Difflugia pyriformis group
Perty (1849, 1852) described Difflugia pyriformis (Fig. 1a) the shell of which is 140-200 µm long, pyriform or sometimes irregular in shape, and with a rough surface. Since then this species has been subject of many discussions and misinterpretations. Authors considered it being identical with D. oblonga Ehrenberg 1832 (Fig. 1b) and even modified the original description of the latter to fit that of D. pyriformis.
Pyriform or pear shaped tests are not rare, but differ in shape and size, ranging from about 60 till over 600 µm. It's not likely that all these shells belong to one species. However it's extremely difficult to distinguish between species solely based on light microscopical characters. DNA sequencing can be very helpful here. Traits which might be important to distinguishing between species are:
- length and breadth of the test
- L/B ratio
- diameter of the aperture
- construction of the pseudostome
- presence or absence of symbiotic algae
- relative length of the neck
- structure of the nucleus
- cyst formation
As long as we don't have sufficient matches between DNA sequences and morphological characters it is nearly impossible to know which species belongs to a specific test.