Ciliate

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

You can find ciliates almost anywhere there is water—whether it’s in lakes, ponds, oceans, rivers, or even in wet soil. They are very common and come in many different shapes and sizes. Some are as tiny as just 10 μm, while others can be as long as 4 mm. Scientists have described about 4,500 different kinds of ciliates, but they believe there could be many more—perhaps as many as 27,000 to 40,000 species still waiting to be discovered!

Ciliates are also very important in nature. Many of them live by helping other organisms, either by staying on their surfaces or living inside them. Some ciliates can even act as parasites, meaning they live off of other creatures. Despite their small size, ciliates are among the most complex single-celled animals, with intricate structures that scientists still study today. They are usually classified under the group called Ciliophora, which is considered a phylum in many systems of taxonomy.

Cell structure

Ciliates have two special kinds of nuclei, or control centers, in their cells. One is small and carries the cell's genetic blueprint for making new cells, while the other is large and helps the cell carry out its daily activities. These nuclei 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 across the cell's surface. Inside the cell, there are also tiny organs that help process food and remove waste, keeping the cell working properly.

Feeding

Most ciliates get their food by eating smaller organisms like bacteria and algae. They use tiny hair-like structures called cilia to sweep food into their oral groove, or mouth. These cilia help move the food into a special part of their body called the gullet, where it forms food vacuoles.

Some ciliates can also get energy from sunlight by living together with tiny plants, while others absorb food directly from their environment. There are even 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 during reproduction, while the DNA in the macronucleus controls what the organism looks like. The macronucleus forms from the micronucleus through a process that rearranges and copies 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 amazing process helps these tiny organisms grow and survive.

Aging

In some tiny water creatures called Paramecium, they can get tired over time if they don't mix with others. Scientists found out 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, which 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 called acritarchs. Another fossil of a ciliate called Vorticella was found inside a leech cocoon from the Triassic period, about 200 million years ago.

Phylogeny

According to a study from 2016, scientists learned that a group called Mesodiniea is closely related 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 several other groups like Colpodea, Oligohymenophorea, and Nassophorea.

In 2018, scientists discovered a new group called Odontostomatea, which is related to Armophorea. This helps us understand how these tiny creatures are connected to each other.

Classification

Further information: Wikispecies:Ciliophora

Ciliates are tiny, single-celled creatures that scientists have tried to sort into different groups in many ways. One way looks at tiny pieces inside the cells from many different types of ciliates.

This helps us understand how these creatures are related. Some groups include Mesodiniea (like Mesodinium), Heterotrichea (like Stentor), and Karyorelictea. Another big group is Intramacronucleata, which has many smaller groups such as Armophorea, Odontostomatea, Cariacotrichea, Muranotrichea, Parablepharismea, Colpodea (like Colpoda), Litostomatea (with subgroups like Didinium and Balantidium), Nassophorea, Phyllopharyngea (including Podophyra), Oligohymenophorea (with groups like Tetrahymena, Paramecium, and Vorticella), Plagiopylea, Prostomatea (like Coleps), Protocruziea, and Spirotrichea (with several subgroups).

Older ways of sorting sometimes put a group called 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. However, it does not make domestic pigs sick, which are the main animals that carry this organism.