Centropyxis aculeata  (Ehrenberg, 1832) Stein, 1859
Basionym: Arcella aculeata Ehrenberg, 1832

Diagnosis: Shell cap‑shaped, yellow to brown, ovoid or circular, typically bearing four or more lateral spines. The shell consists of organic cement, usually densely embedded with quartz grains and diatom frustules, except for a smoother ventral region surrounding the aperture. The aperture is subcentral and invaginated, often supported by two internal struts connecting the apertural rim to the dorsal region. The posterior portion of the apertural rim is frequently visible externally as a flat, belt‑shaped lip.

Dimensions: According to Ehrenberg (1838), specimens reach up to 111 µm in diameter (excluding spines). Other authors report diameters of 92–178 µm, although it is not always clear whether these measurements refer to the shell with or without spines.

Ecology: Freshwater; occurring in Sphagnum, other submerged mosses, and in sediments of ditches and lakes.

Geographical distribution: Cosmopolitan.

Varieties:
Centropyxis aculeata var. grandis Deflandre, 1929 (without spines 150–80 µm)
Centropyxis aculeata var. gigantea Decloitre, 1965 (without spines >300 µm)

Remarks: Centropyxis aculeata was first mentioned by Ehrenberg in 1830 as Arcella aculeata, a nomen nudum because no valid description accompanied the name. In 1832, Ehrenberg provided a formal description including diagnostic morphological characters. Accordingly, the correct authorship and date of the species are Ehrenberg, 1832.

Centropyxis aculeata is the type species of the genus Centropyxis by subsequent monotypy (Stein, 1859):
“…und eine neue, auf Arcella aculeata Ehbg. zu gründende Gattung Centropyxis St.”
(“…and, based on Arcella aculeata Ehbg., a new genus Centropyxis is to be established.”)

Specimens cultured in the absence of extraneous mineral particles produce a proteinaceous shell resembling that of species of Arcella. When encysted, the organism becomes spherical within the broader portion of the shell; the cytoplasm is granulated, colorless, or contains green globules. Cash (1905) noted that specimens vary widely from the type description and also from one another in size and structure. Deflandre (1929), in his revision of the genus, described three varieties of C. aculeata differing in size, shape, and number of spines. In clonal cultures, both large and heavily spined individuals may occur, although such forms represent only a small fraction of the population.

The history of the name Centropyxis aculeata
The first mention of Centropyxis aculeata is by Ehrenberg, who at that time used the name Arcella aculeata, in a presentation to the Royal Academy of Sciences in Berlin in 1830. This presentation was published under the title “Beiträge zur Kenntniss der Organisation der Infusorien und ihrer geographischen Verbreitung, besonders in Sibirien” (in Abhandlungen der Akademie der Wissenschaften zu Berlin, 1830, p. 40). Ehrenberg applied the name with the adjacent al. sp., meaning “alia species”, comparable to the modern use of cf. This notation indicated that the specimen belonged to the same genus but that its precise species identity was uncertain. Because no formal description accompanied the name, it constitutes a nomen nudum.

One year later, in 1831, Ehrenberg presented the following description of Arcella aculeata:

A. aculeata E., stachliches Kapseltierchen. Durchmesser 1/18’’’. Panzer unregelmässig, halbkugelig, mit hornartigen harten Fortsätzen am Rande, die gewöhnlich einseitig sind. Gelbbraun. Berlin. Das Maass giebt den Breitendurchmesser, den grössten, des Panzers.

(Translated: A. aculeata E., spiny little testate animal. Diameter 1/18’’’. Test irregular, hemispherical, with horn‑like, hard projections at the margin, which are usually unilateral. Yellow‑brown. Berlin. The measurement refers to the breadth‑diameter, the largest dimension of the test.)

This presentation was published in 1832 as Über die Entwickelung und Lebensdauer der Infusionsthiere; nebst ferneren Beiträgen zu einer Vergleichung ihrer organischen Systeme in Abhandlungen der Königlichen Akademie der Wissenschaften zu Berlin, Physikalische Klasse 1831: 1–154, pls I–IV.

Because this publication contains the first valid description of the species, 1832 is the official date of publication under the rules of the ICZN.

Six years later, Ehrenberg provided a more detailed account of the species (Ehrenberg, C.G., 1838: Die Infusionsthierchen als vollkommene Organismen. Leipzig, pp. 133–134):

Arcella aculeata, stachliges Kapselthierchen. Tafel IX. Fig. VI.
“A. hemisphaerica, saepe difformis, margine aculeata, lorica e fibris bacillaribus brevibus (paleaceis) constante, flavicans.”
Arcelle épineuse, hémisphérique, souvent difforme, épineuse au bord, ayant la carapace formée de fibres bacillaires courtes, comme de paille menue.
(= Spiny Arcella, hemispherical, often deformed, spiny at the margin, with a shell composed of short rod‑like fibers, like fine straw.)

Ehrenberg added the following description (Translated from German into English, my comments in red.):
This form is generally rarer than the previous one (= Arcella vulgaris), although at times it is also common in Berlin. Since 1830, I have repeatedly encountered it, particularly in large numbers on June 26, 1834 and March 20, 1835, usually together with the other species. Beneath each spine there appeared to be a soft appendage (= pseudopod), as if situated under a semicylindrical projection, although I also observed such appendages singly on the opposite side. The spines are often restricted to one half of the margin of the disc, and the central shell opening is usually off‑center. I observed 6 to 8 mostly curved spines, and occasionally 3, 2, or 1. “Stomach cells” (= food vacuoles) were clearly visible inside, sometimes filled with indigo, which is why the organism was already counted among the true “stomach animals” in 1830. Upon charring, the structure of the shell—composed of short, rigid fibers (= diatom frustules?) resembling chaff—was severely damaged. Size up to 1/18 Linie (without spines) (= up to 111 µm; see my remarks on the Linie in Difflugia oblonga).

Explanation of the figures, Plate IX. Fig. VI.
3 individuals of different shapes are shown, all enlarged 300 times.
The sticks in Fig. A. are not swallowed naviculae but the fibers of the shell wall. The vesicles in between are “stomach cells” (=protoplasm). The large, bright, round spot is the lower shell opening.
Fig. B. has 3 spines, an opposite contractile process and middle “stomach cells”.
Fig. C. is an empty crooked bowl.
The latter 2 figures are not more homogeneous than the first in terms of the shell structure, but just not shown in the drawing. All have the spines on the left.