Euglenids or euglenoids are one of the best-known groups of

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

s: single-celled organisms with

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

, or whip-like tails. They are classified in the phylum Euglenophyta,

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

Euglenida or Euglenoidea. Euglenids are commonly found in fresh water, especially when it is rich in organic materials, but they have a few marine and

endosymbiotic An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root ...

members. Many euglenids feed by

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

, or strictly by

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

. A monophyletic subgroup known as Euglenophyceae have

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

s and produce their own food through

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. This group contains the carbohydrate paramylon. Euglenids split from other

Euglenozoa Euglenozoa are a large group of flagellate Discoba. They include a variety of common free-living species, as well as a few important parasites, some of which infect humans. Euglenozoa are represented by four major groups, ''i.e.,'' Kinetoplastea, ...

(a larger group of flagellates) more than a billion years ago. The

plastid A plastid is a membrane-bound organelle found in the Cell (biology), cells of plants, algae, and some other eukaryotic organisms. Plastids are considered to be intracellular endosymbiotic cyanobacteria. Examples of plastids include chloroplasts ...

s (membranous organelles) in all extant photosynthetic species result from secondary

endosymbiosis An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

between a euglenid and a green alga.

Structure

Euglenoids are distinguished mainly by the presence of a type of cell covering called a pellicle. Within its taxon, the pellicle is one of the euglenoids' most diverse morphological features. The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

s. This varies from rigid to flexible, and gives the cell its shape, often giving it distinctive striations. In many euglenids, the strips can slide past one another, causing an inching motion called

metaboly Spirocuta () is a clade of euglenids, single-celled eukaryotes or protists belonging to the phylum Euglenozoa. They are distinguished from other euglenids by active deformation of their cell shape, a process called euglenid motion or metaboly. Thi ...

. Otherwise, they move using their flagella. 2023 Eugenid

Classification

The first attempt at classifying euglenids was done by Ehrenberg in 1830, when he described the genus ''

Euglena ''Euglena'' is a genus of Unicellular organism, single-celled, flagellate eukaryotes. It is the best-known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. Species of '' ...

'' and placed it in the Polygastrica of family Astasiae, containing other creatures of variable body shape and lacking

pseudopod A pseudopod or pseudopodium (: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filaments and ...

s or lorica. Later, various biologists described additional characteristics for ''Euglena'' and established different classification systems for euglenids based on nutrition modes, the presence and number of

, and the degree of

. The 1942 revision by A. Hollande distinguished three groups, Peranemoidées (flexible phagotrophs), Petalomonadinées (rigid phagotrophs) and Euglenidinées (phototrophs), and was widely accepted as the best reflection of the natural relationships between euglenids, adopted by many other authors. Gordon F. Leedale expanded on Hollande's system, establishing six orders ( Eutreptiales, Euglenales, Rhabdomonadales, Sphenomonadales, Heteronematales and Euglenamorphales) and taking into account new data on their physiology and

ultrastructure Ultrastructure (or ultra-structure) is the architecture of cells and biomaterials that is visible at higher magnifications than found on a standard optical light microscope. This traditionally meant the resolution and magnification range of a c ...

. This scheme endured until 1986, with the sequencing of the

SSU rRNA Small subunit ribosomal ribonucleic acid (SSU rRNA) is the smaller of the two major RNA components of the ribosome. Associated with a number of ribosomal proteins, the SSU rRNA forms the small subunit of the ribosome. It is encoded by SSU- rDNA. ...

gene from '' Euglena gracilis''. Euglenids are currently regarded as a highly diverse clade within

, in the eukaryotic supergroup

Discoba Excavata is an obsolete, extensive and diverse Paraphyly, paraphyletic group of unicellular Eukaryote, Eukaryota. The group was first suggested by Simpson and Patterson in 1999 and the name latinized and assigned a rank by Thomas Cavalier-Smit ...

