Genus Difflugia Leclerc, 1815
Diagnosis: Testate amoeba with an agglutinate shell, with a terminal aperture that is round, oval, lobed or toothed (but never slit-like), sometimes with a collar or necklace but never with an internal diaphragm. The shell is composed of mineral particles or diatom frustules, collectively called xenosomes, that are assembled on structured or sheet-like organic cement. All species of Difflugia acquire their xenosomes from their environment. Many select and arrange these xenosomes according to their size and shape in order to construct a shell with a morphology that is unique to that particular species. The nucleus is usually ovular, but in some species it is vesicular. Several freshwater species have green endosymbionts.
Type species: D. proteiformis Lamarck, 1816, but its nature is questionable (Ogden and Ellison, 1988).
Remarks: Difflugia is the oldest and most species-rich genus of testate amoebae, with more than 300 species and a lot of subspecies and varieties. Therefore it is called an overcrowded genus (Meisterfeld). Many descriptions however are inadequate, based on artificial criteria, very small variations in size, shape, color and shell material or the lack of data on natural variation of shell characters. As the test is often opaque, cytoplasmic characters as shape and structure of the nucleus are rarely used. Identification at species level is for most species extremely difficult. The problem is not only which characters are distinguishable, but also which are diagnostic? Many times even slight deviations in shell shape have resulted in the establishment of new forms or species, regardless of the range of variability that individual Difflugia taxa may exhibit (Mazei and Warren, 2012).
Mazei and Warren (2012, 2014) have classified species in species complexes, primarily based on two collections: Penard's mounted microscope slides and Ogden's SEM micrographs, both in the Natural History Museum in London.
Ecological conditions play an important role in shell morphology. Bobrov et al. (1999) observed a clear decrease of shell size moving from wet to dry conditions in three different species groups. And spined forms within the genera Euglypha and Placocista consistently occurred in wetter habitats whereas spineless forms, or those with short spines, were more prevalent in drier habitats. They concluded that, in order to maximize the ecological indicator value of the assemblages recorded, identifications should be made at the lowest taxonomic rank possible within these groups.
Gauthier-Lièvre and Thomas (1958) divided the genus into ten groups based on the morphology of the shell: lobed, collared, compressed, urceolate, globose, ovoid-globose, elongate, acute angled, horned and pyriform.
Difflugia species inhabit very diverse habitats. Many species are common in freshwater sediments or between water plants; others are planktonic with a benthic phase during winter; some live in dry mosses and soil. Lakes of different trophic status (eutrophic, dystrophic or oligotrophic) have characteristic dominant Difflugia species.
Feeding: mainly algae and fungi. Small species like D. minuta or D. pulex can use bacteria too.