Recent studies suggest that a high and undescribed species diversity of Chaetoceros exist in tropical waters.Based on plankton samples from Chinese tropical waters,Daya Bay of the South China Sea,a new species,Chaetoceros dayaensis sp.nov.,is described.Vegetative cells and resting spores were examined by light microscopy as well as transmission and scanning electron microscopy.The nuclear molecular markers SSU rDNA,ITS rDNA and D1-D3 of LSU rDNA,and the chloroplast rbcL gene were sequenced for information on phylogenetic relationships.The species is characterized by the presence of two 90o rotations of the resting spore during maturation.First a rotation in the apical plane of the mother cell resulting in the valvar plane of the resting spore turning from parallel to perpendicular to that of the mother cell,and a second 90o rotation in the valvar plane of the mother cell resulting in the valve faces of the resting spore turning from facing narrow girdle view to broad girdle view in the mother cell.It is the first report of two 90o rotations of resting spores during maturation in Chaetoceros species.Based on this,the rotation during maturation of the resting spore of C.rotosporus was reevaluated.Chaetoceros dayaensis sp.nov.belongs to the subgenus Hyalochaete Gran,and is based on the analyses of the phylogenetic relationship combined with a morphology similar to C.affinis.And it is difficult to place C.dayaensis in the traditional morphologically-based sections used for grouping Chaetoceros species.