Guide to Actinocyclus

Frustules of Actinocyclus are centric in outline with a relatively narrow mantle. The valve face may be flat or may have concentric undulations in the valve surfaces. Internal openings of rimoportulae are often visible in LM. The genus is distinguished by the presence of a pseudonodulus, a structure found in the three genera Actinocyclus, Hemidiscus and Roperia. The pseudonodulus may only be visible through use of SEM. The areolae on the valve face may give the appearance of being organized into sectors.

Several freshwater species from the western U.S. are extinct and found only in lacustrine sediments. In extant populations, most species are found in marine habitats, primarily in estuaries. Brackish inland waters influenced by anthropogenic nutrients and salts often contain A. normanii, a species considered to be invasive. This species is found in the nutrient-rich habitats as plankton or in the sediment-water interface.