Rotifer

Rotifers, also called wheel animals or wheel animalcules, are tiny animals that make up a group called Rotifera. They are very small, usually between 0.1 and 0.5 mm long, but some can be even smaller or larger. These creatures were first seen a long time ago, in 1696, by a person named Rev. John Harris, and later by Antonie van Leeuwenhoek in 1703.

Most rotifers live in freshwater places all around the world, with just a few living in saltwater. They move in different ways—some swim freely, some crawl along surfaces, and others stay in one place inside tubes or sticky holders. Many rotifers are an important part of the tiny animals in water, helping to feed other creatures and break down organic matter in soil.

Scientists have found that some rotifers thought to be the same everywhere are actually groups of very similar species. In 2021, amazing news came about rotifers called bdelloid rotifers that were frozen for 24,000 years in Siberian permafrost and brought back to life. Rotifers have been around for a very long time, with the oldest known evidence dating back to the Middle Jurassic period in China.

Taxonomy and naming

John Harris first described tiny creatures called rotifers in 1696. He called them animals that looked like small worms but could change shape. Later, Antonie van Leeuwenhoek wrote more detailed descriptions of these creatures and noticed that some could come back to life after drying out. It wasn’t until 1838 that scientists realized rotifers were made of many cells.

Today, we know about around 2,200 types of rotifers. They are grouped into three main categories, with the largest group having about 1,500 species. Scientists are still learning more about how these groups are related to each other.

Etymology

The name “rotifer” comes from a Latin word meaning “wheel-bearer.” This is because the tiny creatures have a ring of cilia around their mouths that move in a way that looks like a spinning wheel.

Anatomy

Rotifers have a simple body shape with bilateral symmetry. Their body is divided into a head, trunk, and foot, and is usually somewhat cylindrical. The trunk holds most of the internal organs, and the foot projects from the rear, often appearing like a tail.

The most noticeable feature of rotifers is the corona on the head, which is a structure covered in tiny hair-like strands called cilia. In most rotifers, this structure is complex and helps them move. The trunk makes up the largest part of the body and contains important organs. The foot, which can sometimes be pulled back into the trunk, often ends in toes that help the rotifer stick to surfaces or move around. Some rotifers can swim using their corona, while others crawl using their foot.

Rotifers have a simple nervous system with a small brain and nerves that extend throughout their body. They also have simple eyes and sensory structures to detect their environment.

The digestive system includes a chewing pharynx called the mastax, which has tiny jaw-like structures. Food enters through the mouth and moves into a stomach where digestion happens. Rotifers can be suspension feeders or carnivores, eating small organisms or particles in the water. Water is expelled from the body through special organs called protonephridia.

Feeding

Rotifers eat tiny pieces of organic material, dead bacteria, algae, and protozoans. They can eat particles up to 10 micrometres in size. Like crustaceans, rotifers help recycle nutrients in nature. Because of this, people often put them in fish tanks to keep the water clean by eating waste. Rotifers also influence which types of algae grow in ecosystems by eating some kinds more than others. Sometimes, they compete with cladocera and copepods for food that floats in the water, called planktonic food.

Reproduction and life cycle

Rotifers can reproduce in two ways: sexually or without males. Females are usually larger than males. Some females can have babies without a male, while others need one.

Females have special parts to make eggs, and males have parts to give sperm. In some species, males are only around for part of the year or not at all. When females reproduce without males, they make clones of themselves. Sometimes, special eggs can become males or resting eggs that can wait until good conditions to hatch.

Most rotifers hatch looking like tiny adults, but some start as swimming babies that grow quickly. Females can live from two days to three weeks. Some rotifers can survive very dry or cold conditions by going into a resting state and then waking up when it gets better.

Predators

Rotifers are eaten by many different animals. These include small water creatures called copepods, fish like herring and salmon, as well as bryozoa, comb jellies, jellyfish, starfish, and tiny animals known as tardigrades.

Genome size

The genome size of a bdelloid rotifer, Adineta vaga, is about 244 million base pairs. The genomes of Monogononts are smaller. In Monogononta, the amount of nuclear DNA in eight different species ranged from 0.12 to 0.46 picograms. Haploid genome sizes in Brachionus species range from 0.056 to 0.416 picograms.