Acanthocystis turfacea  Carter, 1863

Diagnosis: Periplast Ø 10-150 μm, average 60-90 μm. Radial spine-scales of two types: short and long, although a sharp separation between the two varieties is hardly possible as many transitions can be present. The length of the short spine-scales varies between 3.3 and 15 μm, the long spine-scales vary between 12 and 65 μm. The short spine-scales are clearly bifurcated with a relatively long tip: each tip bears two teeth. The longer spine-scales are also branched, but with shorter tips; each tip bears at least three sharp teeth. The length of the shaft appears to be inversely related to the length of the fork. Shaft straight and hollow, sitting centrally on a disc-shaped base. Basal disc 1.5-4.5 μm in diameter. Tangential plate-scales 1.9-5.5 x 3.7-12.0 μm, oval, without structure, only a slight axial thickening in the center. Zoochlorellae often present. Individuals with zoochlorellae are usually larger than those without symbiotic algae. Axopodia vigorously and clearly granulated. Nucleus highly eccentric and shape-changing. Cyst with a siliceous covering of irregular hollow beads, with protoplasm extending far from the skeleton and surrounded by a membran.

Ecology: Freshwater, from oligotrophic to eutrophic water types, especially in those rich in detritus. A. turfacea is by far the most frequently reported Acanthocystis species. Penard found this species at a depth of 30-40 m. in Lake Geneva and Leidy in the Rocky Mountains up to 3300 m. high. World wide distribution.

Remarks: This large species is usually easy to determine with the light microscope. Sometimes there are relatively many small bifurcated spine-scales. Specimens with many long spine-scales and few characteristic forked short spine-scales are light microscopically difficult to distinguish from A. penardi.