Morphological variability and genetic diversity in five species of Skeletonema (Bacillariophyta)

Twelve strains of Skeletonema from Chinese coastal waters and another strain from the West Sea of Korea were examined using light microscopy (LM), scanning and transmission electron microscopy (SEM, TEM), and small subunit (SSU) rDNA sequence analysis. We identified four species of Skeletonema: S. marinoi Sarno and Zingone, S. subsalsum (A.Cleve) Bethge, S. dohrnii Sarno and Kooistra, S. tropicum Cleve, as well as a Skeletonema-like species. These are the first records of S. marinoi, S. dohrnii and S. subsalsum in China. The Skeletonema-like species, MMDL5067, was isolated from the estuary of the Changjiang River. The morphologic and phylogenetic characteristics of this species do not correspond to those of any previously published Skeletonema species. The intercalary fultoportula processes (IFPPs) have long open tubes, and the intercalary rimoportula process (IRP) is short and located inside the rim of the IFPPs. Phylogenetic analysis revealed that MMDL5630 clustered with S. tropicum in the NJ and MP trees, but its IRPs differed from those of previously described specimens of S. tropicum. The IRP is partially open, with an elongated opening at the base of the tube, and is longer than the IFPPs. Some specimens of S. marinoi and S. dohrnii also have longer tubular IRPs than the IFPPs. Compared to previously described specimens of S. marinoi and S. dohrnii, these two specimens we identified have shorter IRPs. Our results reflect the ambiguity and diversity of Skeletonema taxonomy. They are also significant for the future identification and management of red tides caused by Skeletonema in the seas around China.