Euglenoida (u-GLEE-ni-duh) is formed from three Greek roots that mean true (eu -ευ); eyeball or socket (glen-e -γλενε); and form (eidos -είδος).  The reference may be to the eyespot or to the anterior groove from which the flagellum or flagella emerge.



The euglenoids are common aquatic organisms that may be photosynthetic or not.  They include the familiar laboratory icon Euglena (Figure 1) along with other less well known, but important members of the phytoplankton and benthos.  They all have at least two flagella that have different orientations and structures.  The anteriorly-directed flagellum generally is supported by a paraxial rod that runs alongside the 9+2 microtubular array.  The anterior flagellum also has a light sensitive swelling at its base that, in conjunction with the shading by the lipid-filled vesicles called the eyespot, allow the cell to orient itself according to the direction of light.  If a second flagellum emerges from the reservoir, it usually is recurrent and much thinner.  The cells are not covered by a rigid cell wall; rather, they have a pellicle, a series of complex proteinaceous strips that can slide past each other in some taxa like Euglena and allow flexing of the cell in a process called metaboly.  Interpretations of this group, because it exhibits both "plant" and "animal" attributes helped to bring about the demise of the plant-animal dichotomy.

All modern treatments of the euglenoid taxonomy (Bold and Wynne 1985; Dodge 1973; Grell 1976; Kudo 1966; Lee 1980; Lee et al. 1985; Margulis and Schwartz 1988; Sze 1986; Van Den Hoek et al. 1995; Graham and Wilcox 2000; Walne and Kivic 1990; and Rosowski 2003) are very similar. As with the dinoflagellates, the protozoological manuals such as Kudo (1966), Lee et al. (1986), and Grell (1976) tend to lump all of the flagellated unicells into the same phylum. This is a modification of the taxonomy of Margulis and Schwartz (1988) and Walne and Kivic (1990). Margulis and Schwartz (1998) group the euglenoids together with the Schizomastigotes, the Kinetoplastids, and an enigmatic genus, Stephanopogon, in their phylum, Discomitochondria (Pr-12). The taxonomy of Cavalier-Smith (2003) is very similar in combining the kinetoplastids and euglenoids together.  Patterson (1999) groups the kinetoplastids together with them into a group that we interpret as a kingdom.  Baldauf (2003a) also groups the euglenoids with the kinetoplastids as well as the amoeboflagellates, and cellular slime molds.  Furthermore, she suggests a more distant relationship with the cryptomonads and haptomonads.

Details of the relationships of orders within the phylum are difficult to discern.  It appears that Distigmida (Eutreptiales, e.g. Distigma Figure 2) is paraphyletic (Mullner et al. 2001, Leander et al. 2001, and Leander 2008), and until the disposition of taxa within that group are finally ironed out, important characters like pellicular mobility (metaboly), number of emergent flagella, and photosynthesis cannot be designated derived or primitive.  Furthermore, the relationship of Euglenamorpha (Figure 3), an odd 3-flagellated cell that otherwise resembles Euglena but lives in the cloaca of frogs (Wenrich 1924), occupies a position of uncertain status (incertae sedis).

The systematics of this group is in disarray mainly because euglenoids have been treated  as two separate groups: the animal-like eugenoids and plant-like euglenoids.  The standard phylum-wide classification systems are certainly wrong.  The basal taxa, here in the order Sphenomonadales (e.g. Sphenomonas, Figure 4), are bacteriotrophs, but the order is paraphyletic (e.g. Leander 2008, Linton and Treimer 2001).  Furthermore, Astasia, long considered a colorless Euglena, likely is a member of the Eutreptiales-Rhabdomonadales group (Busse and Preisfeld 2003; and Mullner et al. 2001) or Rhabdomonadales alone (Marin et al. 2003).  The consistent message from the molecular/morphological cladistic analyses is that the photosynthetic groups are the most derived in the euglenoids (Leander 2008 and Leander et al. 2001; and Mullner et al. 2001), that the Heteronematales (particularly Peranema, Figure 5) is a sister-group to the photosynthetic euglenoids (Leander 2008, Leander et al. 2001, and Bregalia et al. 2007; Mullner et al. 2001), and that the bacteriotrophic taxa are the most primitive (Leander 2008,  Linton and Treimer 2001, and Leander et al. 2001).

The taxonomy of the euglenoids as given here as a place-holder only.  Rhabdomonadales (e.g. Rhabdomonas, Figure 6), Heteronematales (mostly), and Euglenales are monophyletic groups.  However, much work has yet to be done to work out order-level relationships that include the taxa of Sphenomonadales, and Eutreptiales.   Figure 7 is a simplification of Marin et al. (2003) in which Sphenomonadales is a sister to the rest of the euglenoids with nested taxa to Eutreptiales + Euglenales.





FIGURE 7. A cladogram of the Euglenozoa clade with orders of the Euglenida highlighted in the shaded box. The figure is an abstract of the analysis of Marin et al. (2003).  The results of this study show a series of nested taxa from bacteriotrophs to photosynthetic taxa.  Euglenamorphida was not included in the study.









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By Jack R. Holt.  Last revised: 02/17/2015