Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida,

ICZN The International Code of Zoological Nomenclature (ICZN) is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals. It is also informally known as the ICZN Code, for its publisher, the I ...

, or Euglenophyceae, ICBN) are one of the best-known groups of

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 their ...

s, which are excavate

eukaryote Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...

s of the phylum

Euglenophyta Euglenids (euglenoids, or euglenophytes, formally Euglenida/Euglenoida, ICZN, or Euglenophyceae, ICBN) are one of the best-known groups of flagellates, which are excavate eukaryotes of the phylum Euglenophyta and their cell structure is typic ...

and their cell structure is typical of that group. They are commonly found in freshwater, especially when it is rich in organic materials, with a few marine and endosymbiotic members. Many euglenids feed by phagocytosis, or strictly by diffusion. A monophyletic group consisting of the mixotrophic Rapaza viridis (1 species) and the two groups Eutreptiales (24 species) and Euglenales (983 species) have

chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...

s and produce their own food through photosynthesis. This group is known to contain 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 three major clades, i.e., Kinetoplastea, Di ...

more than a billion years ago. The plastids in all extant photosynthetic species is the result from secondary endosymbiosis between a phagotrophic eukaryovorous euglenid and a

Pyramimonas ''Pyramimonas'' is a genus of green algae in the order Pyramimonadales.See the NCBI The National Center for Biotechnology Information (NCBI) is part of the United States National Library of Medicine (NLM), a branch of the National Institutes ...

-related 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 nm and have an inner diameter between 11 an ...

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 ''Euglena'' is a genus of single cell 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 ''Euglena'' are found in f ...

. Otherwise, they move using their flagella.

Classification

The euglenids were first defined by

Otto Bütschli Johann Adam Otto Bütschli (3 May 1848 – 2 February 1920) was a German zoologist and professor at the University of Heidelberg. He specialized in invertebrates and insect development. Many of the groups of protists were first recognized by him. ...

in 1884 as the flagellate order Euglenida, as an animal. Botanists subsequently created the algal division Euglenophyta; thus, they were classified as both animals and plants, as they share characteristics with both. Conflicts of this nature are exemplary of why the kingdom Protista was adopted. However, they retained their double-placement until the flagellates were split up, and both names are still used to refer to the group. Their

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

s are not masked with accessory pigments.

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. As with other

, the primitive mode of nutrition is phagocytosis. Prey such as bacteria 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 vacuole ...

, 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 (locomotion), the surface over which an organism lo ...

. 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 is 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), poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply t ...

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. 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 fu ...

takes place in the form of binary fission, and the cells replicate and divide during

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

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. 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.

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 ''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 ''Dinema'' is a genus of orchids. It is represented by a single currently accepted species, ''Dinema polybulbon'', native to Mexico, Central America, and the Caribbean. Description They are epiphytes or lithophytes; with pseudobulbs 10 mm l ...

'' and '' Entosiphon'' spp. (Heteronematales)

References

Bibliography

* Ciugulea, I. & Triemer, R. E. (2010) ''A Color Atlas of Photosynthetic Euglenoids''. Michigan State University Press, East Lansing, MI, 204 p.

* Leander, B. S., Triemer, R. E., & Farmer, M. A. (2001). Character evolution in heterotrophic euglenids. ''European Journal of Protistology'', 37(3), 337-356

* Leander, B.S., Lax, G., Karnkowska, A., Simpson, A.G.B. (2017). Euglenida. In: Archibald, J.M., Simpson, A.G.B., Slamovits, C. (Eds.). ''Handbook of the Protists''. Springer, pp. 1–42. * Leedale, G. F. (1978). Phylogenetic criteria in euglenoid flagellates. ''BioSystems'' 10: 183–187

* Wołowski, K & Hindák, F. (2005). ''Atlas of Euglenophytes''. Cracow: VEDA Publishing House of the Slovak Academy of Sciences, 136 p.

External links

*
The Euglenoid Project

Tree of Life: Euglenida
{{Taxonbar, from1=Q134863, from2=Q18916511, from3=Q2356588 Algal taxonomy Euglenozoa Extant Ypresian first appearances