View source for Euglenid

{{Short description|Class of protozoans}}
{{Automatic taxobox
| image = Ehrenberg euglena viridis.jpg
| image_upright = 1.1
| image_caption = ''[[Euglena|Euglena viridis]]'', by [[Christian Gottfried Ehrenberg|Ehrenberg]]
| taxon = Euglenida
| authority = Butschli 1884, [[Emendation (zoology)|emend]]. Simpson 1997
| subdivision_ranks = Major groups
| subdivision = * [[Petalomonadida]]
* [[Alistosa]]
* [[Karavia]]
* [[Spirocuta]]
** [[Anisonemia]]
** [[Peranemida]]
** [[Euglenophyceae]]
| synonyms = * Euglenoidina <small>[[Otto Bütschli|Bütschli]], 1884, Blochmann, 1886</small>
* Euglenoidea <small>Lankester, 1885</small>
* Euglenoida <small>Cavalier-Smith, 1993</small>
| fossil_range = {{fossil range|Middle Ordovician|Present|ref=<ref name="Gray 1989">{{cite journal|last1=Gray|first1=Jane|last2=Boucot|first2=A. J.|title=Is ''Moyeria'' a euglenoid?|journal=Lethaia|volume=22|issue=4|pages=447–456|doi=10.1111/j.1502-3931.1989.tb01449.x|date=1989 |bibcode=1989Letha..22..447G }}</ref><ref name="Strother 2020">{{cite journal|last1=Strother|first1=Paul K.|last2=Taylor|first2=Wilson A.|last3=van de Schootbrugge|first3=Bas|last4=Leander|first4=Brian S.|last5=Wellman|first5=Charles H.|title=Pellicle ultrastructure demonstrates that ''Moyeria'' is a fossil euglenid|journal=Palynology|date=2020|volume=44|issue=3|pages=461–471|doi=10.1080/01916122.2019.1625457|bibcode=2020Paly...44..461S |doi-access=free}}</ref>}}
}}

'''Euglenids''' or '''euglenoids''' are one of the best-known groups of [[Eukaryote|eukaryotic]] [[flagellate]]s: single-celled organisms with [[flagella]], or whip-like tails. They are classified in the phylum [[Euglenozoa]], [[class (biology)|class]] '''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 [[Endosymbiont|endosymbiotic]] members. Many euglenids feed by [[phagocytosis]], or strictly by [[diffusion]]. A monophyletic subgroup known as [[Euglenophyceae]] have [[chloroplast]]s and produce their own food through [[photosynthesis]].<ref>{{cite journal|title=Dynamic evolution of inverted repeats in Euglenophyta plastid genomes|journal=Scientific Reports|year=2018|first1=Anna|last1=Karnkowska|first2=Matthew S.|last2=Bennett|first3=Richard E.|last3=Triemer|volume=8 |issue=1 |page=16071 |doi=10.1038/s41598-018-34457-w |pmid=30375469 |bibcode=2018NatSR...816071K |pmc=6207741}}</ref><ref>{{cite book|url=https://books.google.com/books?id=c93WDgAAQBAJ&dq=Rapaza+viridis+two+genera+Eutreptiales+and+Euglenales&pg=PA323|title=Secondary Endosymbioses|page=323|author=Yoshihisa Hirakawa|publisher=Academic Press|year=2017|isbn=978-0-12-802680-9}}</ref><ref>{{Cite web |url=https://www.algaebase.org/pub_taxonomy/?id=163213 |title=Algaebase :: Subclass: Euglenophycidae |access-date=2019-10-27 |archive-date=2020-07-13 |archive-url=https://web.archive.org/web/20200713195620/https://www.algaebase.org/pub_taxonomy/?id=163213 }}</ref> This group contains the carbohydrate [[paramylon]].

Euglenids split from other [[Euglenozoa]] (a larger group of flagellates) more than a billion years ago. The [[plastid]]s (membranous organelles) in all extant photosynthetic species result from secondary [[endosymbiosis]] between a euglenid and a green alga.<ref>{{cite book|pmid=28429314|doi=10.1007/978-3-319-54910-1_1|volume=979|year=2017|pages=3–17|last1=Zakryś|first1=B|last2=Milanowski|first2=R|last3=Karnkowska|first3=A|title=Euglena: Biochemistry, Cell and Molecular Biology |chapter=Evolutionary Origin of Euglena |series=Advances in Experimental Medicine and Biology|isbn=978-3-319-54908-8}}</ref>

== Structure ==
Euglenoids are distinguished mainly by the presence of a type of cell covering called a [[Pellicle (biology)|pellicle]]. Within its taxon, the pellicle is one of the euglenoids' most diverse morphological features.<ref>{{Cite journal|last1=Leander|first1=Brian S.|last2=Farmer|first2=Mark A.|date=2001-03-01|title=Comparative Morphology of the Euglenid Pellicle. II. Diversity of Strip Substructure|journal=Journal of Eukaryotic Microbiology|language=en|volume=48|issue=2|pages=202–217|doi=10.1111/j.1550-7408.2001.tb00304.x|pmid=12095109|s2cid=2109559 |issn=1550-7408}}</ref> The pellicle is composed of proteinaceous strips underneath the cell membrane, supported by dorsal and ventral [[microtubule]]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]]. Otherwise, they move using their flagella.

