MORPHOLOGICAL STUDIES OF THALASSIOSIRA SPECIES WITH TWO RIMOPORTULAE FROM CHINESE COASTAL WATERS

Morphology and pattern of processes, including three types, fultoportula, rimoportula and occluded process, are key taxonomic characteristics for Thalassiosira. Rimoportula is a very common structure in all Thalassiosira taxa. Most Thalassiosira species only bear one rimoportula, and so far only 8 taxa have been reported to bear two rimoportulae from nearly 400 recorded Thalassiosira taxa. The quantity of rimoportulae has been considered as a different key morphological characteristic between the genus Thalassiosira and Coscinodiscus. Previous reports about Thalassiosira species with two rimoportulae from Chinese coast were rare, and almost no electron microscopy photos about the type of two rimoportulae are available. In order to characterize the species diversity and distribution of the genus Thalassiosira along Chinese coastal waters, some water samples were collected from several representative locations. Thalassiosira specimens within these samples, as well as several clonal strains isolated from natural samples, were studied by scanning electron microscopy. Four Thalassiosira species with two rimoportulae were observed and they were T. bipartita (Rattray) Hallegraeff, T. fragilis Fryxell, T. hendeyi Hasle & Fryxell and T. visurgis Hustedt. Three of them were newly recorded species for China, T. fragilis, T. hendeyi and T. visurgis. The detailed morphological description, habits and ecological distribution of these species were given in this paper. Furthermore, taxonomic comparison among resembling species were made. The transfer of T. bipartita from C. bipartita obscured the morphological difference between the genus Thalassiosira and Planktoniella, and the taxonomic estimation of mucilage membrane should be re-examined in the future. Some morphological variations of marginal rimoportulae and central fultoportulae in T. hendeyi were observed, and analogous variations in other Thalassiosira species were discussed. The taxonomic evaluations of rimoportulae, including their quantities and locations, should be re-analyzed in conjunction with the usage of other modern techniques.