XVI International Botanical Congess

Identification of the domoic acid-producing diatom Pseudo-nitzschia has been based on field observations of colonies forming stepped chains, cell morphometrics, and valve ultrastructure. Which, if any, characters are reliable? Monitoring the morphology of 23 isolates (6 months at 10, 15, 20oC) and their size reduction with time (2 years), indicated the conservative nature of the morphological characters in use. The cultures studied (P. australis, P. delicatissima, P. fraudulenta, P. multiseries, P. pungens, P. pungens var. cingulata, and P. pseudodelicatissima) represented different geographical areas (USA--Gulf of Mexico, E and W coasts, Japan, and China). Significant changes, such as modification of valve outline and number of striae, were observed when the cells reached ca. 50% of their original lengths. The fine structure of the striae (shape, arrangement, and number of poroids) provides key characters in species identification.