A vegetative body ( thallus ) is a multinucleated cell that is completely transformed into an organ of sexual or asexual reproduction ( holocapric thallus) or mycelium from branching unseptic hyphae ( eukarpic thallus).
The organs of asexual reproduction are zoosporangia, in which pear-shaped primary zoospores with 2 flagella at the anterior end are formed. Zoospores that have reached maturity emerge from zoosporangium, swim for some time in water (usually 5-10 minutes), then stop swimming and form a resting cyst : they are rounded, covered with a membrane and lose flagella (possibly pull them in). After a few hours, the cyst grows into the secondary zoospore , usually of a kidney-shaped form with two flagella on the side, one of which is short, pinnate, directed forward, the other longer, smooth, directed back. If primary and secondary zoospores are present in the life cycle, they are called dimorphic . Secondary zoospores actively move in the direction of a suitable substrate and, upon reaching it, sprout a vegetative hypha. Zoospores can leave zoosporangia through the apical opening or through a collapsing membrane or grow through the membrane bypassing the mobile phase (cysts and either secondary or primary zoospores are absent in such species - they are called monomorphic zoospores). Secondary zoospores can be monoplanetic or di- and polyplanetic - monoplanetic can only sprout a vegetative body, polyplanetic can encyst a second time, sometimes several times. The importance of dimorphism and di- and polyplanetism in the life cycle of saprolegniene has not been elucidated.
During sexual reproduction (sexual process - oogamy ), oogonia and anteridia are formed . Round ogonies contain one or several (up to 40) eggs , the formation of which is the entire content of the oogonium. Anteridial cells are processes of hyphae of various shapes: filiform, swollen or irregular, sometimes branching, adjacent to the oogonium one or several (up to 20). Fertilizing tubes are formed at the point of contact, through which the contents of the anteridia overflow into oogonium. Fertilized oocytes - oospores - contain a supply of nutrients and has a two-layer membrane.
Most often they are parasites of fish and other freshwater and marine animals, protozoa, algae and higher plants. On their tissues, saprolegnic lushly develop, forming zoosporangia, oogonia, and anteridia. Non-parasitic species - water and soil saprotrophs . The most famous genera are saprolegnia ( Saprolegnia ) and achlia ( Achlya ), which develop saprophytically on the corpses of insects or parasitize on fish eggs and fish, often causing their epizootics and death.
Saprolegnievye - article from the Great Soviet Encyclopedia .
