Genus Centropyxis Stein, 1857
Centropyxis is a genus of testate amoebae with a discoid, flattened test, somewhat beret‑shaped. The dorsal surface is rounded, while the ventral side is flat to concave, with a ventral aperture that may be circular or irregular, and is displaced toward one end. The shell may be slightly flattened at the apertural end. At the posterior end, or sometimes around the entire periphery, spines may be present. Within a population, individuals may bear anywhere from zero to thirteen spines. The shell surface is smooth: dorsally it carries varying amounts of mineral particles or diatoms cemented together by organic material, whereas ventrally it appears polished due to a higher proportion of cement and much smaller grains.
Two shell types can be distinguished.
The first type is bilaterally symmetrical, rounded, and more strongly flattened anteriorly than posteriorly. The ventral surface is flat. Spines are often present on both sides and at the rear. The aperture is ventral, anterior, roundish, and has ventral lips that are recurved and invaginated. The ventral margin of the aperture is frequently connected to the dorsal face by flat pillars or bridges. The shell may be entirely organic or constructed with mineral particles or diatoms. Shape, number of spines, and size are extremely variable. This type occurs mainly in freshwater habitats.
The second type has a circular shell or is elongated in ventral view. In lateral view, the oral region is slightly flattened; the aperture is subterminal, with only the ventral lip incurved. No connections or bridges occur between the ventral and dorsal faces. Some species of this type are common in freshwater habitats and Sphagnum, but most are found in drier mosses and humus.
More than 130 species and many varieties have been described, but many descriptions are inadequate. Identifying Centropyxis species is often extremely difficult or even impossible.
Taxonomic position of bridged Centropyxis species
The position and number of bridges in a Centropyxis shell may serve as valuable characters for distinguishing and describing species more accurately. Lahr et al. (2008) demonstrated, through their description and SEM images, that the specimens they identified as C. aculeata possessed four support points, externally visible as small indentations under SEM. Siemensma (2025) discovered a large population of a small C. aculeata (or a subspecies thereof) in a sample from the Odenwald region, Germany. All specimens exhibited two frontal struts attached to the anterior part of the inner dorsal region. In shells belonging to C. spinosa, two pillars were consistently observed on each side of the aperture. This feature is also evident in the images published by Todorov & Bankov (2019) under the name C. spinosa.
Dekhtyar (2009) proposed the genus Armipyxis to accommodate Centropyxis species with bridges, designating Armipyxis discoides as the type species. This was an elegant attempt to introduce some order into the large assemblage of species currently placed within Centropyxis. However, the type species of Centropyxis is C. aculeata (Ehrenberg, 1830). Although bridges were not mentioned in the original description, it is highly likely that they were present but could not be observed with the techniques available at the time, as Leidy (1879) had already noted.
Leidy (1879) described and illustrated C. aculeata specimens in which these bridges are clearly visible. It is probable that Leidy also included under C. aculeata certain species that had not yet been formally described, such as C. spinosa, C. lapponica, and C. discoides. Penard (1902) likewise referred to internal bridges in his description of C. aculeata, and Deflandre (1929), in his monograph, similarly indicated that partitions might be present in this species.
Based on these observations, together with more recent findings by Lahr et al. (2008) and Siemensma (2025), it can be concluded that C. aculeata possesses internal bridges. Consequently, the validity of the genus Armipyxis is undermined, as C. aculeata is the type species of Centropyxis and therefore cannot be transferred to Armipyxis.
