According to the literature, C. curvatulus is smaller than A. curvatulus and has no pseudonodulus, except for C. curvatulus var. subocellatus, which has been transferred to Actinocyclus (Rattray, 1890b). Grunow (1884) recorded C. curvatulus var. inermis, var. genuitta, var. karianus, and var. minor from the Kara Sea and Franz Josef Land. Clonal cultures isolated from ice in the Barents Sea showed the whole range of variation indicated by these varieties within one clone, and all clones had a more or less obscure pseudonodulus (E. Syvertsen, personal observations). Our proposal is therefore that C. curvatulus should be put into synonomy with A. curvatulus as a species with a much wider range of size of cells and areolae than is evident from the current literature. Other diatomists would regard A. curvatulus as defined here as more than one single species (Fryxell, 1990).
Actinocyclus exiguus G. Fryxell & Semina (Table 25) Reference: Fryxell & Semina, 1981. Small species. Areolae in irregular rows, sometimes with a tendency toward fasciculation. Valve usually heavily silicified and the labiate processes not easily seen with LM. Pseudonodulus obscure with irregular outline as in A. curvatulus.
A. exiguus—southern cold water region.
A. curvatulus—cosmopolitan (it should be noted that our circumscription of A. curvatulus includes diatoms common on Arctic sea ice).
C. Areola rows parallel to central and/or edge row.
Actinocyclus spiritus T. P. Watkins in Watkins &c Fryxell (Table 26) Reference: Watkins & Fryxell, 1986, p. 302, Figs. 17-23.
Similar to A. actinochilus but differs by the pseudonodulus being clearly visible with LM, by the lack of a hyaline band between valve areola rows
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