Euglenids or euglenoids are one of the best-known groups of
flagellates. They are
excavateeukaryotes of the phylum
Euglenophyta, classified as
classEuglenida or Euglenoidea. Euglenids 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 known as
Euglenophyceae have
chloroplasts and produce their own food through
photosynthesis.[2][3][4] This group is known to contain the carbohydrate
paramylon.
Euglenids split from other
Euglenozoa 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-related green alga.[5] In January 2024, some fossilized forms of euglenid had been found to be mistakenly classified as Pseudoschizaea shells.[6]
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.[7] 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. Otherwise, they move using their flagella.
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
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.[8] 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.[8]
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
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:[10][11][12][13][14]
Family
EuglenaceaeDujardin 1841, 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.[18]
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
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.[19]
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 –
discuss]
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.[20] Because of the way that this reproduction takes place and the axis of separation, it is called longitudinal
cell division or longitudinal binary fission.[21]
^Zakryś, B; Milanowski, R; Karnkowska, A (2017). "Evolutionary Origin of Euglena". Euglena: Biochemistry, Cell and Molecular Biology. Advances in Experimental Medicine and Biology. Vol. 979. pp. 3–17.
doi:
10.1007/978-3-319-54910-1_1.
ISBN978-3-319-54908-8.
PMID28429314.
Leander, B. S.; Triemer, R. E.; Farmer, M. A. (2001). "Character evolution in heterotrophic euglenids". European Journal of Protistology. 37 (3): 337–356.
doi:
10.1078/0932-4739-00842.
S2CID4181281.
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.
doi:
10.1007/978-3-319-32669-6_13-1.
ISBN978-3-319-32669-6.