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 ...

flagellates: single-celled organisms with flagella, 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 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 (a larger group of flagellates) more than a billion years ago. The plastids (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 microtubules. 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'' and placed it in the Polygastrica of family Astasiae, containing other creatures of variable body shape and lacking pseudopods 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 flagella, 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. This scheme endured until 1986, with the sequencing of the SSU rRNA gene from '' Euglena gracilis''. Euglenids are currently regarded as a highly diverse clade within Euglenozoa, in the eukaryotic supergroup Discoba. They are traditionally organized into three categories based on modes of nutrition: the phototrophs ( Euglenophyceae), the osmotrophs (mainly the 'primary osmotrophs' known as Aphagea), and the phagotrophs, 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'', '' 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 **** 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 Euglenozoa, 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. 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 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 '' Euglena'') 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. This process occurs in a very distinct order. First, the basal bodies 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 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, '' Euglena'' 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, 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