though a few are also found in saltwater. Some are also found on land, in puddles, rainwater drains, on algae, moss, moist soil; typically anywhere a thin film of water is present.

❍ The outer covering is called the 'cuticle', which decides its appearance. Rotifers with a hard cuticle have a box-like appearance, while those with a flexible one seem worm-like. However, most species are long and somewhat cylindrical.

❍ The body wall is colorless, and therefore, all the organs are visible. The food present in the digestive system decides what color the animal takes on temporarily.

❍ The corona, or the rings of cilia on the head, are used to suck food into the mouth, or to move by creating water currents.

❍ Depending on the species, rotifers may either move through the water (called 'planktonic' rotifers), stay fixed permanently (called 'sessile'), or creep like a worm.

❍ Planktonic rotifers use their foot to hold on to something while feeding. Sessile forms secrete an adhesive substance to stay attached permanently with their foot.

❍ Rotifers consume dead and decomposing substances, algae, bacteria, smaller rotifers; typically anything they can fit inside their mouth. After ingesting, their trophi (jaws) crush the particles to make them smaller.

❍ These animals are eaten by fish and insect larvae, crustaceans, copepods (small crustaceans), tadpoles, and fish.

❍ The bodies are soft, and hence, do not turn into fossils. However, this property, along with their nutritious food habits, have made them a popular feed for fish larvae in aquaculture.

❍ Rotifers have a number of survival strategies. They can draw their corona and foot inside the cuticle on facing danger; folding like a telescope. Some species also produce 'resting eggs', which can survive cold and drought for as long as 9 years. Other species survive the drying of ponds by contracting their bodies, and get revived as soon as conditions improve.

❍ Owing to their resilience, this group of animals has survived for 40 - 80 million years.

❍ Rotifers have extremely high reproductive rates. Many species reproduce without males, when eggs produced by the females directly hatch into juveniles (called parthenogenesis). All rotifers of the class Bdelloidea reproduce only by parthenogenesis, and males are absent altogether. Other species reproduce both, by sexual and asexual methods.

❍ In most species, females are much larger than males. The only function of males is to reproduce, and they lack any digestive organs. For this reason, they cannot eat, and survive only for a few hours.

❍ Rotifers are unique, in that, they are born with all their cells. So, unlike most animals, which grow by adding new cells, rotifers grow by increasing the size of their cells.
As can be seen, these animals clean up waste in water bodies, and provide food to various creatures in the wild. Thus, rotifers play an important ecological role, which cannot be overstated.