Genus Difflugia Leclerc, 1815

Testate amoeba with an agglutinated shell, featuring a terminal aperture that is round, oval, lobed, or toothed—but never slit-like. The aperture may sometimes have a collar or necklace-like structure but never contains an internal diaphragm. The test (or shell) is composed of mineral particles, such as quartz fragments or diatom frustules, collectively termed xenosomes, which are embedded in a structured or sheet-like organic cement.
All known Difflugia species obtain their xenosomes from the surrounding environment. Many species selectively arrange these particles by size and shape to construct a shell with a morphology characteristic of their particular species.
The nucleus is typically ovular, though in larger species, it may be vesicular. Some species are multinucleated. Certain larger freshwater species inhabiting deeper waters harbor green endosymbionts (zoochlorellae).’

Type species: D. proteiformis Lamarck, 1816, but its nature is questionable (Ogden and Ellison, 1988).

The drawing on the right shows the main characteristics of a Difflugia, in this case of D. capreolata. Its pseudopodia are finger-shaped and therefore called lobopodia. They can have different shapes:  
a stretching pseudopodium, b retracting,
c branching and
d broad so-called lamellipodium.

Remarks: Difflugia is the oldest and most species-rich genus of testate amoebae, comprising over 300 species along with numerous subspecies and varieties. Due to this high diversity, it has been termed an “overcrowded genus” (Meisterfeld). However, many descriptions are inadequate, relying on artificial criteria, limited specimens, or minor variations in size, shape, color, and shell composition—often without accounting for natural intraspecific variability. Additionally, the opacity of the test often obscures cytoplasmic features such as nuclear morphology, making these characters rarely useful for identification.

Species-level identification remains extremely challenging, not only due to ambiguous distinguishing characters but also because of uncertainty over which traits are truly diagnostic. Even slight deviations in shell morphology have frequently led to the establishment of new species or forms, often without considering the full range of variability within Difflugia taxa (Mazei & Warren, 2012).
Historically, Gauthier-Lièvre & Thomas (1958) categorized the genus into ten morphological groups based on shell shape: lobed, collared, compressed, urceolate, globose, ovoid-globose, elongate, acute-angled, horned, and pyriform. More recently, Mazei & Warren (2012, 2014) reclassified species into species complexes, primarily using two key collections: Penard’s microscope slides and Ogden’s SEM micrographs, both housed at the Natural History Museum in London.

Ecological conditions: Shell morphology is strongly influenced by environmental conditions. Bobrov et al. (1999) documented a clear reduction in shell size across three species groups along a gradient from wet to dry habitats. Similarly, within Euglypha and Placocista, spined forms predominated in wetter environments, while spineless or short-spined variants were more common in drier habitats. The authors emphasized that ecological interpretations require identifications at the finest possible taxonomic level.
Difflugia species occupy highly diverse habitats. Many are benthic, found in freshwater sediments or among aquatic vegetation, while others are planktonic with seasonal benthic phases (e.g., winter dormancy). Some inhabit terrestrial environments, such as dry mosses and soils. Lakes of varying trophic status (eutrophic, dystrophic, or oligotrophic) often host distinct dominant Difflugia species.

Feeding:
Primarily algivorous and fungivorous, though smaller species (e.g., D. minuta, D. pulex) also consume bacteria.