Difflugia acuminata species complex
Diagnosis: Shell long, cylindrical, slightly tapering from the posterior to the anterior end; posterior end pointed or with a more or less long, hollow and open tube or spine; shell composed of mineral grains, more or less mixed with diatom frustules; in sphagnum the test can be completely composed of diatoms; aperture circular.
Dimensions: Shells 150 µm, up to 400 µm or more. The late Dr. A.A. de Groot (pers. comm.) has found acuminated shells in Baarn, the Netherlands, which were 800 µm large!
Ecology: all kind of watertypes, ranging from oligotrophic till eutrophic.
Remarks: Difflugia acuminata is a polymorphic species. Several variations of Difflugia acuminata has been described as new species. On the other hand the name covers probably several real species which are lightmicroscopically hard to distinguish from the main morphotype. The most discriminating character is the shape en length of the spine, but in fact we don't know precisely how variable it is. Shells with relatively long spines have been described as Difflugia giganteacuminata and with curved spines as Difflugia curvicaulis.
This species complex covers all large Difflugia species with a shell length of 150-550 µm.
Mazei and Warren (2012) divide this group into three subgroups:
- Species with relatively short (150-250 µm) and narrow (60-100 µm) shells,
including D. acuminata typical form, D. curvicaulis, D. venusta, and D. ventricosa;
- Species with relatively short (175-270 µm) and broad (90-135 µm) shells,
including D. distenda and D. acutissima;
- Species with long (270-550 µm) shells, including D. acuminata var. magna and D. giganteacuminata.
However, the original description of D. acuminata by Ehrenberg (1838) gives a length of 330 µm. Therefore this species cannot be part of the first subgroup!