. They are traditionally organized into three categories based on modes of nutrition: the

phototroph Phototrophs () are organisms that carry out photon capture to produce complex organic compounds (e.g. carbohydrates) and acquire energy. They use the energy from light to carry out various cellular metabolic processes. It is a list of common m ...

s ( Euglenophyceae), the

osmotroph Osmotrophy is a form of heterotrophic nutrition and a cellular feeding mechanism involving the direct absorption of dissolved organic compounds by osmosis. Organisms that use osmotrophy are called osmotrophs. Osmotrophy is used by diverse groups ...

s (mainly the 'primary osmotrophs' known as Aphagea), and the

phagotroph Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis i ...

s, from which the first two groups have evolved. The phagotrophs, although

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, have historically been classified under the name of Heteronematina. In addition, euglenids can be divided into inflexible or rigid euglenids, and flexible or metabolic euglenids which are capable of '

' or 'euglenid motion'. Only those with more than 18 protein strips in their pellicle gain this flexibility. Phylogenetic studies show that various clades of rigid phagotrophic euglenids compose the base of the euglenid tree, namely Petalomonadida and the paraphyletic ' Ploeotiida'. In contrast, all flexible euglenids belong to a

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

group known as Spirocuta, which includes Euglenophyceae, Aphagea and various phagotrophs ( Peranemidae, Anisonemidae and Neometanemidae). The current classification of class Euglenida, as a result of these studies, is as follows: * Euglenida ''incertae sedis'': '' Atraktomonas'', '' Calycimonas'', ''

Dolium A dolium (plural: dolia) is a large earthenware vase or vessel used in ancient Roman times for the fermentation of alcoholic beverages, as well as storage or transportation of goods. They are similar to kvevri, large Georgian vessels used to fer ...

'', '' Dylakosoma'', '' Tropidoscyphus'', '' Michajlowastasia'', '' Parastasiella'', '' Dinemula'', '' Paradinemula'', '' Mononema'', '' Ovicola'', '' Naupliicola'', '' Embryocola'', '' Copromonas''. * Order Petalomonadida * Order " Ploeotiida" (paraphyletic) ** Clade Alistosa ** '' Entosiphon'' ** '' Gaulosia'' ** Clade Karavia ** '' Chelandium'' ** '' Olkasia'' * Clade Spirocuta elicales ref name="Lax 2021"/> ** Clade

Anisonemia Anisonemia is a clade of single-celled protists belonging to the phylum Euglenozoa, relatives of the Euglenophyceae algae. They are flagellates, with two flagella for locomotion. Anisonemia includes various phagotrophic species and a group of prim ...

*** Order Anisonemida **** Family Anisonemidae *** Order Natomonadida **** Suborder Metanemina ***** Family Neometanemidae **** Suborder Aphagea habdomonadina ref name="Higher Euglenozoa"> ***** Family Astasiidae ***** Family Distigmidae ** Order Peranemida *** Family Peranemidae ** Clade Euglenophyceae uglenea *** Euglenophyceae ''incertae sedis'': '' Ascoglena'', '' Euglenamorpha'', '' Euglenopsis'', '' Glenoclosteroium'', '' Hegneria'', '' Klebsina'', '' Euglenocapsa''. *** Order

Rapazida ''Rapaza viridis'' (Latin for 'green grasper') is a species of single-celled flagellate within the Euglenophyceae, a group of algae. It is the only species within the genus ''Rapaza'', family Rapazidae and order Rapazida. It was discovered in a ...

**** Family Rapazidae *** Order Eutreptiales **** Family Eutreptiaceae *** Order Euglenales **** Family Phacaceae **** Family Euglenaceae

Nutrition

The classification of euglenids is still variable, as groups are being revised to conform with their molecular

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

. Classifications have fallen in line with the traditional groups based on differences in nutrition and number of flagella; these provide a starting point for considering euglenid diversity. Different characteristics of the euglenids' pellicles can provide insight into their modes of movement and nutrition. As with other

, the primitive mode of nutrition is

. Prey such as

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and smaller flagellates is ingested through a

cytostome A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacu ...

, supported by microtubules. These are often packed together to form two or more rods, which function in ingestion, and in ''Entosiphon'' form an extendable siphon. Most phagotrophic euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

. In some, such as '' Peranema'', the leading flagellum is rigid and beats only at its tip.

