New insights into the morphology and phylogeny of Heterosigma akashiwo (Raphidophyceae), with the description of Heterosigma minor sp. nov.

Abstract Heterosigma akashiwo is one of the smallest raphidophytes and is known for its ability to form dense, fish-killing blooms. In many parts of the world, H. akashiwo is a permanent resident of the algal community, and blooms have caused extensive financial losses for the aquaculture industry. Different aspects of its biology have been the focus of numerous investigations over the last two decades but, perhaps because of its small size and fragile nature, few morphological studies have been carried out. We conducted phylogenetic and morphological studies of 24 strains of Heterosigma from the coastal waters of Europe, North America, Australia and New Zealand. Complete 18S, ITS region and the D1–D2 region of 28S rDNA were sequenced and compared with all available raphidophyte sequences, revealing a novel species of Heterosigma, Heterosigma minor sp. nov. All strains of H. akashiwo were genetically very similar across the rDNA region (> 99.8% similarity) and morphologically very similar in light microsc...