Abstract:Because of the variation of some key morphological characteristics and the subjective influences from researchers,the mis-identification has occurred commonly to Chaetoceros brevis.Meanwhile,there are still some debate on the species sharing very similar morphology with C.brevis,for example whether they could be ranked as a separated species or just a variety as C.brevis var.pseudobrevis.To solve the above questions,several monoclonal stains were established,their morphology and systematic were studied based on the combination of continuously morphological observation in the life cycle and molecular data from some target genes.One new morphological characteristic was found in C.brevis,i.e.,the cap-like structure on the top of spins on the surface of resting spore,which has never been reported in other Chaetoceros species,and could be a unique feature to identify C.brevis.A newly recorded diatom species for China was observed,C.pseudobrevis.Combine the morphology and molecular data,it was shown that C.pseudobrevis should be a separate taxa,not a variety of C.brevis.In the phylogenetic trees,the taxa in section Laciniosa clustered in the same clade,which shown the monophygeny and matching with the morphological grouping.But so far it is difficult to confirm the common-sharing features for this section.