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Actinocyclus normanii Ehrenberg(1837)

Ber. Berkanntm. Verh. Koenigl. Preuss. Akad. Wiss. Berlin, 2: 61.

valve view brightfield A. normanii, marginal focus

valve view brightfield A. normanii, rimoportule focus

valve view brightfield A. normanii, central focus

valve view brightfield A. normanii, marginal focus

valve view brightfield A. normanii, central focus

Frustules or valves are usually seen in valve view. Valves are centric, flat or domed, with central and marginal areas composed of the same pattern. The valve mantle is shallow and appears striate. A pseudonodulus is visible at the valve margin in LM with careful focussing. There are no spines and no central strutted processes. Valve face areolae are large, equally spaced and distinctly hexagonal in shape. They occur in straight radial rows of variable length. The framework of silica is foam-like or bubbly (known as bullulate).

If circular, diameter 25-110 Ám 
If circular, mantle depth 4.5-6 Ám 
If striae resolvable, density 10-13 /10Ám 
Spines Absent True
Striae, nature (centrics) Straight True
If circular, central area relative to marginal zone Not differentiated True
If circular, striae on valve face Extend almost to valve centre True
If circular, central ornamentation Regularly punctate True
If circular, extent of valve face areolae Continue over valve mantle True
Rimoportulae Present True
Rimoportulae, position Around valve margin True
Fultoportulae, marginal Present True
Fultoportulae, valve face Absent True


Colonies Solitary cells True
Attachment Not attached (floating or freely motile) True

True Rarely True Unknown
Only by Misinterpretation

Taxonomic Comment
A. normanii has been recorded under various names (e.g. Coscinodiscus normanii) during the last century and caused much confusion. It is basically a marine species which has become established in freshwaters, probably due to increasing eutrophication (Round et al., 1990).

Ecological Information
Actinocyclus normanii and its variety subsalsus are planktonic, freshwater taxa and bloom typically in late summer and autumn.

Comments to: Diatom Key Development Team