Lacogromia pawlowskii Siemensma and Holzmann, 2023
Diagnosis: Shell elongated to broadly pyriform or lens- or spindle-shaped, dark, with a relatively thick agglutinated wall and a large aperture. Large single nucleus present or several smaller ones, with nucleolar material arranged in a thick layer of small granules close to the nuclear membrane.
Description. The agglutinated tests are elongate ovoid, pyriform or oblong. Shell length varies from 146 to 334 µm (mean 233 µm, n=13). In cross-section, the tests are round, with a diameter varying from 80 to 157 µm (mean 120 µm, n=13). The length/width ratio is 1.5-2.5 (mean 2.0). The aperture is located terminally, roughly circular, straight, or slightly oblique and enlarged when protoplasmic strands flow out of the cell or larger food particles are ingested. The aperture is surrounded by a flexible slightly widened agglutinated collar with relatively large diameter, ranging from 29-71 µm. In two specimens, a single nucleus was observed with a diameter of 61.2 and 29.2 µm respectively. In optical cross-section a 4.0-5.7 µm thick ring of nucleolar granules located along the nuclear membrane). In a third specimen, at least five nuclei were counted, with a diameter of 15.8-17.4 µm (mean 16.6 µm). The test wall is brown to blackish in transmitted light, opaque, composed of small, mostly flat colorless mineral particles ranging in size from about 0.7 to 2.2 µm. The thickness of the test wall was measured in two specimens embedded in Hydro-Matrix© and varied from 10.7-13.8 µm in the first specimen and 8.2-11.9 µm in the second specimen. Due to the opacity of the test wall, the presence of a peduncle could not be detected. The cytoplasm contains food vacuoles, some contractile vacuoles and numerous small yellow, bi-refringent rod-shaped crystalline inclusions attached to a spherical non-refractive body.
Observed food. Diatoms.
Habitat: In organic sediment of stagnant mesotrophic freshwater bodies. Type locality: A ditch along nature reserve Naardermeer, near the town of Bussum, the Netherlands (52°16’52.4″N 5°07’07.9″E).
Differential diagnosis. Morphologically, L. pawlowskii can be distinguished from L. cassipara by its thicker test wall and wider aperture. It can be distinguished from L. brunneri and L. squamosa by the structure of its nucleus. L. pawlowskii branches as sister to L. cassipara (86% BV) and several environmental clones. The environmental clone Leman 6 (AJ318011) from Lake Geneva (Holzmann and Pawlowski, 2002) is branching close to L. pawlowskii, but the sequences are not identical. Pairwise genetic distance between L. pawlowskii and environmental clone Leman 6 is 0.002. Lacogromiidae are supported by high bootstrap values (93%). The obtained sequences of L. pawlowskii are identical, sequence length is 849 nucleotides, the GC content is 44%.
