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Canadian coastal waters of the Pacific Ocean and the Atlantic Ocean as well as inland freshwater habitats in Ontario were sampled for centrohelid heliozoans (free-living heterotrophic single-celled organisms) over a period of nearly five decades. More than 60 species and subspecies were revealed, including 1 Triangulopteris, 1 Raphidocystis, 2 Pseudoraphidocystis, 3 Raineriophrys, 4 Pseudoraphidiophrys, 12 Choanocystis, 15 Pterocystis, and 25 Acanthocystis taxa. Of these, 22 were officially named and described as new to science based primarily on the morphology of the siliceous scales that cover the cell. New species were compared with images and/or descriptions of close "relatives" to validate their new species assignments. New data on five species of Acanthocystis and one species of Choanocystis required revisions of their descriptive taxonomy that in some cases resulted in the splitting off of separate species or subspecies. Very little can be concluded about global distribution of centrohelid heliozoans, owing to the paucity of records. For many of the taxa reported here, previous records consist of just one or two findings from other parts of the world. One example is Choanocystis antarctica Tikhonenkov and Mylnikov, 2011 that was previously known only from Antarctic seawater, but is reported here from an Ontario softwater lake. Key words: heliozoans, silica-scaled protists, taxonomy, new species, Haptista, Centroplasthelida, Introduction The centrohelid heliozoans comprise a relatively large group of over 100 free-living species with filose pseudopodia and endogenously produced siliceous scales that cover the cell. Where found, in a [...]

Light and scanning electron microscopy were used to describe Neidium petersiveri sp. nov., a previously unnamed diatom found in the Canadian waters of Lake Superior. It had been reported previously as single cells of an unidentified Neidium species from four widely separated regions in North America: Michipicoten Island (Lake Superior, Ontario, Canada), Cape Cod (Massachusetts, United States), a Quebec river (Quebec, Canada), and the Back River (Nunavut, Canada). There are superficial resemblances to Neidium mirum Krasske and Neidium calvum Østrup, but there are significant differences in frustule size and shape as well as the pattern of striae on the valve face that render N. petersiveri unique. There was not a significant correlation between frustule lengths and widths in N. petersiveri, owing to a relatively small range within the width data that resulted in much higher width-to-length ratios among shorter cells than for longer cells (74 cells measured). Other significant aspects of morphologic variation include various shapes and configurations of the lacinial fissures and of the proximal raphe fissures, and the sporadic occurrence of pit-like depressions in the axial and central regions of the valves, which suggest that variations in these characters likely have limited value as discriminating characters at the species or subspecies levels. Key words: Lake Superior, bi-raphid diatom, Neidium, frustules. La microscopie optique et la microscopie électronique à balayage ont été utilisées pour décrire Neidium petersiveri sp. nov., une diatomée jusqu'alors sans nom trouvée dans les eaux canadiennes du lac Supérieur. Elle avait été signalée précédemment comme cellules uniques d'une espèce de Neidium non identifiée provenant de quatre régions très éloignées en Amérique du Nord : l île Michipicoten (lac Supérieur, Ontario, Canada), Cape Cod (Massachusetts, Etats-Unis), une rivière du Québec (Québec, Canada) et la rivière Back (Nunavut, nord du Canada). Il existe des ressemblances superficielles avec Neidium mirum Krasske et Neidium calvum Østrup, mais il y a des différences significatives dans la taille et la forme des frustules ainsi que dans le motif des stries sur la face de la valve qui rendent N. petersiveri unique. Il n'y avait pas de corrélation significative entre les longueurs et les largeurs des frustules chez N. petersiveri, en raison d'une gamme relativement faible dans les données de largeur qui a entraîné des rapports largeur-à-longueur beaucoup plus élevés parmi les cellules courtes que pour les cellules plus longues (74 cellules mesurées). D'autres aspects significatifs de la variation morphologique comprennent les différentes formes et configurations des fissures laciniales et des fissures du raphé proximal, ainsi que l'apparition sporadique de dépressions en forme de puits dans les régions axiales et centrales des valves, ce qui suggère que les variations de ces caractères ont probablement une valeur limitée en tant que caractères de discrimination au niveau de l'espèce ou de la sous-espèce. [Traduit par la Rédaction] Mots-clés : lac Supérieur, diatomée biraphide, Neidium, frustules., Introduction Diatoms are microscopic, single-celled photoautotrophs and are an important component of aquatic food webs in both freshwater and marine ecosystems. Marine diatoms appear to have been responsible (during the [...]

Abstract: We examined long-term changes in phytoplankton composition from 1981 to 2003 in seven intensively studied lakes on the southern Canadian Shield in Ontario, Canada. Significant (P < 0.05), temporally [...]