Coelomoron pusillum is a microscopic marine protist in the class Dinophyceae or a related group of planktonic microorganisms, recorded from temperate coastal and oceanic waters of the North Sea and adjacent Atlantic regions, including waters off Denmark, Norway, and Sweden. The genus Coelomoron comprises small, poorly studied flagellate organisms whose taxonomy and life history remain incompletely understood, partly due to difficulties in culturing and consistently identifying these diminutive cells in preserved plankton samples. Planktonic microorganisms of this type occupy an important position in marine food webs, contributing to the microbial loop that recycles dissolved organic matter and transfers energy to zooplankton grazers. Their distribution is largely governed by water temperature, salinity gradients, and seasonal productivity cycles characteristic of boreal and sub-arctic shelf seas. As with the majority of marine microbial species, Coelomoron pusillum has not been formally evaluated by the IUCN, reflecting both the practical difficulty of assessing conservation status for planktonic organisms and the general assumption that cosmopolitan or widely distributed microbes are not at immediate extinction risk. However, changes in ocean temperature and chemistry associated with climate change and ocean acidification may affect the species composition and geographic ranges of planktonic communities over the coming decades.