Further observations on the fine structure of marine collared flagellates (Choanoflagellata) from arctic Canada and west Greenland: species of Parvicorbicula and Pleurasiga
New observations amplifying and sometimes correcting existing descriptions have been compiled for Parvicorbicula socialis (Meunier) Deflandre, Parvicorbicula quadricostata Throndsen, Pleurasiga minima Throndsen, and P. reynoldsii Throndsen from arctic material prepared as whole mounts at the time of gathering. Introductory details on localities, techniques, terminology, etc. relevant to these and other organisms to be considered later are recorded here. Special attention has been paid in each case to previously unrecorded details of lorica construction and, where material has been abundant (P. socialis and P. quadricostata), also to the range of variability in wild populations which, in these two taxa, is shown to be greater at the hind end of the lorica than elsewhere. Some differences from the type descriptions, notably up to 50% larger lorica size in three species and other differences which are interpreted as seasonal in one species (P. socialis), are thought to be ecologically but not taxonomically significant. Other details such as the shapes of costal tips, the nature of the junctions, and the mode of suspension of the protoplast within the lorica, most fully explored in P. quadricostata, are thought to be of possible phyletic significance though full discussion is deferred pending availability of additional taxa. The only nomenclatural recommendation given here is to confer specific rank on Pleurasiga minima var. minuta Leadbeater (as P. minuta Leadbeater) to separate it from P. minima Throndsen itself, which is recorded electron microscopically here for the first time and is shown to be a clearly defined and different taxon. This work is continuing.