INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

FLAGELLATES

PHYLUM EUGLENOZOA

CLASS EUGLENOIDEA

Euglenoids are widespread in nature. Both autotrophic and heterotrophic species inhabit freshwater bodies. Heterotrophic forms predominate in the coastal benthic zone of the World Ocean down to depths of 200 m (the shelf zone). Over a thousand species are known.

Morphology. Euglenoids are characterized by an elongated body containing a single nucleus and two flagella of unequal length: one is short, barely noticeable, and occasionally entirely reduced; the other is long, quite thick, and visible even under a Light Microscope (Fig. 7). As a rule, the flagella emerge from a depression at the anterior end of The Cell known as the flagellar pocket. Near its base, the long flagellum bears a basal Swelling—the paraflagellar body—which is associated with the "eye spot", or stigma, an aggregation of granules containing red carotenoid pigments (Fig. 8). It is hypothesized that the paraflagellar body acts as a photoreceptor (detecting light stimuli), while the stigma serves as a reflector for it.

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Fig. 7. Diagram of euglenoid Structure (after Hausmann):

a - Euglena gracilis; b - Phacus:

1 - long flagellum; 2 - flagellar pocket; 3 - short flagellum;

4 - contractile vacuole; 5 - kinetosomes; 6 - nucleus; 7 - paramylon grains;

8 - chromatophore; 9 - stigma; 10 - paraflagellar body

Fig. 8. Diagram of the Fine Structure of the euglenoid flagellar apparatus (after Karpov):

1 - locomotory flagellum; 2 - mastigonemes; 3 - paraxial rod; 4 - short flagellum; 5 - flagellar pocket;

6 - kinetosome; 7 - microtubule bands; 8 - paraflagellar body; 9 - stigma; 10 - cytostome;

11 - intermediate microtubular ROOT

Beneath The Plasma Membrane lies an elastic pellicle composed of protein strips arranged in overlapping spiral bands. Supported by microtubules, these strips can slide past one another, producing a distinctive movement known as euglenoid movement (metaboly) and enabling the cell to slightly alter its body shape. Some species possess a rigid pellicle and a fixed body shape (e.g., Phacus, Fig. 7, b).

Numerous Mitochondria with lamellar cristae are distributed throughout the Cytoplasm. Species inhabiting anaerobic environments lack mitochondria.

One-third of euglenoid species are autotrophs bearing Plastids—chromatophores similar to the METABOLISM/14.html">Chloroplasts of green Algae (Chlorophyta)—with chlorophylls a and b among their pigment composition. Chromatophores vary in shape and size, ranging from a few large plates to numerous small discs. The end product of Photosynthesis is paramylon, grains of which accumulate in the cytoplasm.

Many euglenoid species are mixotrophs. A striking example is Euglena viridis: in the light, it Functions as a typical autotroph, whereas in the dark, it loses chlorophyll and feeds heterotrophically on dissolved organic matter in the Water via pinocytosis, which takes place in the region of the flagellar pocket containing a rudimentary cytostome that functions as a pinocytotic channel.

Other euglenoids have secondarily lost their chromatophores and shifted to heterotrophic feeding via phagocytosis and pinocytosis. Many predatory forms (such as Peranema) have evolved a complex food-capturing apparatus—the cytostome—whose walls are reinforced with pharyngeal rods made of microtubules connected to fibrillar structures, enabling them to engulf relatively large Protozoans, rotifers, and other organisms.

A contractile vacuole with a reservoir is located at the anterior end of the body; its contents empty into the flagellar pocket and subsequently to the exterior.

Reproduction. In euglenids, only asexual reproduction by longitudinal Cell Division is known.

Numerous species of the genus Euglena inhabit freshwater plankton: Euglena viridis is found in small, organic-rich water bodies, while Euglena sanguinea, which possesses the red pigment astaxanthin In addition to chlorophyll, inhabits clean waters. Colorless heterotrophic euglenids are widespread in benthic shallows of seas, and freshwater species also exist (Peranema). Some species have transitioned to a parasitic lifestyle on rotifers, crustaceans, frogs, and fish gills.

Euglenids serve as food for carp fry and play a role in the biological purification of water bodies, as they can absorb dissolved organic matter and enrich the water with oxygen released during photosynthesis. During mass multiplication, some euglenid species, along with green algae, cause the green "blooming" of fresh water.