Centropyxis discoides

Centropyxis discoides, 231 and 245 µm wide

Centropyxis discoides (Penard, 1902) Deflandre, 1929
Basionym: Centropyxis aculeata var. discoides Penard, 1890
Synonym: Centropyxis discoides var. grandistoma Chardez, 1966

Diagnosis: Shell usually more or less circular in outline, sometimes strongly deformed, distinctly flattened, and disc-shaped. Fundus area with 0-9 hollow spines. Shell composed of granular organic material embedded with some flat xenosomes, though rarely completely covered. Some larger xenosomes are present in the fundus area. Aperture round to oval, eccentric or nearly central, irregular or lobed, with two lateral pillars and 1-2 frontal pillars connecting the invaginated apertural border to the inner dorsal surface. Contains a single granular nucleus.

Dimensions: 135-320 µm (including spines). My measurements: 159-286 µm (excluding spines).

Habitat: Freshwater. On aquatic plants and in sediment.

Geographical distribution: Probably cosmopolitan.

Remarks: This is a relatively large species, with an average size of about 250 µm and a relatively flattened shell compared to other Centropyxis species. Smaller specimens can easily be mistaken for C. aculeata, a species that has not been sufficiently typified. Originally, C. discoides was described by Penard (1890) as a variety of C. aculeata. The images below show variations in shell shape and structure among specimens from the same location. It is quite possible that deviant forms have been described in the past as new species or varieties, such as C. aculeata var. grandis Deflandre, 1929, and C. discoides var. grandistoma Chardez, 1966.

This image shows the building plan of the pillars around the aperture. There are always two lateral pillars and one or two frontal pillars. When there is one frontal pillar, it is usually broad, otherwise there are two smaller pillars.

Aperture with two lateral pillars and two frontal pillars, all arrowed.

Aperture with two lateral pillars and one frontal pillar.

Aperture with two lateral pillars and one broad frontal pillar.

C. discoides, lateral view with two pillars or bridges visible.

A lateral pillar. The pillar is an extension of the organic lip.

Shell in lateral view.

All these shells were found in the same sample from Laegieskamp, Netherlands, July 2022. Their size ranges from 159-250 µm (mean 212 µm). Note the differences in shape, size and number of spines.

Shell in lateral (L) and dorsal view (R); the arrows indicate the position of the four bridges.

C. discoides, after Penard (1902)

Some specimens collect a lot of debris around their shell.

Nuclei

C. discoides, both from the same sample, on submerged plants in lakes near Kortenhoef, Netherlands

Specimen (235 µm) from Crailoo

C. discoides, 286 and 260 µm, both from the same sample, ditch in ‘s-Graveland, Netherlands. Notice the difference in the size of the apertures.

C. discoides, 225 and 245 µm in diameter, Crailoo, Netherlands

C. discoides, without spines, feeding on an algae

C. discoides, with 9 spines – Laegieskamp

C. discoides, 226 µm, with 8 spines – Laegieskamp

Centropyxis cf. discoides, 196 µm – Naardermeer

C. discoides, 235 µm

C. discoides, 224 µm – Crailoo

C. discoides, 248 µm – Lagieskamp

C. discoides, 245 µm. Similar shells with large apertures, but much smaller (147-155 µm), were described by Chardez (1966) as var. grandistoma – Naardermeer.