TAXONOMY AND MORPHOLOGY OF THE MARINE PLANKTON DIATOMSTHALASSIONEMAANDTHALASSIOTHRIX

Publisher Website
Google Scholar
Abstract
The needle-shaped frustules of the diatoms Thalassionema Grunow and Thalassiothrix Cleve et Grunow are furnished with uniquely structured areolae which appear as a row of depressions along each edge of the valve. In Thalassionema the areolae open freely outwards and are surmounted by one to three simple silica branches. In Thalassiothrix the areolae are closed by a reticulate velum and in addition they can be divided into compartments by transverse bars (T. heteromorpha) or may carry superimposed marginal spines (T. longissima). Shape of the valve poles and the presence of marginal and/or apical spines, which have commonly been used to differentiate the species of these diatoms, show considerable variability. A scanning and transmission electron microscopic survey of world-wide sample collections (including cultures) has identified fine structure of marginal areolae as a more consistent diagnostic character. The following taxonomic revisions are proposed: Thalassiothrix frauenfeldii Grunow is transferred to the genus Thalassionema; Thalassiothrix pseudonitzschioides Schuette et Schräder is shown to be a form of Thalassionema nitzschioides Grunow; Thalassiothrix antarctica Schimper (ex Karsten), T. gibberula Hasle and T. lanceolata Hustedt are relegated to varieties of T. longissima Cleve et Grunow; T. vanhoeffenii Heiden is regarded as a synonym of T. acuta Karsten, and T. mediterránea Pavillard and T. delicatula Cupp are synonyms of T. heteromorpha Karsten. The six species now recognised (Thalassionema bacillaris, T. frauenfeldii, T. nitzschioides, Thalassiothrix acuta, T. heteromorpha, T. longissima) can be adequately distinguished in the light microscope by the density of their marginal areolae (‘transapical striae’). Amended generic diagnoses and a key to the genera and species of these diatoms are provided.