[[File:2023 Eugenid.svg|center|thumb|upright=2|{{center|'''Euglenid Body Plan'''}}{{ordered list|Dorsal [[flagellum]]|
[[Axoneme]]|
Paraflagellar rod|
[[Mastigoneme]]s, "hairs" attached to flagellum|
Flagellar pocket vestibulum|
Feeding apparatus|
Paraxial swelling|
[[Eyespot apparatus|Eyespot]], photoreceptor used to sense light direction and intensity|
[[Contractile vacuole]], regulates the quantity of water inside a cell|
Ventral flagellum|
Ventral root|
[[Golgi apparatus]]; modifies [[protein]]s and sends them out of the cell|
[[Endoplasmic reticulum]], the transport network for molecules going to specific parts of the cell|
[[Phagosome]]|
[[Lysosome]], holds enzymes|
[[Cell nucleus|Nucleus]]|
[[Nucleolus]]|
[[Plastid]] membranes (3, secondary)|
[[Thylakoid]]s, site of the [[light-dependent reactions]] of [[photosynthesis]]|
[[Pyrenoid]], center of [[carbon fixation]]|
[[Paramylon]] [[Granule (cell biology)|granules]]|
[[Protozoa#Walls, pellicles, scales, and skeletons|Pellicular]] strip|
Muciferous body|
[[Mitochondria|Mitochondrion]], creates [[Adenosine triphosphate|ATP]] (energy) for the cell (discoid [[crista]]e)}}]]

== Classification ==

[[File:Algen I. (Schizophyceen, Flagellaten, Peridineen) (1910) (17950163265).jpg|right|thumb|upright=1.2|Examples of euglenid diversity.<br/>1—2. ''Ascoglena'' sp. (Euglenales);<br /> 3–4. ''Cryptoglena'' sp. (idem);<br /> 5–9, 14–15, 24–25, 27–29. ''[[Trachelomonas]]'' spp. (id.);<br /> 10. ''Eutreptia'' sp. (Eutreptiales);<br /> 11, 20. ''Astasia'' spp. (Euglenales);<br /> 12. ''Distigma'' sp. (Eutreptiales);<br /> 13. ''Menoid[i]um'' sp. (Rhabdomonadales);<br /> 16–18. ''Colacium'' sp. (Euglenales);<br /> 19, 26. ''Petalomonas'' spp. (Sphenomonadales);<br /> 21. ''Sphenomonas'' sp. (id.);<br /> 22–23. ''Euglenopsis'' sp. (Euglenales);<br /> 30. ''[[Peranema]]'' sp. (Heteronematales)]]

The first attempt at classifying euglenids was done by [[Christian Gottfried Ehrenberg|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 [[pseudopod]]s or [[lorica (biology)|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 [[metaboly]]. 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.<ref name="Bicudo 2016">{{cite Q|Q57898656|doi-access=free }}</ref> 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]]''.<ref name="Bicudo 2016"/>

Euglenids are currently regarded as a highly diverse clade within [[Euglenozoa]], in the [[eukaryotic supergroup]] [[Discoba]].<ref name="Adl 2019">{{cite Q|Q57086550}}</ref> They are traditionally organized into three categories based on modes of nutrition: the [[phototroph]]s ([[Euglenophyceae]]), the [[osmotroph]]s (mainly the 'primary osmotrophs' known as [[Aphagea]]), and the [[phagotroph]]s, from which the first two groups have evolved.<ref name="Lax 2020">{{cite Q|Q101127864}}</ref> The phagotrophs, although [[paraphyletic]], have historically been classified under the name of [[Heteronematina]].<ref name="Adl 2019"/>

