Ciliate

Ciliates are a group of tiny, single-celled organisms. They belong to a larger family called alveolates. What makes ciliates special is that they have many tiny hair-like parts called cilia. These cilia look similar to little whips called flagella, but they are usually shorter and move in wave-like patterns. Ciliates use their cilia for many jobs, like moving around, grabbing food, and sticking to surfaces.

You can find ciliates almost anywhere there is water—lakes, ponds, oceans, rivers, or even wet soil. They are very common and come in many shapes and sizes. Some are as tiny as 10 μm, while others can be as long as 4 mm. Scientists have described about 4,500 different kinds of ciliates. They think there could be many more species still waiting to be discovered.

Ciliates are important in nature. Many live by helping other organisms, either on their surfaces or inside them. Some ciliates can live off of other creatures. Despite their small size, ciliates are very complex single-celled animals. They are usually classified under the group called Ciliophora. This group is considered a phylum in many systems of taxonomy.

Cell structure

Ciliates have two special control centers in their cells. One is small and helps make new cells. The other is large and helps the cell do its daily work. These work together to keep the cell healthy and growing.

Ciliates also have tiny hair-like structures called cilia that help them move and eat. These cilia are arranged in special patterns on the cell's surface. Inside the cell, there are tiny parts that help process food and remove waste, keeping the cell working properly.

Feeding

Most ciliates eat smaller organisms like bacteria and algae. They use tiny hair-like structures called cilia to move food into their mouth. These cilia help push the food into a special part of their body called the gullet. There, the food forms food vacuoles.

Some ciliates get energy from sunlight by living with tiny plants. Others take food directly from their environment. There are ciliates that live on animals, but only one type, Balantidium coli, can make humans sick.

Reproduction and sexual phenomena

Ciliates can reproduce without needing another organism. They split into two parts during a process called fission. One half becomes one new organism, and the other half becomes another. Sometimes, small offspring called "swarmers" grow out from the parent, or the parent splits into a chain of new organisms.

Ciliates can also share genetic material with another ciliate in a process called conjugation. Two ciliates connect and exchange tiny parts called micronuclei. After they separate, each ciliate uses its new micronucleus to grow a full set of parts needed for reproduction. This sharing helps mix their genetic material.

DNA rearrangements (gene scrambling)

Ciliates have two types of nuclei: the somatic "macronucleus" and the germline "micronucleus". The DNA in the micronucleus is passed on when they reproduce, while the DNA in the macronucleus controls what the organism looks like. The macronucleus forms from the micronucleus by rearranging and copying the DNA.

In some ciliates like Tetrahymena, the micronucleus starts with 10 chromosomes, but the macronucleus ends up with over 20,000. This happens because the DNA is broken into tiny pieces and then put back together. In other ciliates like Oxytricha, this process is even more complex, with the pieces needing to be flipped and moved to the right spot. This helps these tiny organisms grow and survive.

Aging

Some tiny water creatures called Paramecium can get tired over time if they don’t mix with others. Scientists found that a special part inside these creatures, called the macronucleus, is what causes them to get tired. They also learned that when these creatures get older, their DNA can get damaged. This seems to be why they eventually stop working well.

Fossil record

Scientists used to think the oldest fossils of ciliates were from a time called the Ordovician period. In 2007, they found fossils that might be ciliates from an even older time, called the Ediacaran period, about 580 million years ago. These fossils were found in a place named the Doushantuo Formation. Later, some scientists decided these fossils might actually be a different kind of tiny organism. Another fossil of a ciliate called Vorticella was found inside a leech cocoon from the Triassic period, about 200 million years ago.

Phylogeny

Scientists found that a group called Mesodiniea is close to all other ciliates. Inside a bigger group called Intramacronucleata, there are two main branches. One branch includes Spirotrichea, Armophorea, and Litostomatea. The other branch includes groups like Colpodea, Oligohymenophorea, and Nassophorea.

In 2018, scientists found a new group called Odontostomatea. It is related to Armophorea. This helps us learn how these tiny creatures are connected.

Classification

Further information: Wikispecies:Ciliophora

Ciliates are tiny, single-celled creatures. Scientists sort them into different groups. They look at tiny pieces inside the cells.

Some groups are Mesodiniea (like Mesodinium), Heterotrichea (like Stentor), and Karyorelictea. Another big group is Intramacronucleata. It has many smaller groups. These include Armophorea, Odontostomatea, Cariacotrichea, Muranotrichea, Parablepharismea, Colpodea (like Colpoda), Litostomatea (with Didinium and Balantidium), Nassophorea, Phyllopharyngea (with Podophyra), Oligohymenophorea (with Tetrahymena, Paramecium, and Vorticella), Plagiopylea, Prostomatea (like Coleps), Protocruziea, and Spirotrichea.

Older ways sometimes put Opalinidae with ciliates. But they are different because only ciliates have special big cells called macronuclei.

Pathogenicity

The only type of ciliate that can make humans sick is Balantidium coli. This tiny organism causes a disease called balantidiasis. It does not make domestic pigs sick, which are the main animals that carry this organism.