Ciliate
Ciliates are microscopic, single-celled organisms distinguished by their hair-like appendages known as cilia. These structures enable ciliates to move through aquatic environments and facilitate feeding by capturing food particles. Classified within the phylum Ciliophora, ciliates are part of the broader category of protozoans, which are unicellular eukaryotes containing a nucleus and organelles. There are over 12,000 identified ciliate species, though many more may exist. These organisms exhibit considerable biological and behavioral complexity, with some species, such as Stentor, being large enough to be seen without a microscope. Ciliates primarily inhabit water bodies, including ponds and oceans, and play a critical role in aquatic ecosystems, often used as indicators of environmental health.
Ciliates reproduce both asexually through fission or budding, and sexually via conjugation, a process that allows for genetic exchange. While most ciliates are free-living, some are parasitic, affecting fish and other animals. Notably, Balantidium coli is the only ciliate known to cause disease in humans. Overall, ciliates are vital to ecological balance and provide insights into the health of their environments.
Subject Terms
Ciliate
A ciliate (pronounced SIL-ee-it) is a microscopic single-cell organism characterized by cilia, hair-like appendages, on its outer surface. Ciliates use these appendages to propel themselves through water and to feed. According to the taxonomic system used to organize all living creatures by their shared characteristics, ciliates are classified as protozoans. Protozoans are unicellular (single-cell) organisms with both a nucleus and organelles, which are small cellular structures that function the way organs do in larger creatures. All ciliates are classified together as part of the phylum Ciliophora. The word Ciliophora is taken from the Latin word for eyelash, which many ciliates closely resemble. At least 8,000 named species and 12,000 identified ciliates are known to science although many more likely exist.
While they are mostly microscopic in size, some ciliates, such those in the genus Stentor, can reach up to .08 inches (2 millimeters) in length, which makes them visible to the naked eye. Ciliates are regarded as some of the most biologically and behaviorally complex protozoans. Many ciliates, such as paramecia, are used in laboratory experiments and can be valuable indicators of the health of an aquatic ecosystem.
Background
According to most taxonomic systems, the three forms of life are eukaryotes (of the domain Eukaryota), bacteria, and archaea. Ciliates are eukaryotes. Eukaryota is a broad taxonomic category that includes both multicellular organisms (such as human beings, animals, and plants) and unicellular organisms (such as ciliates). All eukaryotes have cells that have both a nucleus and chromosomes composed of DNA tied together by proteins. Additionally, the cells of all eukaryotes contain organelles such as Golgi bodies, chloroplasts, lysosomes, and mitochondria that control different biological functions in cells.
Ciliates are protists. This category of microscopic life consists of any organism that is not part of the animal, fungus, or plant kingdoms. Protists have no tissues and are either unicellular or composed of unicellular colonies. Beyond these simple shared traits, protists vary dramatically between species and are grouped together only because they do not fit into the other taxonomic kingdoms.
Ciliates differ from other protists because of their cilia, which differ between species. Such traits as the number, distribution, and array of cilia around the outer shell of the organism are used to differentiate and taxonomically organize the many species of ciliates.
Ciliates are mostly found in aquatic environments around the world. Virtually any body of water ranging from ponds to oceans contains ciliates in large numbers. Some species of ciliates have also been found in soil.
Scientists know of only a few examples of ciliate fossils. This is mainly because ciliates have few hard parts that are capable of becoming fossilized. Many of the known examples of fossilized ciliates were discovered trapped in amber. However, a few types of ciliates, such as the Tintinnids, form barrel-shaped outer shells called loricae. These shells often contain minerals that enable them to become microfossils. Some of these microfossils have been dated back to the Ordovician period (488 to 443 million years ago).
Cell Structure of a Ciliate
The most distinctive features of ciliates are their cilia. The two forms of cilia are somatic (body) cilia, and oral cilia. These two forms may be fused together as a compound structure called a membranelle that functions as a membrane. They may also appear as cirri, rows or tufts of cilia that are used for movement and food gathering. Somatic cilia are spread across the cells of ciliates in rows called kineties that are found on the external surface of the rigid pellicle that forms the outermost covering of the cell. The cilia are attached to the cell body through a set of fibers on the outer surface of the ciliate called the infraciliature or kinetidal system. The infraciliature of ciliates is part of a complex structure located beneath a thin skin.
Ciliates use the somatic cilia to propel themselves through water. While larger organisms find it easy to navigate through water, tiny organisms like ciliates find it more difficult because to them, water seems thick, like jelly. However, the powerful cilia enable the ciliate to move through water quickly and easily.
The oral cilia are used to help feed the ciliate. They are located around an opening in the ciliate called an oral groove. Food is converted to food vacuoles in the gullet found at the end of the oral groove. Ciliates feed on algae, bacteria, and other ciliates.
Ciliates have two types of nuclei: at least one diploid micronuclei, which contains two sets of chromosomes, and a single polyploid macronuclei, which contains more than two sets of chromosomes. The smaller micronuclei are used in reproduction while the macronucleus maintains the cell's biological functions.
Overview
Ciliates reproduce both asexually and sexually. Asexual reproduction occurs either through fission or budding. In fission, a ciliate divides into two copies after undergoing a process of cell division called mitosis. Budding occurs among ciliates called suctorians. In this process, a new genetically identical organism is created from an outgrowth of the parent cell.
Sexual reproduction occurs through conjugation. In this process, two members of the same species rub against each other. This creates a cytoplasmic bridge between the two organisms that allows them to partially fuse together. Next, the two ciliates share parts of their cytoplasm with one another while their macronuclei disintegrate. The two cells exchange micronuclei before separating. This form of sexual reproduction is reliant upon meiosis, a process that allows them to exchange genetic material. Conjugation does not create any new offspring. Rather, conjugation occurs because fission is an inherently flawed process. After reproducing in this manner over several hundred generations, genetic material becomes increasingly defective. Conjugation allows ciliates to revitalize themselves with new genetic material.
Most ciliates are free-living organisms. This means that they do not require another host animal for survival. However, some species are parasitic; many of these species are found in fish that share their aquatic environment. Only one known species, Balantidium coli, has been identified as causing disease in humans. Still other species have been found living in the digestive systems of hooved animals. These ciliates help manage the bacteria that allow these animals to safely break down and digest the cellulose in plant matter.
Eric Bullard
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