In addition, euglenids can be divided into inflexible or rigid euglenids, and flexible or metabolic euglenids which are capable of '[[metaboly]]' 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 [[basal (phylogenetics)|base]] of the euglenid tree, namely [[Petalomonadida]] and the paraphyletic '[[Ploeotiida]]'. In contrast, all flexible euglenids belong to a [[monophyletic]] 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:<ref name="Lax 2020"/><ref name="Lax 2021">{{cite Q|Q110667805}}</ref><ref name="Kostygov 2021">{{cite Q|Q125548575}}</ref><ref name="Lax 2023">{{cite Q|Q123348233}}</ref><ref name="Lax Keeling 2023">{{cite journal|vauthors=Lax G, Keeling PJ|title=Molecular phylogenetics of sessile ''Dolium sedentarium'', a petalomonad euglenid|journal=The Journal of Eukaryotic Microbiology|volume=70|number=e12991|date=2023|article-number=e12991 |doi=10.1111/jeu.12991|doi-access=free|pmid=37424051}}</ref>
* Euglenida ''incertae sedis'': ''[[Atraktomonas]]'', ''[[Calycimonas]]'', ''[[Dolium]]'', ''[[Dylakosoma]]'', ''[[Tropidoscyphus]]'', ''[[Michajlowastasia]]'', ''[[Parastasiella]]'', ''[[Dinemula]]'', ''[[Paradinemula]]'', ''[[Mononema]]'', ''[[Ovicola]]'', ''[[Naupliicola]]'', ''[[Embryocola]]'', ''[[Copromonas]]''.
* Order [[Petalomonadida]] {{au|Cavalier-Smith 1993}}
* Order "[[Ploeotiida]]" {{au|Cavalier-Smith 1993}} (paraphyletic)
** Clade [[Alistosa]] {{au|Lax & Simpson 2021}}
** ''[[Entosiphon (excavate)|Entosiphon]]'' {{au|Cavalier-Smith, Chao & Vickerman 2016}}
** ''[[Gaulosia]]'' {{au|Lax, Cho & Keeling 2023}}
** Clade [[Karavia]] {{au|Lax, Cho & Keeling 2023}}
** ''[[Chelandium (genus)|Chelandium]]'' {{au|Lax, Cho & Keeling 2023}}
** ''[[Olkasia]]'' {{au|Lax, Lee, Eglit & Simpson 2019}}<ref name="Lax 2019">{{cite Q|Q92132357}}</ref>
* Clade [[Spirocuta]] {{au|Cavalier-Smith, Chao & Vickerman 2016}}<ref name="Spirocuta">{{cite Q|Q31135651}}</ref> [Helicales {{au|Perschke et al. 2017}}]<ref name="Lax 2021"/>
** Clade [[Anisonemia]] {{au|Cavalier-Smith 2016}}
*** Order [[Anisonemida]] {{au|Cavalier-Smith 2016}}
**** Family [[Anisonemidae]] {{au|Kent 1880 [[Emendation (zoology)|emend]]. Cavalier-Smith 2016}}
*** Order [[Natomonadida]] {{au|Cavalier-Smith 2016}}
**** Suborder [[Metanemina]] {{au|Cavalier-Smith 2016}}
***** Family [[Neometanemidae]] {{au|Cavalier-Smith 2016}}
**** Suborder [[Aphagea]] {{au|Cavalier-Smith 1993 [[Emendation (zoology)|emend]]. Busse & Preisfeld 2003}} [Rhabdomonadina {{au|Leedale 1967 [[Emendation (zoology)|emend]]. Cavalier-Smith 1993}}]<ref name="Higher Euglenozoa">{{cite Q|Q39151632}}</ref>
***** Family [[Astasiidae]] {{au|Kent 1884}}
***** Family [[Distigmidae]] {{au|Hollande 1942}}
** Order  [[Peranemida]] {{au|Bütschli 1884}}
*** Family [[Peranemidae]] {{au|Bütschli 1884}}
** Clade [[Euglenophyceae]] {{au|Schoenichen 1925, [[Emendation (zoology)|emend]]. Marin & Melkonian 2003}} [Euglenea {{au|Butschli 1884, [[Emendation (zoology)|emend]]. Busse & Preisfeld 2002}}]
*** Euglenophyceae ''incertae sedis'': ''[[Ascoglena]]'', ''[[Euglenamorpha]]'', ''[[Euglenopsis]]'', ''[[Glenoclosteroium]]'', ''[[Hegneria]]'', ''[[Klebsina]]'', ''[[Euglenocapsa]]''.
*** Order [[Rapazida]] {{au|Cavalier-Smith 2016}}
**** Family [[Rapazidae]] {{au|Cavalier-Smith 2016}}
*** Order [[Eutreptiales]] {{au|Leedale 1967, [[Emendation (zoology)|emend]]. Marin & Melkonian 2003}}
**** Family [[Eutreptiaceae]] {{au|Hollande 1942}}
*** Order [[Euglenales]] {{au|Leedale 1967, [[Emendation (zoology)|emend]]. Marin & Melkonian 2003}}
**** Family [[Phacaceae]] {{au|Kim et al. 2010}}
**** Family [[Euglenaceae]] {{au|Dujardin 1841, [[Emendation (zoology)|emend]]. Kim et al. 2010}}

