Arcellinida

Arcellinida is a class of testate (cased) amoebae within the phylum Amoebozoa. Members of this group construct protective shells, or tests, from particles such as soil clays, sand grains, or biological materials. The tests are typically polymerized from secreted agglutinated material and vary in shape from globular to elongated forms.

Taxonomically, Arcellinida is divided into several orders, the most prominent being Arcellinida sensu stricto, for which the type genus is Arcellinida. Other groups include the order Trichochaeridae and the family Cyclopyxidae, all unified by the presence of a test and a lobopodial pseudopodium. Morphological diversity is greatest in the arrangement of the tests, with species exhibiting radial or concentric symmetry, and some having decorative carapaces with ribs or spines.

Ecologically, arcellinid amoebae inhabit a range of moist habitats, from forest leaf litter and peat bogs to freshwater sediments. They feed on bacteria and small detrital particles, thereby contributing to nutrient cycling and soil fertility. Their shells provide protection against desiccation, predators, and environmental stress, and the agglutinated composition allows them to adapt to local mineral availability.

The life cycle of most Arcellinida consists of a single trophozoite stage that can encyst when conditions become unfavorable. Encysted cells form a thick test and resist extreme conditions until favorable moisture and temperature return, at which point the cyst wall dissolves and the cell resumes active feeding. Some species also reproduce by binary fission, while others may form temporary cysts in response to stress.

Arcellinida have been used in paleoenvironmental reconstructions because test assemblages preserve well in sediment archives. Their morphological simplicity and robust tests make them useful indicators of past climate and substrate conditions. The group remains a focus of research in protist taxonomy, phylogenomics, and functional ecology.