Clypeolina marginata Penard, 1902
Diagnosis: Shell flexible, circular to elliptical, strongly compressed, and lenticular in cross-section, consisting of a double envelope. The outer envelope is composed of two opposed chitinoid valves covered with small scales and particles, usually with a keel. The inner envelope is a membranous sac closely investing the cell. Aperture flexible, oval. Filopodia long, thin, and often branching.
Dimensions: Penard (1902) reported a length of 80–140 µm, while De Groot (1979) measured specimens between 110–115 µm. My own measurements range from 30–145 µm.
Habitat: Freshwater. Found in sediment or floating debris of oligotrophic to eutrophic waters. Observed feeding on green and blue algae and diatoms.
Geographical distribution: Switzerland (Penard, 1902), Hungary (Török, pers. comm.), China, and the Netherlands (Siemensma, pers. observations.). This species was recorded in the Netherlands earlier by De Groot (1979).
In the Netherlands, I found this rare and remarkable species in floating sapropel in a ditch of the Naardermeer nature reserve (2011, 2015, 2024, 2025), together with Paramphitrema lemanense and Hyalosphenia punctata. In 2012, I found this species in a shallow forest ditch in Gaasterland (GPS: 52.88529, 5.56147), in a shallow ditch in Den Treek (Amersfoort), and between basalt blocks in the large eutrophic lake Gooimeer. Additionally, I found it in a sample from China (2015).
Remarks: Clypeolina marginata is a rare species but is very easily overlooked due to its resemblance to a piece of detritus. Small specimens are difficult to detect among detritus. They can be confused with Pseudodifflugia and Diaphoropodon mobile. Its taxonomic status is uncertain.