==Nutrition==
The classification of euglenids is still variable, as groups are being revised to conform with their molecular [[phylogeny]].  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.<ref>{{Cite journal|last=Leander|first=Brian Scott|date=May 2001|title=Evolutionary morphology of the euglenid pellicle|url=https://athenaeum.libs.uga.edu/handle/10724/20165|journal=University of Georgia Theses and Dissertations}}</ref>

As with other [[Euglenozoa]], the primitive mode of nutrition is [[phagocytosis]].  Prey such as [[bacteria]] and smaller flagellates is ingested through a [[cytostome]], 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 [[Phagocytosis|phagotrophic]] euglenids have two flagella, one leading and one trailing.  The latter is used for gliding along the [[Substrate (biology)|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 [[Taxonomy (biology)|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.<ref>{{cite journal|title=Systematics of primary osmotrophic euglenids: a molecular approach to the phylogeny of Distigma and Astasia (Euglenozoa)|first1=Ingo|last1=Busse|first2=Angelika|last2=Preisfeld|date=14 April 2018|journal=International Journal of Systematic and Evolutionary Microbiology|volume=53|issue=2|pages=617–624|doi=10.1099/ijs.0.02295-0|pmid = 12710635|doi-access=free}}</ref>

A prolonged absence of light or exposure to harmful chemicals may cause [[atrophy]] 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]]. This is one of the reasons they could no longer be classified as animals.{{dubious|1=Animals? Really?|date=February 2024}}

For euglenids to reproduce, [[asexual reproduction]] takes place in the form of [[binary fission]], and the cells replicate and divide during [[mitosis]] and [[cytokinesis]]. This process occurs in a very distinct order. First, the [[basal bodies]] and flagella replicate, then the [[cytostome]] and microtubules (the feeding apparatus), and finally the nucleus and remaining [[cytoskeleton]]. 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.<ref>{{Cite web|url=http://tolweb.org/Euglenida/97461|title=Euglenida|website=tolweb.org|access-date=2017-03-30}}</ref> Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal [[cell division]] or longitudinal binary fission.<ref>{{Cite web|url=http://euglenabiology.weebly.com/reproduction.html|title=Reproduction|website=Euglena|access-date=2017-03-31}}</ref>

== 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]] rocks, making it the oldest fossil evidence of euglenids.<ref name="Gray 1989"/><ref name="Strother 2020"/>

==Gallery==
<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 (alga)|Dinema]]'' and ''[[Entosiphon (excavate)|Entosiphon]]'' spp. (Heteronematales)
</gallery>

== References ==
{{Reflist}}

==Bibliography==
* {{cite book|last1=Ciugulea|first1=I.|last2=Triemer|first2=R. E.|year=2010|title=A Color Atlas of Photosynthetic Euglenoids|publisher=Michigan State University Press|location=East Lansing, MI|isbn=978-0-87013-879-9 |url=https://books.google.com/books?id=0FVFAQAAIAAJ}}
* {{cite journal|last1=Leander|first1=B. S.|last2=Triemer|first2=R. E.|last3=Farmer|first3=M. A.|year=2001|title=Character evolution in heterotrophic euglenids|journal=European Journal of Protistology|volume=37|issue=3|pages=337–356|doi=10.1078/0932-4739-00842 |s2cid=4181281 }}
* {{cite book|author1=Leander, B.S.|author2=Lax, G.|author3=Karnkowska, A.|author4=Simpson, A.G.B.|year=2017|chapter=Euglenida|editor1=Archibald, J.M.|editor2=Simpson, A.G.B.|editor3=Slamovits, C.|title=Handbook of the Protists|publisher=Springer|pages=1–42|doi=10.1007/978-3-319-32669-6_13-1|isbn=978-3-319-32669-6 }}
* {{cite journal|last=Leedale|first=G. F.|year=1978|title=Phylogenetic criteria in euglenoid flagellates|journal=BioSystems|volume=10|issue=1–2 |pages=183–187|doi=10.1016/0303-2647(78)90040-0 |pmid=656566 |bibcode=1978BiSys..10..183L }}
* {{cite book|last1=Wołowski|first1=K|last2=Hindák|first2=F|year=2005|title=Atlas of Euglenophytes|location=Krakow|publisher=VEDA Publishing House of the Slovak Academy of Sciences|isbn=978-80-224-0836-3|url=https://books.google.com/books?id=4lVFAQAAIAAJ}}

== External links ==
* {{Wikispecies-inline|Euglenoidea}}
* [https://web.archive.org/web/20130721115857/http://euglena.msu.edu/ The Euglenoid Project]
* [http://tolweb.org/Euglenida/97461 Tree of Life: Euglenida]

{{Discoba}}
{{Taxonbar|from1=Q134863|from2=Q18916511}}
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[[Category:Algal taxonomy]]
[[Category:Euglenozoa]]
[[Category:Extant Ypresian first appearances]]