Comparative Morphologic and Taxonomic Studies in the Genus Thraustochytrium

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Abstract
The genus Thraustochytrium consists of 10 spp. Characters used to separate these taxa include: zoospore and zoosporangial size; nature of the rhizoids; proliferation characteristics; discharge methods; pigmentation of thalli in mass; and wall thickness of reproductive bodies. Although these taxa appear distinct when various aspects of the above characters are used, comparative morphological studies on a Thraustochytrium roseum-T. visurgense complex and a T. motivum-T. aureum complex point up the difficulty in using many of the characters which presumably separate the thraustochytriaceous taxa. The difficulty arises because the characters are susceptible to variation, the characters tend to overlap, and the characters of the isolates of this study, in conjunction with the characters of the already established taxa form a character gradient or cline. More specifically, pigmentation, sporangial discharge characteristics, zoospore behavior, rhizoidal nature, and wall thickness, characters used to separate Thraustochytrium roseum and T. visurgense, are either highly variable or clinal so as to suggest that the 2 spp. are in fact but a single taxon, T. roseum. Also some of the characters used to distinguish T. aureum and T. motivum are so variable to clinal that their value is questionable.