Meet Delphineis surirella (Ehrenberg) G.W. Andrews 1981, a cute little thing that lives attached to sand grains. The species is widespread in the North Pacific and didn't pose a lot of problems except for one: it was hard to figure out its size diminution series. Moving along their life cycle, diatoms become smaller and smaller after each mitotic division. During that process the proportions of cells are often change as well, not only the size.
There are two common species of Delphineis in the region and both look quite differently when they are large and when they are small. Naturally, the samples are full of the extremes, with no transitional forms. So which tiny version belongs to whom? Now I know! Delphineis surirella's second name in the counts is Delphineis small. It becomes small and round with no central area to speak of.
The most useful paper: Andrews, G.W. (1981). Reviosion of the diatoms genus Delphineis and morphology of Delphineis surirella. In: Proceedings of the 6th Symposium on Recent and Fossil Diatoms. Budapest, September 1-5, 1980. (Ross, R. Eds), pp. 81-92. Königstein: O. Koeltz, Science Publishers.
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