Acanthocystis turfacea Carter, 1863
1863 Acanthocystis turfacea Carter, Ann. Nat. Hist., 3:12: 263.
Diagnosis: Periplast Ø 10-150 μm, average 60-90 μm. Radial scales of two kinds: short and long, although a sharp separation between the two varieties is hardly possible, since many transitions may be present. The length of the short spines varies between 3.3 and 15 μm, the long spines vary between 12 and 65 μm. The short spines are clearly bifurcated with a relatively long tip: each tip carries two teeth. The longer spines are also branched, but with shorter tips; each tip carries 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 scales 1.9-5.5 x 3.7-12.0 μm, oval, without structure, only in the center a slight axial thickening. Zoochlorella often present. Individuals with zoochlorella usually larger than those without symbiotic algae. Axopodia vigorously and distinctly granulated. Nucleus strongly eccentric and shape-changing. Cyst with a siliceous covering of irregular hollow pearls, with protoplasm protruding far from the skeleton, and surrounded by a membran.
Ecology: Fresh water, 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 spines. Specimens with many long spines and few characteristic forked short spines are hard to distinguish from A. penardi light microscopically.