830 AQUATIC PHYCOMYCETES 



Known only on marine algae. 



The establishment and development of the thallus are of the Chytrid- 

 ium type (see p. 47). 



Observations on zoospores of Thraustochytrium proliferum have re- 

 vealed that approximately three hours after liberation, the spores in the 

 freed clumps separate from one another. Individual movement of the 

 spore body is initiated, and it loses its angularity and becomes more 

 ellipsoidal. After a few preliminary twists it then leaves the group as a 

 free-swimming body. The motile spore is somewhat gibbose-pyriform, 

 with a small refractive anterior granule and a central vacuole. Movement 

 is slow and even rather than hopping and chytridiaceous.. No evidence 

 of encystment of the zoospore during the period of quiescence succeed- 

 ing discharge can be found. 



A peculiarity of Thraustochytrium proliferum is its method of internal 

 proliferation. After cleavage of the zoospores (Fig. 60 E, p. 806) there 

 can generally be seen in the sporangium a larger basal protoplasmic 

 unit. This persists after spore liberation, enlarges, and becomes the new 

 secondary zoosporangium (Fig. 60 D). Thus, in contrast to the prolifer- 

 ating sporangia of Saprolegnia, Phytophthora, and so on, the rudiment 

 of the secondary sporangium of Thraustochytrium appears to be delim- 

 ited at the time of zoospore cleavage in the primary body rather than 

 after discharge of the spores (Fig. 60 E-F). Furthermore, it is formed 

 as part of the contents of the primary sporangium, not as an outgrowth 

 from the base. The subsequent enlargement of the secondary sporan- 

 gium undoubtedly occurs as a result of materials received from the 

 vegetative system. 



In Thraustochytrium globosum Kobayasi and Ookubo (1953) the zoo- 

 spores become completely mature within the sporangium and move 

 about actively inside prior to liberation. Once the wall breaks, the 

 spores remain together for a few minutes before swimming away. 

 After a swarming period of about five hours, they come to rest on the 

 algal wall and germinate. No internal proliferation of the sporangium 

 occurs in this species. 



Watson (in Vishniac, 1955b) has studied certain physiological aspects 

 of Thraustochytrium proliferum in pure culture. 1 



1 See also Adair and Vishniac (1958). 



