Anisonema acinus

Cell outline is like a grain of barley. Cells are 21 to 40 microns long, 13 to 17 microns wide, flattened with a ventral groove, which diminishes posteriorly. There are about nine longitudinal pellicular grooves on each of the ventral and dorsal faces of the cell. In some cells, the grooves are fine and difficult to see but in a few cells they are deeper. There is debate as to whether this species has an ingestion organelle, it was described without such an organelle, in some cells no organelle is seen, but in other cells otherwise indistinguishable from mouthless cells, a mouth can be seen. The anterior flagellum is about 1.5 times cell length and beats freely from side to side. The cells may eat large particles of food, such as diatoms. The trailing posterior flagellum is about 1.7 to 3.2 times the length of the cell, is thicker than the anterior flagellum, lies in the ventral groove, and tapers posteriorly. The flagellar pocket is located in the left side of the cell and the nucleus is in the right side. The cells glide smoothly, but jerk backwards when changing direction.

Gliding euglenid, cell outline oval or ovate with a slight indentation on left anterior margin, flattened and rigid, 20 µm - 40 µm long and 11µm - 18 µm wide. Pellicle with about 10 longitudinal broadly spaced (spacing 2 µm) ridges on each face of the cell; they may be hard to see,. A granular nucleus is located in the right hand side near the middle of the cell. Two emergent flagella arising from a flagellar pocket on the left. The anterior flagellum beats actively with a sweeping motion, and is about one or one and a half times the length of cell. The posterior flagellum is twice or three times the length of the cell and the distal region is broad and forms a hook after leaving the flagellar pocket. This flagellum lies in a ventral depression and trails along the substrate as the cell moves. The gliding of cells may be interrupted by sudden stops and cells jerk backward and then moves in a new direction. No ingestion apparatus was visible but cells contain diatoms indicating the presence of an ingestion apparatus. A contractile vacuole is present on the left hand side and empties into the flagellar pocket. Excretion of undigested residues of food from the posterior region of the cell. Anisonema acinus was first described by Dujardin (1841) from freshwater sites in Europe. The genus Anisonema contains 31 species. Anisonema acinus is distinguished from Anisonema gaciale Larsen and Patterson, 1990 and Anisonema trepidum Larsen, 1987 because of size and lack of ‘frozen’ stages in movement. Anisonema prosgeobium Skuja, 1939 and Anisonema obliquum Roskin, 1931 have no surface groove but are otherwise similar to Anisonema acinus - the grooves may be very hard to see. Anisonema platysomum is similar in size but is described as having a mouth (Skuja, 1939). All four species may subsequently prove to be conspecific. Anisonema alpinum (Deflandre, 1925) is distinguished from Anisonema acinus because this species has a distinct flat cell shape, pellicle is smooth without ridges or grooves and the posterior flagellum is during swimming often directed straight anteriorly as well as the anterior flagellum of cell. Anisonema truncatum Stein, 1878 with a larger size (60 µm x 20 µm) and with an distinct ovate-acuminated posterior end of cell. Anisonema costatum Christen, 1962 with a size about 57 µm x 60 µm is distinguished because the surface of cell has well developed spiral ribs like species of the genus Gyropaigne (Christen, 1962).   The organisms described by Dujardin (1841), Playfair (1921) and Skuja (1939) as well as those from tropical marine sediments (Larsen and Patterson 1990) - are of a similar size. Those of Maskell (1887) are somewhat smaller, and those from the Wadden Sea are considerably larger (Larsen, 1987). Freshwater isolates have a contractile vacuole. Anisonema acinus is cosmopolitan