Studies of polycystine radiolarians (marine planktonic protists) have shown that great faunistic differences occur between the different Norwegian fjord systems. Plankton studies by Swanberg and Bjørklund in the early 1980′s identified more than 55 polycystine radiolarian species, including two unidentified forms of Actinomma noted as sp. A and sp. B. The discovery of fully developed adult individuals of the same taxa in plankton material from the Sogndalsfjord in 2010 revealed that the sp. A form in fact was a juvenile stage of the sp. B form. Its general morphologic skeletal structure differs so much from the other local Actinomma species that the whole concept of how to define, and what to include in, this genus has been taken up here for discussion.

This new species, described herein as Actinomma sogndalensis, has mainly been found in the Sognefjord, which leads us to conclude that A. sogndalensis might be an example of a real endemic fjord species. Due to the fragility of the latticed cortical shells, only the two thick primary medullary shells are preserved in the sediment. This led us to include a discussion regarding the number of concentric shells and their relevance or usefulness as a taxonomic criterion at the genus level in conjunction with this description of the new species. The genus Actinomma is emended to include species with two or multiple lattice spheres. Cromyechinus is by us still regarded as a junior synonym of Actinomma.