Difflugia curvicaulis Penard, 1899
Diagnosis: Shell tubular, narrowing from the fundus to the aperture, usually transparent, colorless or brown, with a distinct aboral horn, often curved and sometimes perforated at the apex. The horn is never perfectly terminal, due to the fact that on one of the shell sides it continues the general curvature of the fundus. Shell usually with a smooth outline, composed of small and medium sized pieces of flat quartz and fragments of diatoms, embedded in an organic cement which forms a regular network; irregular thicker pieces of quartz as common in most other Difflugia species, are rare. Aperture circular, surrounded by regular arrangement of small particles. Nucleus spherical, with some irregularly shaped nucleoli laying close to the nuclear membrane.
Dimensions: Penard 170-200 µm (without horn); Length 146-232 µm; my measurements: 197-428 µm (including horn); nucleus 25-27 µm, nucleoli c. 6-8 µm.
Ecology: Freshwater; sediments of ditches and lakes. Locally abundant, otherwise rare.
Remarks: According to Penard (1899), D. curvicaulis differs from D. acuminata and D. elegans in the smoothness of the shell and the curved aboral horn. In his words: “This beautiful species is rather rare, and sporadic; it can be distinguished at first glance from Difflugia acuminata by a very great transparency due to its thin and hyaline plates, and by its posterior horn which is never perfectly terminal. The paradoxical position of this horn comes from the fact that on one of its sides it continues the general curvature of the fundus of the shell, while on the other side it forms with this shell a re-entrant angle which can be very pronounced. There is a perfectly constant character here, and one that I have recognized in all the individuals observed; but it should be noted that, seen from the front or from behind, this horn appears as terminal, so it is necessary to examine it from the side to fully appreciate its position. This horn is generally short and tubular, but often deformed in different ways by the position of the small scales that constitute it.”
This species was considered by Cash (1909) to differ from D. acuminata only in the shape of the terminal horn. Mazei and Warren (2012) consider this form as a junior synonym of D. acuminata. The main distinguishing character however is the structure of the nucleus. Penards description is quite clear, though his drawing is confusing. This drawing shows the surface of the nucleus, giving the impression that the nucleus is filled with nucleoli.
I have found shells of this species among D. acuminata shells. They could be distinguished from the latter by their transparency, caused by transparent and flat xenosomes and the absence of thicker sand grains, and the structure of the nucleus. A large part of these flat xenosomes were fragments of diatoms. another feature, well visible in typical shapes, as in the shells above, is the smooth transition from shell to spine on one side, similar to the transition from head to snout in an elephant fish.