INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

FLAGELLATES

PHYLUM LABYRINTHOMORPHA

Labyrinthomorphs are colonial marine Protozoans, parasitic on marine Algae and Mollusks (oysters). About 30 species are known.

They have a complex life cycle comprising the following phases: a colony-plasmodium consisting of an extracellular ectoplasmic network of tubes with spindle-shaped and amoeboid (amoeba-like) Cells inside; cysts; sporocysts; sporangia; biflagellated zoospores. The zoospore is the only free-swimming phase of The life cycle. The Cell/35.html">Mitochondria within the cells feature tubular cristae. The spindle-shaped cells contain a unique organelle, the sagenogenetosome, which is responsible for forming the extracellular ectoplasmic network. This phylum includes a single Class, Labyrinthomorphea.

Structure. The vegetative phase of the labyrinthomorph life cycle is a colony resembling a reticular plasmodium made of thin, branching thread-like tubes enclosing spindle-shaped cells (Fig. 17). The tubes are ectoplasmic outgrowths of the spindle-shaped cells that form the extracellular ectoplasmic network, which constitutes the body of the plasmodium. The tubes are covered with a Plasmalemma bearing scales On the surface; the ectoplasm filling them lacks cellular Organelles. The spindle-shaped cells, 10-30 µm in length, possess a single nucleus and are covered by a plasmalemma beneath which lies a specialized organelle of complex ultrastructure known as the sagenogenetosome, whose function is to form the extracellular ectoplasmic network. Spindle-shaped cells lack locomotory organelles, yet they glide within the tubes. Phagocytosis and pinocytosis have not been observed in them; they presumably feed across their entire body surface (saprophytically).

Fig. 17. Diagram of The structure of Labyrinthula caenocystis (from Sukhanova):

1 - cell membrane; 2, 3 - inner and outer membrane of the tube, respectively;

4 - sagenogenetosome; 5 - spindle-shaped cell

In the colonies of many species, alongside spindle-shaped cells, there are amoeboid cells with thin or lobose pseudopodia which, unlike the spindle-shaped cells, phagocytose Bacteria and other small organisms. Amoeboid cells represent the life cycle stage that initiates The Development of the colony. They encyst and, in the cyst state, transform into spindle-shaped cells that form the ectoplasmic network.

Life cycle. Both spindle-shaped and amoeboid cells reproduce by binary fission, which drives colony growth; this process forms long tubes that may break apart into separate fragments, giving rise to new colonies. During colony growth, expansions containing clusters of spindle-shaped cells develop inside the plasmodium tubes, eventually forming sporocysts. Within each of these, several sporangia develop, where Cell Division produces very small (3-5 µm) biflagellated zoospores. Zoospores are asexual cells, yet Meiosis is observed during their formation within the sporangium, indicating the presence of a cryptic sexual process that remains unstudied. Zoospores emerge through ruptures in the sporangium wall, actively swim in the Water, settle on plants or mollusks, and transform into spindle-shaped or amoeboid cells from which a new colony originates.

Colonizing algae or oysters, labyrinthomorph colonies destroy their Tissues, leading to their death. A particularly dangerous oyster parasite is Labyrinthomyxa marina, which can cause mass mortality of mollusks in mariculture.