We present the first report and description of the pinnate diatom Haslea sp. from the northeastern Adriatic Sea, Croatia, producing a blue pigment. This organism is very similar to the well-known Haslea ostrearia, the first described “blue” diatom producing marennine, i.e. the pigment involved in the greening of oysters, and recently described H. provincialis. However, the Croatian diatom slightly differs from other Haslea species in its morphology and 18S rRNA sequence. The discovery of Haslea sp. from Croatia confirmed the possible existence of more species among the representatives of blue Haslea species, as previously assumed. The discovery of several genetically distinct populations of Haslea ostrearia, new species H. karadagensis, H. provincialis and Haslea sp. from Croatia, suggests that species richness in the group of “blue” diatoms is probably underestimated and still more new blue diatoms remain undiscovered. This also raises questions about previously published reports and observations of Haslea distribution in the Mediterranean Sea whether these organisms really belong to H. ostrearia.
