Ciliate

Ciliates are a group of tiny, single-celled organisms. They belong to the alveolates. What makes them special is that they have many hair-like structures called cilia on their surface. These cilia look like little whips called flagella, but they are usually shorter and move in a wave-like way. Ciliates use their cilia for many jobs, such as moving around, grabbing food, and sensing their surroundings.

You can find ciliates almost anywhere there is water—whether it’s in lakes, oceans, rivers, or even in wet soil. There are many known species of ciliates, and scientists believe there could be even more waiting to be discovered. These creatures come in many sizes, from very tiny ones to ones that can grow up to 4 mm, about the width of a thin pencil.

Ciliates are considered protists, which means they are not plants, animals, or fungi. They live in many different ways—some float freely in water, while others live on or inside other organisms. Because of their complex structures and behaviors, ciliates are very interesting to scientists who study tiny living things.

Cell structure

Ciliates have two special parts in their cells that help them stay alive. One part is small and holds the instructions the cell needs to make new cells. The other part is larger and helps the cell do its everyday jobs.

Ciliates also have tiny hair-like parts called cilia. These cilia help the cell move and find food. The cilia are arranged in special patterns on the cell's surface. Inside the cell, there are pockets called food vacuoles that help break down food. Other parts of the cell help keep the right amount of water inside.

Feeding

Most ciliates eat smaller organisms like bacteria and algae. They sweep these foods into their mouth using special cilia. Inside, the food forms pockets called food vacuoles where it is digested.

Some ciliates can also get energy from sunlight by working with tiny plants or algae. One type, Halteria, has been seen eating viruses. Other ciliates absorb food directly through their bodies, while a few hunt other tiny creatures. Only one ciliate, Balantidium coli, can make humans sick.

Main article: Heterotroph
Main articles: Bacteria, Algae
Further information: Membranelle
Main article: Mixotroph
Further information: Phagotrophy, Phototrophy
Further information: Kleptoplasty
Further information: Halteria
Further information: Chlorovirus
Further information: Osmotrophy
Further information: Animal
Further information: Balantidium coli

Reproduction and sexual phenomena

Ciliates can reproduce without using another organism, called asexual reproduction, through a process called fission. During fission, the small part of the cell called the micronucleus divides, and the larger part, the macronucleus, changes shape before the cell splits into two new cells.

Ciliates can also reproduce sexually through a process called conjugation. In conjugation, two ciliates connect and exchange tiny parts of their micronuclei. After they separate, each cell uses this exchanged part to build a new macronucleus. This mixing of genetic material helps keep the group healthy and strong.

DNA rearrangements (gene scrambling)

Ciliates have two types of nuclei: a somatic "macronucleus" and a germline "micronucleus". The micronucleus passes DNA during reproduction, while the macronucleus controls the organism’s traits. The macronucleus forms from the micronucleus through DNA rearrangement and amplification.

In Tetrahymena, the micronucleus starts with 10 chromosomes, but the macronucleus ends up with over 20,000. This happens because micronuclear DNA is broken into small pieces and rearranged. Some parts, called "internal eliminated sequences" (IESs), are removed, and the remaining pieces are joined together. In spirotrich ciliates like Oxytricha, this process is more complex, needing DNA to be inverted and moved to unscramble the genes.

Aging

In some tiny creatures called Paramecium, aging happens over many generations. They lose energy as they get older. But they can feel better again through a process called conjugation or self-fertilization. Scientists found that damage to their DNA increases as they age. This seems to be the reason for the aging process.

Fossil record

Scientists once thought the oldest ciliate fossils were from the Ordovician period, called tintinnids. In 2007, they found older fossils that looked like ciliates in the Doushantuo Formation from the Ediacaran period, about 580 million years ago. They later found out these were not true ciliates. Another fossil of a creature named Vorticella was found inside a leech cocoon from the Triassic period, about 200 million years ago.

Phylogeny

Ciliates have an interesting family tree. In 2016, scientists found 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 is SAL. It includes Spirotrichea, Armophorea, and Litostomatea. The other branch is called CONthreeP or Ventrata. It has smaller groups like Colpodea and Oligohymenophorea.

Later, in 2018, scientists discovered a new group called Odontostomatea. It is related to Armophorea. This helps us understand how different types of ciliates are connected.

Classification

Further information: Wikispecies:Ciliophora

Ciliates are grouped into several classes based on their genes. Some main groups include Mesodiniea, Heterotrichea (like Stentor), and Karyorelictea.

Other classes are Armophorea, Odontostomatea (including Discomorphella and Saprodinium), Colpodea (like Colpoda), Litostomatea (with subgroups such as Didinium and Balantidium), and Oligohymenophorea (including Tetrahymena, Paramecium, and Vorticella). Many more classes and subclasses help scientists learn about these tiny, hair-like organisms.

Pathogenicity

The only type of ciliate that can make humans sick is called Balantidium coli. It causes a disease known as balantidiasis. This disease does not harm domestic pigs, which often carry this organism.