INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

FLAGELLATES

PHYLUM CHOANOMONADA

CLASS CHOANOMONADEA

The Class Choanomonadea is represented by a single order, Choanoflagellida. About 200 species are known.

Structure. The choanoflagellate Cell is pear-shaped or oval (Fig. 5). A flagellum emerges from the apical region, surrounded by a collar composed of ultrafine cytoplasmic projections called microvilli. Inside each microvillus runs a bundle of microfilaments that maintain it in an extended state. Adjacent to the flagellar kinetosome lies a second, non-flagellar kinetosome, with both connected by a fibrillar bridge. The Nucleus is located in the anterior half of The Cell, while one or two contractile vacuoles and digestive vacuoles lie closer to the posterior end. Choanomonads feed on Bacteria.

Fig. 5. Salpingoeca amphoroideum in a lorica (from Hausmann):

1 - lorica; 2 - flagellum; 3 - microvillar collar; 4 - nucleus

Flagellar movements direct Water currents around the cell in such a way that food particles hit the outer surface of the collar and adhere to it. At the Base of the collar, a Tongue-shaped pseudopodium protrudes and grows toward the collar, engulfing the bacteria. It then closes to form a digestive vacuole, which is drawn into the cell (Fig. 6).

Fig. 6. Mechanism of food capture in choanomonads (from Zhukov and Karpov):

1 - food particle; 2 - pseudopodium

Reproduction in choanomonads is asexual, occurring via longitudinal Cell Division. In sessile forms, one of the daughter Cells becomes a dispersal stage; it swims freely before eventually settling on a substrate and attaching via a posterior cellular extension.

The majority of choanoflagellates inhabit planktonic Algae, aquatic macrophytes, crustaceans, hydra tentacles, and other aquatic organisms. Colonial species are either planktonic (free-swimming in the water Column) or sessile. Choanomonads are active bacterivores. During certain seasons, they multiply rapidly, reaching high population densities and thereby playing a significant ecological role in aquatic ecosystems.

Choanoflagellates bear a striking morphological resemblance to the choanocytes of Sponges, which makes them a subject of considerable scientific interest.