Euglenid

Euglenids or euglenoids are one of the best-known groups of eukaryotic flagellates: single-celled organisms with flagella, or whip-like tails.

Euglenids are commonly found in fresh water, especially when it is rich in organic materials, but they have a few marine and endosymbiotic members.

The plastids (membranous organelles) in all extant photosynthetic species result from secondary endosymbiosis between a euglenid and a green alga.

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.

[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.

In contrast, all flexible euglenids belong to a monophyletic group known as Spirocuta, which includes Euglenophyceae, Aphagea and various phagotrophs (Peranemidae, Anisonemidae and Neometanemidae).

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.

[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.

First, the basal bodies and flagella replicate, then the cytostome and microtubules (the feeding apparatus), and finally the nucleus and remaining cytoskeleton.

Euglenid Body Plan
  1. Dorsal flagellum
  2. Axoneme
  3. Paraflagellar rod
  4. Mastigonemes , "hairs" attached to flagellum
  5. Flagellar pocket vestibulum
  6. Feeding apparatus
  7. Paraxial swelling
  8. Eyespot , photoreceptor used to sense light direction and intensity
  9. Contractile vacuole , regulates the quantity of water inside a cell
  10. Ventral flagellum
  11. Ventral root
  12. Golgi apparatus ; modifies proteins and sends them out of the cell
  13. Endoplasmic reticulum , the transport network for molecules going to specific parts of the cell
  14. Phagosome
  15. Lysosome , holds enzymes
  16. Nucleus
  17. Nucleolus
  18. Plastid membranes (3, secondary)
  19. Thylakoids , site of the light-dependent reactions of photosynthesis
  20. Pyrenoid , center of carbon fixation
  21. Paramylon granules
  22. Pellicular strip
  23. Muciferous body
  24. Mitochondrion , creates ATP (energy) for the cell (discoid cristae )
Examples of euglenid diversity.
1—2. Ascoglena sp. (Euglenales);
3–4. Cryptoglena sp. (idem);
5–9, 14–15, 24–25, 27–29. Trachelomonas spp. (id.);
10. Eutreptia sp. (Eutreptiales);
11, 20. Astasia spp. (Euglenales);
12. Distigma sp. (Eutreptiales);
13. Menoid[i]um sp. (Rhabdomonadales);
16–18. Colacium sp. (Euglenales);
19, 26. Petalomonas spp. (Sphenomonadales);
21. Sphenomonas sp. (id.);
22–23. Euglenopsis sp. (Euglenales);
30. Peranema sp. (Heteronematales)