Udgivelsesdato: November 2005 ; Pseudo-nitzschia seriata f. obtusa (Hasle) Hasle is raised in rank to P. obtusa (Hasle) Hasle & Lundholm, based on morphological, phylogenetic and distributional features. The most prominent distinctive morphological feature is the shape of the valve ends, which in girdle view are truncate in P. obtusa and pointed in P. seriata. Other differences are the transapical width, the number of striated bands and the fine structure of bands and valve striae. Phylogenetically, P. obtusa is found in a clade comprising species lacking a larger central interspace and a central nodule but possessing simple hymenate poroids. Within that clade, P. obtusa is most closely related to P. seriata and P. australis, which make up the sister group to P. obtusa. Pseudo-nitzschia obtusa is recorded from the Arctic regions of the Atlantic and the Pacific Oceans, whereas P. seriata is recorded from the Arctic and Temperate regions but only from the Atlantic Ocean. The relationship between P. obtusa and other morphologically and phylogenetically similar species of the genus is discussed. In growth studies under replete nutrient conditions, P. obtusa had a growth rate of 0.55-0.66 day -1 and an upper pH limit for growth of 9.1. No P. obtusa clones have yet been shown to produce the toxin, domoic acid.