Shionodiscus gaarderae sp. nov. (Thalassiosirales, Thalassiosiraceae), a bloom-producing diatom from the southwestern Atlantic Ocean, and emendation of Shionodiscus bioculatus var. bioculatus

The recently established diatom genus Shionodiscus is characterized as possessing a labiate process on the valve face and strutted processes with long internal and reduced, or no, external extensions. Many Thalassiosira species with these features were transferred to this new genus, as well as some with short internal, strutted process extensions. Examination of samples from the Argentine continental shelf and slope waters, and from the Beagle Channel, revealed the presence of a small centric diatom that formed an extensive bloom in slope waters during spring 2005 and 2006. Analysis by light and electron microscopy revealed high similarity to specimens previously referred as Thalassiosira bioculata var. raripora. This name was never effectively published; nevertheless, the name has been adopted and observations referring to it have been reported on several occasions. Here we trace the usage of the (nomenclaturally invalid) name Thalassiosira bioculata var. raripora in the literature, and formally describe...