Osmotrophic euglenoids

Osmotrophic euglenids are euglenids which have undergone osmotrophy. Due to a lack of characteristics that are useful for

taxonomical In biology, taxonomy () is the scientific study of naming, defining ( circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa (singular: taxon), and these groups are given ...

purposes, the origin of osmotrophic euglenids is unclear, though certain morphological characteristics reveal a small fraction of osmotrophic euglenids are derived from phototrophic and phagotrophic ancestors. A prolonged absence of light or exposure to harmful chemicals may cause

atrophy Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations (which can destroy the gene to build up the organ), malnutrition, poor nourishment, poor circulatory system, circulation, loss of hormone, ...

and absorption of the chloroplasts without otherwise harming the organism. A number of species exists where a chloroplast's absence was formerly marked with separate genera such as ''Astasia'' (colourless ''

'') and '' Hyalophacus'' (colourless '' Phacus''). Due to the lack of a developed cytostome, these forms feed exclusively by osmotrophic absorption.

Reproduction

Although euglenids share several common characteristics with animals, which is why they were originally classified as so, no evidence has been found of euglenids ever using

sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...

. This is one of the reasons they could no longer be classified as animals. For euglenids to reproduce,

asexual reproduction Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the f ...

takes place in the form of

binary fission Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical o ...

, and the cells replicate and divide during

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

and

cytokinesis Cytokinesis () is the part of the cell division process and part of mitosis during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division ...

. This process occurs in a very distinct order. First, the

basal bodies A basal body (synonymous with basal granule, kinetosome, and in older cytological literature with blepharoplast) is a protein structure found at the base of a eukaryotic undulipodium (cilium or flagellum). The basal body was named by Theodor ...

and flagella replicate, then the

and microtubules (the feeding apparatus), and finally the nucleus and remaining

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

. Once this occurs, the organism begins to cleave at the basal bodies, and this cleavage line moves towards the center of the organism until two separate euglenids are evident. Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

or longitudinal binary fission.

Evolution

The earliest fossil of euglenids is attributed to '' Moyeria'', which is interpreted as possessing a pellicle composed of proteinaceous strips, the defining characteristic of euglenids. It is found in Middle Ordovician and

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

rocks, making it the oldest fossil evidence of euglenids.

Gallery

File:Euglena.gracilis.jpg, ''

'' sp. (Euglenales) File:Phacus pleuronectes - 400x (13263445584).jpg, '' Phacus'' sp. (Euglenales) File:Trachelomonas sp.jpg, '' Trachelomonas'' sp. (Euglenales) File:Bioconvection Euglena Concentration series Colour.jpg, Euglenoid cultures in Petri dishes File:Euglena scheme no arrows.svg, Cell diagram FMIB 40826 Astasia contorta--Two Aspects.jpeg, '' Astasia'' sp. (Euglenales) File:Album général des Cryptogames, Pl. 23.jpg, ''Euglena'', ''Astasia'' and ''Phacus'' spp. (Euglenales) File:Algen I. (Schizophyceen, Flagellaten, Peridineen) (1910) (17762559370).jpg, ''Euglena, Phacus'' and ''Lepocinclis'' spp. (Euglenales) File:Algen I. (Schizophyceen, Flagellaten, Peridineen) (1910) (17947077272).jpg, ''

Anisonema ''Anisonema'' is a genus of colorless flagellates that occur in marine, brackish, and freshwater habitats. The cell is typically ovoid, somewhat flattened, and rigid. The name ''Anisonema'' derives from Greek for "unequal thread", in reference t ...

, Petalomonas, Notosolenus, Scytomonas'' and '' Tropidoscyphus'' spp. (Sphenomonadales); '' Heteronema, Dinema'' and '' Entosiphon'' spp. (Heteronematales)

References

Bibliography

* * * * *

External links

*
The Euglenoid Project

Tree of Life: Euglenida
{{Authority control Algal taxonomy Euglenozoa Extant Ypresian first appearances