Karenia selliformis is a species from the genus Karenia, which are dinoflagellates. It was first discovered in New Zealand. Karenia selliformis produces the highly toxic gymnodimine, and as such is a potentially harmful ocean dweller. Gymnodimine is a nicotinic acetylcholine receptor-blocking phycotoxin, a source of shellfish poisoning.
Common to the genus Karenia, this species shares morphological characters such as a smooth theca and a linear apical groove on its apex. At the same time, this species can be distinguished from its cogenerates on the basis of morphological characteristics within its , including the location and shape of its nucleus; the excavation of its ; the characteristics of its apical and sulcal groove extensions on the epitheca; the shape of its cells, as well as their size and symmetry; the degree of dorsoventral compression; and the presence of an apical .
Species that present said dorsoventral compression are shown to swim in a distinctive fluttering motion.
Molecular phylogenetic analyses of rDNA indicates Karenia selliformis, together with K. papilionacea and K. bicuneiformis, is closely related to K. mikimotoi and K. brevis.