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Chaetoceros debilis is considered one of the most abundant and widespread diatoms in the coastal marine phytoplankton, and is often used in research studies ranging from ecophysiology and molecular biology to oceanography and aquaculture. To clarify the species delineation of C. debilis and explore the diversity among C. debilis sensu lato taxa, monoclonal strains were established from different geographical regions, including the Danish coast, close to the type locality of C. debilis, the Denmark Strait (between Iceland and Greenland) as well as the Taiwan Strait, East China Sea and Daya Bay, South China Sea. Vegetative cells and resting spores were examined using light and electron microscopy. The hypervariable D1–D3 region of the nuclear large subunit ribosomal gene and the small subunit ribosomal gene were sequenced to address phylogenetic relationships. In both SSU and LSU trees, the C. debilis sensu lato strains clustered in four distinct clades. Culture material from the type locality was, along with molecular data, used to delineate and emend the description of C. debilis. Based on molecular data and detailed morphological features, one of the clades originating from Chinese waters was described as C. galeatus sp. nov., characterized by curved and helical chains, elliptical valves, and smooth and helmet‐shaped primary valves of the resting spores. The remaining two clades, which also represent two novel taxa, C. cf. debilis‐1 and C. cf. debilis‐2, were not formally described as new species due to the insufficient information on their morphology. [ABSTRACT FROM AUTHOR]

Chaetoceros debilis is considered one of the most abundant and widespread diatoms in the coastal marine phytoplankton, and is often used in research studies ranging from ecophysiology and molecular biology to oceanography and aquaculture. To clarify the species delineation of C. debilis and explore the diversity among C. debilis sensu lato taxa, monoclonal strains were established from different geographical regions, including the Danish coast, close to the type locality of C. debilis, the Denmark Strait (between Iceland and Greenland) as well as the Taiwan Strait, East China Sea and Daya Bay, South China Sea. Vegetative cells and resting spores were examined using light and electron microscopy. The hypervariable D1–D3 region of the nuclear large subunit ribosomal gene and the small subunit ribosomal gene were sequenced to address phylogenetic relationships. In both SSU and LSU trees, the C. debilis sensu lato strains clustered in four distinct clades. Culture material from the type locality was, along with molecular data, used to delineate and emend the description of C. debilis. Based on molecular data and detailed morphological features, one of the clades originating from Chinese waters was described as C. galeatus sp. nov., characterized by curved and helical chains, elliptical valves, and smooth and helmet‐shaped primary valves of the resting spores. The remaining two clades, which also represent two novel taxa, C. cf. debilis‐1 and C. cf. debilis‐2, were not formally described as new species due to the insufficient information on their morphology. [ABSTRACT FROM AUTHOR]