Acanthocystis tubata   Dürrschmidt, 1987

Diagnosis: Periplast, ca. 25 µm in diameter, with two types of siliceous scales. Details of protoplast unknown. Plate-scales ovoid or elliptical, 2.9-3.7 µm long and 2.2 µm wide, patternless, except for an axial thickening. Spine-scales, 6-11 µm long and 0.23 µm wide at the narrowest point. Shaft cylindrical, gradually expanding towards the distal end. Apex, 0.4 µm in dia­meter, covered by a thin siliceous layer. A circle of about 12 conical teeth surrounds the apical margin. Base of the shaft supported by a circular, centrally depressed base­plate (diameter 1.2 µm) with peripheral rim.

Ecology: Surface sample collected 1982 in a swamp near Maramaranui, North Island, New Zealand.

Remarks: Dürrschmidt: A. tubata specimens reveal in the light microscope a striking similarity to A. mimetica Penard (1904), in the gradually expanded spine scale shafts, their circular base-plates and the dimensions of cells and scales. The only feature which contradicts an identification with A. mimetica is the absence of a conspicuous horse­shoe-like rim reported by Penard from the surface of the plate-scales. Such a ‘rim’ appears sometimes during the LM observation of uneven structured plate-scales, especially when these are somewhat tilted. Those scales are present, for instance in A. rotundata Nicholls ssp. rotoairense and A. pantopodeoides Nicholls (syn. A. cuneiformis Dürrschmidt. Thus it is assumed to be only an optical effect. However, the plate-scales of A. tubata are flat. As no type material has been left by Penard, as inquiries at the following institutes revealed (British Museum, London; Museum D’Histoire Naturelle, Geneva; The Royal Microscopical Society, London; Museum National d’Histoire Naturelle, Paris) it will remain unclear on which specimens Penard based the diagnosis of A. mimetica.
A. tubata is easily recognized under the electron microscope by the shape of the spine-scales. No variation in scale structure has been observed in specimens from different localities.