Arcella excavata Cunningham, 1919
Diagnosis: The shell is colorless or yellowish in young specimens, transitioning to brown in older cells. It is oval or circular in apertural view and croissant-like in lateral view. The shell’s major axis (length) is typically perpendicular to its protuberances when the apertural view is not circular, though in some cases, it may align parallel to the protuberances. The apertural surface is deeply invaginated, forming two pronounced protuberances, with a circular or elliptical aperture located at the bottom of the invagination. This aperture is bordered by a small lip. The aboral hemispherical region of the shell is either smooth or marked by regular depressions. The shell wall comprises numerous alveoli made of a proteinaceous material, arranged in a single layer. The shell surface is either smooth or irregular, with numerous small pores. Specimens typically contain 3—4 nuclei, though some may have 5—6, each featuring a central nucleolus.
According to Cunningham (1919), the shell’s shape resembles “a quarter-section of cantaloupe, with the mouth situated in the cup.” The shell color ranges from brown to nearly black, with the following dimensions: Length 55 μm, width 50 μm, total depth 45 μm, depth of depression 25 μm, mouth 15 × 20 μm.
Dimensions: According to Cunningham 55 µm. Stepanek 63—67 µm. Todorov and Golemansky 60—70 µm. My measurements 44 µm (average).
Habitat: Freshwater, on aquatic plants; also reported from waste water (Todorov and Golemansky, 2003).
Geographical distribution: Rare. Small swamp near Durham, N. Carolina (USA) (Cunningham, 1919); Bulgaria ( Todorov and Golemansky, 2003); Czech Republic (Štěpánek 1953, 1954); Russia (Geltzer and Korganova, 1976, and Geltzer et al., 1985); Azerbaijan (Alekperov and Snegovaya (1999, 2000). I found this species once in considerable numbers in a sample from Crailoo, The Netherlands.
References: Todorov, M. and Golemansky, V., 2003. Morphology, Biometry and Ecology of Arcella excavata Cunningham, 1919 (Rhizopoda: Arcellinida). Acta Protozool. 42:105-111.
