Hustedt (1957) erected two forms, Actinocyclus normanii fo. normanii and Actinocyclus normanii fo. subsalsa, based primarily on ecology and size. According to Hasle (1977) no substantial differences were observed between A. normanii fo. normanii and A. normanii fo. subsalsa aside from size and ecology so there appears to be no taxonomic reason to keep the forms separate; however she suggested for ecological perspectives the two forms may have some meaning and did not propose combining the taxa.
Stoermer et al. (1985) stated it is one of the few Great Lakes diatoms with its maximum abundance near 20 degrees C. Hohn (1969) identified this taxon as Coscinodiscus radiatus in Lake Erie and reported it was most abundant in August and September while it was absent in the colder months of the year. Its late summer pulse first appeared in 1964 in the western basin of Lake Erie. It appears to be common in eutrophic waters (Mills et al. 1993), Green Bay (Stoermer and Yang 1969), Saginaw Bay (Stoermer and Theriot 1983) but uncommon in the offshore waters of Lake Ontario (Stoermer et al. 1974).
Actinocyclus normanii fo. normanii appears to be more common in strongly eutrophic marine to brackish waters while A. normanii fo. subsalsa appears to be more of a fresh water to brackish form with its greatest concentrations in low Chloride waters (Hustedt 1957).
Coscinodiscus subsalsus Juhlin-Dannfelt 1882
Coscinodiscus subtilis var. rothii (Grun.) V.H. Type No. 533, 1855
Coscinodiscus subtilis var. fluviatilis Lemmermann 1898
Coscinodiscus rothii var. subsalsa (Juhl.-Dannf.) Hust. 1928
Coscinodiscus rothii fo. minor Grun. in Van Heurck 1885
Actinocyclusis characterized by the presence of a pseudonodulus. Hustedt (1957) recognized the pseudonodulus (= ocellus in his terminology). He recognized the pseudonodulus (=ocellus) on the larger specimens stating it is difficult to see the pseudonodulus (=ocellus) on smaller specimens. The SEM's of Hasle (1977) show the pseudonodulus at the transition zone between the valve face and the mantle.
Another characteristic is the prominent labiate processes (=rimoportulae) situated on the mantle. These structures can be seen in normal mounts.