Thalassiosira laevis Gao et Cheng
This species has a circular valve and is distinctive in SEM by the areolar pattern. Most of the valve is smooth, lacking areolae, except for two rows of small areolae external to the fultoportulae, and one or two irregular rows radially interior to the fultoportulae (Fig. 1, SEM). The former are smaller and more numerous than the latter. There appear to be areolae near the valve center surrounding the single central fultoportula that do not connect with the outer surface (Fig. 2, detail of Fig. 1). A rimoportula with a short projecting exterior tube (Fig. 2, white arrow) is present in the marginal fultoportula ring, slight closer to one fultoportula (Fig. 2, black arrow).
The species was originally described from Xiamen, China, as 3-4.5µm in diameter with the fultoportulae 7-9 in 1µm, and recently found elsewhere in China’s coastal waters (Li et al. 2013). Seen only once in the IRL, this specimen is 5.5µm in diameter with 7 fultoportulae in 10µm; it has commonly been seen in Atlantic coastal samples (AKSK Prasad, personal communication, 23 Feb 2013). Gao and Cheng (1992) report that the areolae number 7-9 in 1µm, but as measured from their Figure 2, the number is ~5/µm for the larger interior areolae, and ~9/µm for the smaller areolae at the margin. The specimen shown here has ~4.5 larger interior areolae in 1µm and ~7 smaller areolae in 1µm.
Gao Y, Cheng Z. 1992. A new species and two new varieties of Thalassiosira. J Xiamen Univ (Natural Science) 31(3):291-294.
Li Y, Zho Q, Lü S. 2013. The genus Thalassiosira off the Guangdong coast, South China Sea. Bot Mar 56:83-110.