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 The ''International Code of Nomenclature for algae, fungi, and plants'' (ICN) is the set of rules and recommendations dealing with the formal botanical names that are given to plants, fungi and a few other groups of organisms, all those "trad ...

) 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 thei ...

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. Bacter ...

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 typica ...

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 An ''endosymbiont'' or ''endobiont'' is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον ''endon'' "within ...

members. Many euglenids feed by

phagocytosis 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 is ...

, 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 ...

. A monophyletic group consisting of the

mixotroph A mixotroph is an organism that can use a mix of different sources of energy and carbon, instead of having a single trophic mode on the continuum from complete autotrophy at one end to heterotrophy at the other. It is estimated that mixotrophs comp ...

ic 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 i ...

s and produce their own food through

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...

. 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, D ...

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 Institu ...

-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 1 ...

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 ...

. 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

Protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the e ...

a 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 ...

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 A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological s ...

. 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 (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

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 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 is c ...

euglenids have two flagella, one leading and one trailing. The latter is used for gliding along the substrate. In some, such as ''

Peranema ''Peranema'' is a genus of free-living phagotrophic euglenids (Euglenida; Euglenozoa; Excavata). There are more than 20 nominal species, varying in size between 8 and 200 micrometers. ''Peranema'' cells are gliding flagellates found in freshwat ...

'', 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 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 ...

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 ''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 ...

'') and '' Hyalophacus'' (colourless ''

Phacus ''Phacus'' is a genus of unicellular excavates, of the phylum Euglenozoa (also known as Euglenophyta), characterized by its flat, leaf-shaped structure, and rigid cytoskeleton known as a pellicle. These eukaryotes are mostly green in colour, an ...

''). 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 th ...

. 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 ...

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 digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...

, 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 maintai ...

and

cytokinesis Cytokinesis () is the part of the cell division process 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 in mitosis and mei ...

. 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 W ...

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 co ...

. 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 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 eukaryotes, there ...

or longitudinal binary fission.

Gallery

File:Euglena.gracilis.jpg, ''

'' sp. (Euglenales) File:Phacus pleuronectes - 400x (13263445584).jpg, ''

'' sp. (Euglenales) File:Trachelomonas sp.jpg, ''

Trachelomonas ''Trachelomonas'' is a genus of swimming, free-living euglenoids characterized by the presence of a shell-like covering called a lorica. Details of lorica structure determine the classification of distinct species in the genus. The lorica can e ...

'' 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 ''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 ...

'' 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