Abstract
Oswaldella is the most speciose genus, and one of the most characteristic, of hydrozoans inhabiting the Antarctic benthic marine ecosystem. Its species have relatively many important taxonomic characters allowing for their identification. Some of them, however, are difficult to study properly with a compound microscope. With the aim of improving scientific knowledge concerning species of the genus, a SEM survey of species of Oswaldella was carried out to study key morphological characters. Fourteen out of the 27 known nominal species were considered. The study has revealed unknown important characters, such as the presence of nematothecae associated with the nematophores at the cauline apophyses, and has provided a better understanding of others, such as the shape of the nematotheca associated with the medial infrathecal nematophore. A final general discussion on some of the characters studied and on the bathymetrical and geographical distribution of all known species is also included. Finally, an annotated checklist including all known nominal species has also been assembled.